| <!DOCTYPE HTML> |
| <html lang="en" class="sidebar-visible no-js light"> |
| <head> |
| <!-- Book generated using mdBook --> |
| <meta charset="UTF-8"> |
| <title>Testcase: map-reduce - Rust By Example</title> |
| <!-- Custom HTML head --> |
| |
| |
| <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> |
| <meta name="description" content="Rust by Example (RBE) is a collection of runnable examples that illustrate various Rust concepts and standard libraries."> |
| <meta name="viewport" content="width=device-width, initial-scale=1"> |
| <meta name="theme-color" content="#ffffff" /> |
| |
| <link rel="icon" href="../../favicon.svg"> |
| <link rel="shortcut icon" href="../../favicon.png"> |
| <link rel="stylesheet" href="../../css/variables.css"> |
| <link rel="stylesheet" href="../../css/general.css"> |
| <link rel="stylesheet" href="../../css/chrome.css"> |
| <link rel="stylesheet" href="../../css/print.css" media="print"> |
| <!-- Fonts --> |
| <link rel="stylesheet" href="../../FontAwesome/css/font-awesome.css"> |
| <link rel="stylesheet" href="../../fonts/fonts.css"> |
| <!-- Highlight.js Stylesheets --> |
| <link rel="stylesheet" href="../../highlight.css"> |
| <link rel="stylesheet" href="../../tomorrow-night.css"> |
| <link rel="stylesheet" href="../../ayu-highlight.css"> |
| |
| <!-- Custom theme stylesheets --> |
| </head> |
| <body> |
| <!-- Provide site root to javascript --> |
| <script type="text/javascript"> |
| var path_to_root = "../../"; |
| var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light"; |
| </script> |
| |
| <!-- Work around some values being stored in localStorage wrapped in quotes --> |
| <script type="text/javascript"> |
| try { |
| var theme = localStorage.getItem('mdbook-theme'); |
| var sidebar = localStorage.getItem('mdbook-sidebar'); |
| |
| if (theme.startsWith('"') && theme.endsWith('"')) { |
| localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1)); |
| } |
| |
| if (sidebar.startsWith('"') && sidebar.endsWith('"')) { |
| localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1)); |
| } |
| } catch (e) { } |
| </script> |
| |
| <!-- Set the theme before any content is loaded, prevents flash --> |
| <script type="text/javascript"> |
| var theme; |
| try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { } |
| if (theme === null || theme === undefined) { theme = default_theme; } |
| var html = document.querySelector('html'); |
| html.classList.remove('no-js') |
| html.classList.remove('light') |
| html.classList.add(theme); |
| html.classList.add('js'); |
| </script> |
| |
| <!-- Hide / unhide sidebar before it is displayed --> |
| <script type="text/javascript"> |
| var html = document.querySelector('html'); |
| var sidebar = 'hidden'; |
| if (document.body.clientWidth >= 1080) { |
| try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { } |
| sidebar = sidebar || 'visible'; |
| } |
| html.classList.remove('sidebar-visible'); |
| html.classList.add("sidebar-" + sidebar); |
| </script> |
| |
| <nav id="sidebar" class="sidebar" aria-label="Table of contents"> |
| <div class="sidebar-scrollbox"> |
| <ol class="chapter"><li class="chapter-item affix "><a href="../../index.html">Introduction</a></li><li class="chapter-item "><a href="../../hello.html"><strong aria-hidden="true">1.</strong> Hello World</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../hello/comment.html"><strong aria-hidden="true">1.1.</strong> Comments</a></li><li class="chapter-item "><a href="../../hello/print.html"><strong aria-hidden="true">1.2.</strong> Formatted print</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../hello/print/print_debug.html"><strong aria-hidden="true">1.2.1.</strong> Debug</a></li><li class="chapter-item "><a href="../../hello/print/print_display.html"><strong aria-hidden="true">1.2.2.</strong> Display</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../hello/print/print_display/testcase_list.html"><strong aria-hidden="true">1.2.2.1.</strong> Testcase: List</a></li></ol></li><li class="chapter-item "><a href="../../hello/print/fmt.html"><strong aria-hidden="true">1.2.3.</strong> Formatting</a></li></ol></li></ol></li><li class="chapter-item "><a href="../../primitives.html"><strong aria-hidden="true">2.</strong> Primitives</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../primitives/literals.html"><strong aria-hidden="true">2.1.</strong> Literals and operators</a></li><li class="chapter-item "><a href="../../primitives/tuples.html"><strong aria-hidden="true">2.2.</strong> Tuples</a></li><li class="chapter-item "><a href="../../primitives/array.html"><strong aria-hidden="true">2.3.</strong> Arrays and Slices</a></li></ol></li><li class="chapter-item "><a href="../../custom_types.html"><strong aria-hidden="true">3.</strong> Custom Types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../custom_types/structs.html"><strong aria-hidden="true">3.1.</strong> Structures</a></li><li class="chapter-item "><a href="../../custom_types/enum.html"><strong aria-hidden="true">3.2.</strong> Enums</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../custom_types/enum/enum_use.html"><strong aria-hidden="true">3.2.1.</strong> use</a></li><li class="chapter-item "><a href="../../custom_types/enum/c_like.html"><strong aria-hidden="true">3.2.2.</strong> C-like</a></li><li class="chapter-item "><a href="../../custom_types/enum/testcase_linked_list.html"><strong aria-hidden="true">3.2.3.</strong> Testcase: linked-list</a></li></ol></li><li class="chapter-item "><a href="../../custom_types/constants.html"><strong aria-hidden="true">3.3.</strong> constants</a></li></ol></li><li class="chapter-item "><a href="../../variable_bindings.html"><strong aria-hidden="true">4.</strong> Variable Bindings</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../variable_bindings/mut.html"><strong aria-hidden="true">4.1.</strong> Mutability</a></li><li class="chapter-item "><a href="../../variable_bindings/scope.html"><strong aria-hidden="true">4.2.</strong> Scope and Shadowing</a></li><li class="chapter-item "><a href="../../variable_bindings/declare.html"><strong aria-hidden="true">4.3.</strong> Declare first</a></li><li class="chapter-item "><a href="../../variable_bindings/freeze.html"><strong aria-hidden="true">4.4.</strong> Freezing</a></li></ol></li><li class="chapter-item "><a href="../../types.html"><strong aria-hidden="true">5.</strong> Types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../types/cast.html"><strong aria-hidden="true">5.1.</strong> Casting</a></li><li class="chapter-item "><a href="../../types/literals.html"><strong aria-hidden="true">5.2.</strong> Literals</a></li><li class="chapter-item "><a href="../../types/inference.html"><strong aria-hidden="true">5.3.</strong> Inference</a></li><li class="chapter-item "><a href="../../types/alias.html"><strong aria-hidden="true">5.4.</strong> Aliasing</a></li></ol></li><li class="chapter-item "><a href="../../conversion.html"><strong aria-hidden="true">6.</strong> Conversion</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../conversion/from_into.html"><strong aria-hidden="true">6.1.</strong> From and Into</a></li><li class="chapter-item "><a href="../../conversion/try_from_try_into.html"><strong aria-hidden="true">6.2.</strong> TryFrom and TryInto</a></li><li class="chapter-item "><a href="../../conversion/string.html"><strong aria-hidden="true">6.3.</strong> To and from Strings</a></li></ol></li><li class="chapter-item "><a href="../../expression.html"><strong aria-hidden="true">7.</strong> Expressions</a></li><li class="chapter-item "><a href="../../flow_control.html"><strong aria-hidden="true">8.</strong> Flow of Control</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../flow_control/if_else.html"><strong aria-hidden="true">8.1.</strong> if/else</a></li><li class="chapter-item "><a href="../../flow_control/loop.html"><strong aria-hidden="true">8.2.</strong> loop</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../flow_control/loop/nested.html"><strong aria-hidden="true">8.2.1.</strong> Nesting and labels</a></li><li class="chapter-item "><a href="../../flow_control/loop/return.html"><strong aria-hidden="true">8.2.2.</strong> Returning from loops</a></li></ol></li><li class="chapter-item "><a href="../../flow_control/while.html"><strong aria-hidden="true">8.3.</strong> while</a></li><li class="chapter-item "><a href="../../flow_control/for.html"><strong aria-hidden="true">8.4.</strong> for and range</a></li><li class="chapter-item "><a href="../../flow_control/match.html"><strong aria-hidden="true">8.5.</strong> match</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../flow_control/match/destructuring.html"><strong aria-hidden="true">8.5.1.</strong> Destructuring</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../flow_control/match/destructuring/destructure_tuple.html"><strong aria-hidden="true">8.5.1.1.</strong> tuples</a></li><li class="chapter-item "><a href="../../flow_control/match/destructuring/destructure_enum.html"><strong aria-hidden="true">8.5.1.2.</strong> enums</a></li><li class="chapter-item "><a href="../../flow_control/match/destructuring/destructure_pointers.html"><strong aria-hidden="true">8.5.1.3.</strong> pointers/ref</a></li><li class="chapter-item "><a href="../../flow_control/match/destructuring/destructure_structures.html"><strong aria-hidden="true">8.5.1.4.</strong> structs</a></li></ol></li><li class="chapter-item "><a href="../../flow_control/match/guard.html"><strong aria-hidden="true">8.5.2.</strong> Guards</a></li><li class="chapter-item "><a href="../../flow_control/match/binding.html"><strong aria-hidden="true">8.5.3.</strong> Binding</a></li></ol></li><li class="chapter-item "><a href="../../flow_control/if_let.html"><strong aria-hidden="true">8.6.</strong> if let</a></li><li class="chapter-item "><a href="../../flow_control/while_let.html"><strong aria-hidden="true">8.7.</strong> while let</a></li></ol></li><li class="chapter-item "><a href="../../fn.html"><strong aria-hidden="true">9.</strong> Functions</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../fn/methods.html"><strong aria-hidden="true">9.1.</strong> Methods</a></li><li class="chapter-item "><a href="../../fn/closures.html"><strong aria-hidden="true">9.2.</strong> Closures</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../fn/closures/capture.html"><strong aria-hidden="true">9.2.1.</strong> Capturing</a></li><li class="chapter-item "><a href="../../fn/closures/input_parameters.html"><strong aria-hidden="true">9.2.2.</strong> As input parameters</a></li><li class="chapter-item "><a href="../../fn/closures/anonymity.html"><strong aria-hidden="true">9.2.3.</strong> Type anonymity</a></li><li class="chapter-item "><a href="../../fn/closures/input_functions.html"><strong aria-hidden="true">9.2.4.</strong> Input functions</a></li><li class="chapter-item "><a href="../../fn/closures/output_parameters.html"><strong aria-hidden="true">9.2.5.</strong> As output parameters</a></li><li class="chapter-item "><a href="../../fn/closures/closure_examples.html"><strong aria-hidden="true">9.2.6.</strong> Examples in std</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../fn/closures/closure_examples/iter_any.html"><strong aria-hidden="true">9.2.6.1.</strong> Iterator::any</a></li><li class="chapter-item "><a href="../../fn/closures/closure_examples/iter_find.html"><strong aria-hidden="true">9.2.6.2.</strong> Searching through iterators</a></li></ol></li></ol></li><li class="chapter-item "><a href="../../fn/hof.html"><strong aria-hidden="true">9.3.</strong> Higher Order Functions</a></li><li class="chapter-item "><a href="../../fn/diverging.html"><strong aria-hidden="true">9.4.</strong> Diverging functions</a></li></ol></li><li class="chapter-item "><a href="../../mod.html"><strong aria-hidden="true">10.</strong> Modules</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../mod/visibility.html"><strong aria-hidden="true">10.1.</strong> Visibility</a></li><li class="chapter-item "><a href="../../mod/struct_visibility.html"><strong aria-hidden="true">10.2.</strong> Struct visibility</a></li><li class="chapter-item "><a href="../../mod/use.html"><strong aria-hidden="true">10.3.</strong> The use declaration</a></li><li class="chapter-item "><a href="../../mod/super.html"><strong aria-hidden="true">10.4.</strong> super and self</a></li><li class="chapter-item "><a href="../../mod/split.html"><strong aria-hidden="true">10.5.</strong> File hierarchy</a></li></ol></li><li class="chapter-item "><a href="../../crates.html"><strong aria-hidden="true">11.</strong> Crates</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../crates/lib.html"><strong aria-hidden="true">11.1.</strong> Creating a Library</a></li><li class="chapter-item "><a href="../../crates/using_lib.html"><strong aria-hidden="true">11.2.</strong> Using a Library</a></li></ol></li><li class="chapter-item "><a href="../../cargo.html"><strong aria-hidden="true">12.</strong> Cargo</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../cargo/deps.html"><strong aria-hidden="true">12.1.</strong> Dependencies</a></li><li class="chapter-item "><a href="../../cargo/conventions.html"><strong aria-hidden="true">12.2.</strong> Conventions</a></li><li class="chapter-item "><a href="../../cargo/test.html"><strong aria-hidden="true">12.3.</strong> Tests</a></li><li class="chapter-item "><a href="../../cargo/build_scripts.html"><strong aria-hidden="true">12.4.</strong> Build Scripts</a></li></ol></li><li class="chapter-item "><a href="../../attribute.html"><strong aria-hidden="true">13.</strong> Attributes</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../attribute/unused.html"><strong aria-hidden="true">13.1.</strong> dead_code</a></li><li class="chapter-item "><a href="../../attribute/crate.html"><strong aria-hidden="true">13.2.</strong> Crates</a></li><li class="chapter-item "><a href="../../attribute/cfg.html"><strong aria-hidden="true">13.3.</strong> cfg</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../attribute/cfg/custom.html"><strong aria-hidden="true">13.3.1.</strong> Custom</a></li></ol></li></ol></li><li class="chapter-item "><a href="../../generics.html"><strong aria-hidden="true">14.</strong> Generics</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../generics/gen_fn.html"><strong aria-hidden="true">14.1.</strong> Functions</a></li><li class="chapter-item "><a href="../../generics/impl.html"><strong aria-hidden="true">14.2.</strong> Implementation</a></li><li class="chapter-item "><a href="../../generics/gen_trait.html"><strong aria-hidden="true">14.3.</strong> Traits</a></li><li class="chapter-item "><a href="../../generics/bounds.html"><strong aria-hidden="true">14.4.</strong> Bounds</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../generics/bounds/testcase_empty.html"><strong aria-hidden="true">14.4.1.</strong> Testcase: empty bounds</a></li></ol></li><li class="chapter-item "><a href="../../generics/multi_bounds.html"><strong aria-hidden="true">14.5.</strong> Multiple bounds</a></li><li class="chapter-item "><a href="../../generics/where.html"><strong aria-hidden="true">14.6.</strong> Where clauses</a></li><li class="chapter-item "><a href="../../generics/new_types.html"><strong aria-hidden="true">14.7.</strong> New Type Idiom</a></li><li class="chapter-item "><a href="../../generics/assoc_items.html"><strong aria-hidden="true">14.8.</strong> Associated items</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../generics/assoc_items/the_problem.html"><strong aria-hidden="true">14.8.1.</strong> The Problem</a></li><li class="chapter-item "><a href="../../generics/assoc_items/types.html"><strong aria-hidden="true">14.8.2.</strong> Associated types</a></li></ol></li><li class="chapter-item "><a href="../../generics/phantom.html"><strong aria-hidden="true">14.9.</strong> Phantom type parameters</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../generics/phantom/testcase_units.html"><strong aria-hidden="true">14.9.1.</strong> Testcase: unit clarification</a></li></ol></li></ol></li><li class="chapter-item "><a href="../../scope.html"><strong aria-hidden="true">15.</strong> Scoping rules</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../scope/raii.html"><strong aria-hidden="true">15.1.</strong> RAII</a></li><li class="chapter-item "><a href="../../scope/move.html"><strong aria-hidden="true">15.2.</strong> Ownership and moves</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../scope/move/mut.html"><strong aria-hidden="true">15.2.1.</strong> Mutability</a></li><li class="chapter-item "><a href="../../scope/move/partial_move.html"><strong aria-hidden="true">15.2.2.</strong> Partial moves</a></li></ol></li><li class="chapter-item "><a href="../../scope/borrow.html"><strong aria-hidden="true">15.3.</strong> Borrowing</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../scope/borrow/mut.html"><strong aria-hidden="true">15.3.1.</strong> Mutability</a></li><li class="chapter-item "><a href="../../scope/borrow/alias.html"><strong aria-hidden="true">15.3.2.</strong> Aliasing</a></li><li class="chapter-item "><a href="../../scope/borrow/ref.html"><strong aria-hidden="true">15.3.3.</strong> The ref pattern</a></li></ol></li><li class="chapter-item "><a href="../../scope/lifetime.html"><strong aria-hidden="true">15.4.</strong> Lifetimes</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../scope/lifetime/explicit.html"><strong aria-hidden="true">15.4.1.</strong> Explicit annotation</a></li><li class="chapter-item "><a href="../../scope/lifetime/fn.html"><strong aria-hidden="true">15.4.2.</strong> Functions</a></li><li class="chapter-item "><a href="../../scope/lifetime/methods.html"><strong aria-hidden="true">15.4.3.</strong> Methods</a></li><li class="chapter-item "><a href="../../scope/lifetime/struct.html"><strong aria-hidden="true">15.4.4.</strong> Structs</a></li><li class="chapter-item "><a href="../../scope/lifetime/trait.html"><strong aria-hidden="true">15.4.5.</strong> Traits</a></li><li class="chapter-item "><a href="../../scope/lifetime/lifetime_bounds.html"><strong aria-hidden="true">15.4.6.</strong> Bounds</a></li><li class="chapter-item "><a href="../../scope/lifetime/lifetime_coercion.html"><strong aria-hidden="true">15.4.7.</strong> Coercion</a></li><li class="chapter-item "><a href="../../scope/lifetime/static_lifetime.html"><strong aria-hidden="true">15.4.8.</strong> Static</a></li><li class="chapter-item "><a href="../../scope/lifetime/elision.html"><strong aria-hidden="true">15.4.9.</strong> Elision</a></li></ol></li></ol></li><li class="chapter-item "><a href="../../trait.html"><strong aria-hidden="true">16.</strong> Traits</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../trait/derive.html"><strong aria-hidden="true">16.1.</strong> Derive</a></li><li class="chapter-item "><a href="../../trait/dyn.html"><strong aria-hidden="true">16.2.</strong> Returning Traits with dyn</a></li><li class="chapter-item "><a href="../../trait/ops.html"><strong aria-hidden="true">16.3.</strong> Operator Overloading</a></li><li class="chapter-item "><a href="../../trait/drop.html"><strong aria-hidden="true">16.4.</strong> Drop</a></li><li class="chapter-item "><a href="../../trait/iter.html"><strong aria-hidden="true">16.5.</strong> Iterators</a></li><li class="chapter-item "><a href="../../trait/impl_trait.html"><strong aria-hidden="true">16.6.</strong> impl Trait</a></li><li class="chapter-item "><a href="../../trait/clone.html"><strong aria-hidden="true">16.7.</strong> Clone</a></li><li class="chapter-item "><a href="../../trait/supertraits.html"><strong aria-hidden="true">16.8.</strong> Supertraits</a></li><li class="chapter-item "><a href="../../trait/disambiguating.html"><strong aria-hidden="true">16.9.</strong> Disambiguating overlapping traits</a></li></ol></li><li class="chapter-item "><a href="../../macros.html"><strong aria-hidden="true">17.</strong> macro_rules!</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../macros/syntax.html"><strong aria-hidden="true">17.1.</strong> Syntax</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../macros/designators.html"><strong aria-hidden="true">17.1.1.</strong> Designators</a></li><li class="chapter-item "><a href="../../macros/overload.html"><strong aria-hidden="true">17.1.2.</strong> Overload</a></li><li class="chapter-item "><a href="../../macros/repeat.html"><strong aria-hidden="true">17.1.3.</strong> Repeat</a></li></ol></li><li class="chapter-item "><a href="../../macros/dry.html"><strong aria-hidden="true">17.2.</strong> DRY (Don't Repeat Yourself)</a></li><li class="chapter-item "><a href="../../macros/dsl.html"><strong aria-hidden="true">17.3.</strong> DSL (Domain Specific Languages)</a></li><li class="chapter-item "><a href="../../macros/variadics.html"><strong aria-hidden="true">17.4.</strong> Variadics</a></li></ol></li><li class="chapter-item "><a href="../../error.html"><strong aria-hidden="true">18.</strong> Error handling</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../error/panic.html"><strong aria-hidden="true">18.1.</strong> panic</a></li><li class="chapter-item "><a href="../../error/option_unwrap.html"><strong aria-hidden="true">18.2.</strong> Option & unwrap</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../error/option_unwrap/question_mark.html"><strong aria-hidden="true">18.2.1.</strong> Unpacking options with ?</a></li><li class="chapter-item "><a href="../../error/option_unwrap/map.html"><strong aria-hidden="true">18.2.2.</strong> Combinators: map</a></li><li class="chapter-item "><a href="../../error/option_unwrap/and_then.html"><strong aria-hidden="true">18.2.3.</strong> Combinators: and_then</a></li></ol></li><li class="chapter-item "><a href="../../error/result.html"><strong aria-hidden="true">18.3.</strong> Result</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../error/result/result_map.html"><strong aria-hidden="true">18.3.1.</strong> map for Result</a></li><li class="chapter-item "><a href="../../error/result/result_alias.html"><strong aria-hidden="true">18.3.2.</strong> aliases for Result</a></li><li class="chapter-item "><a href="../../error/result/early_returns.html"><strong aria-hidden="true">18.3.3.</strong> Early returns</a></li><li class="chapter-item "><a href="../../error/result/enter_question_mark.html"><strong aria-hidden="true">18.3.4.</strong> Introducing ?</a></li></ol></li><li class="chapter-item "><a href="../../error/multiple_error_types.html"><strong aria-hidden="true">18.4.</strong> Multiple error types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../error/multiple_error_types/option_result.html"><strong aria-hidden="true">18.4.1.</strong> Pulling Results out of Options</a></li><li class="chapter-item "><a href="../../error/multiple_error_types/define_error_type.html"><strong aria-hidden="true">18.4.2.</strong> Defining an error type</a></li><li class="chapter-item "><a href="../../error/multiple_error_types/boxing_errors.html"><strong aria-hidden="true">18.4.3.</strong> Boxing errors</a></li><li class="chapter-item "><a href="../../error/multiple_error_types/reenter_question_mark.html"><strong aria-hidden="true">18.4.4.</strong> Other uses of ?</a></li><li class="chapter-item "><a href="../../error/multiple_error_types/wrap_error.html"><strong aria-hidden="true">18.4.5.</strong> Wrapping errors</a></li></ol></li><li class="chapter-item "><a href="../../error/iter_result.html"><strong aria-hidden="true">18.5.</strong> Iterating over Results</a></li></ol></li><li class="chapter-item "><a href="../../std.html"><strong aria-hidden="true">19.</strong> Std library types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../std/box.html"><strong aria-hidden="true">19.1.</strong> Box, stack and heap</a></li><li class="chapter-item "><a href="../../std/vec.html"><strong aria-hidden="true">19.2.</strong> Vectors</a></li><li class="chapter-item "><a href="../../std/str.html"><strong aria-hidden="true">19.3.</strong> Strings</a></li><li class="chapter-item "><a href="../../std/option.html"><strong aria-hidden="true">19.4.</strong> Option</a></li><li class="chapter-item "><a href="../../std/result.html"><strong aria-hidden="true">19.5.</strong> Result</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../std/result/question_mark.html"><strong aria-hidden="true">19.5.1.</strong> ?</a></li></ol></li><li class="chapter-item "><a href="../../std/panic.html"><strong aria-hidden="true">19.6.</strong> panic!</a></li><li class="chapter-item "><a href="../../std/hash.html"><strong aria-hidden="true">19.7.</strong> HashMap</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../std/hash/alt_key_types.html"><strong aria-hidden="true">19.7.1.</strong> Alternate/custom key types</a></li><li class="chapter-item "><a href="../../std/hash/hashset.html"><strong aria-hidden="true">19.7.2.</strong> HashSet</a></li></ol></li><li class="chapter-item "><a href="../../std/rc.html"><strong aria-hidden="true">19.8.</strong> Rc</a></li><li class="chapter-item "><a href="../../std/arc.html"><strong aria-hidden="true">19.9.</strong> Arc</a></li></ol></li><li class="chapter-item expanded "><a href="../../std_misc.html"><strong aria-hidden="true">20.</strong> Std misc</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../std_misc/threads.html"><strong aria-hidden="true">20.1.</strong> Threads</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../std_misc/threads/testcase_mapreduce.html" class="active"><strong aria-hidden="true">20.1.1.</strong> Testcase: map-reduce</a></li></ol></li><li class="chapter-item "><a href="../../std_misc/channels.html"><strong aria-hidden="true">20.2.</strong> Channels</a></li><li class="chapter-item "><a href="../../std_misc/path.html"><strong aria-hidden="true">20.3.</strong> Path</a></li><li class="chapter-item "><a href="../../std_misc/file.html"><strong aria-hidden="true">20.4.</strong> File I/O</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../std_misc/file/open.html"><strong aria-hidden="true">20.4.1.</strong> open</a></li><li class="chapter-item "><a href="../../std_misc/file/create.html"><strong aria-hidden="true">20.4.2.</strong> create</a></li><li class="chapter-item "><a href="../../std_misc/file/read_lines.html"><strong aria-hidden="true">20.4.3.</strong> read lines</a></li></ol></li><li class="chapter-item "><a href="../../std_misc/process.html"><strong aria-hidden="true">20.5.</strong> Child processes</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../std_misc/process/pipe.html"><strong aria-hidden="true">20.5.1.</strong> Pipes</a></li><li class="chapter-item "><a href="../../std_misc/process/wait.html"><strong aria-hidden="true">20.5.2.</strong> Wait</a></li></ol></li><li class="chapter-item "><a href="../../std_misc/fs.html"><strong aria-hidden="true">20.6.</strong> Filesystem Operations</a></li><li class="chapter-item "><a href="../../std_misc/arg.html"><strong aria-hidden="true">20.7.</strong> Program arguments</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../std_misc/arg/matching.html"><strong aria-hidden="true">20.7.1.</strong> Argument parsing</a></li></ol></li><li class="chapter-item "><a href="../../std_misc/ffi.html"><strong aria-hidden="true">20.8.</strong> Foreign Function Interface</a></li></ol></li><li class="chapter-item "><a href="../../testing.html"><strong aria-hidden="true">21.</strong> Testing</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../testing/unit_testing.html"><strong aria-hidden="true">21.1.</strong> Unit testing</a></li><li class="chapter-item "><a href="../../testing/doc_testing.html"><strong aria-hidden="true">21.2.</strong> Documentation testing</a></li><li class="chapter-item "><a href="../../testing/integration_testing.html"><strong aria-hidden="true">21.3.</strong> Integration testing</a></li><li class="chapter-item "><a href="../../testing/dev_dependencies.html"><strong aria-hidden="true">21.4.</strong> Dev-dependencies</a></li></ol></li><li class="chapter-item "><a href="../../unsafe.html"><strong aria-hidden="true">22.</strong> Unsafe Operations</a></li><li class="chapter-item "><a href="../../compatibility.html"><strong aria-hidden="true">23.</strong> Compatibility</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../compatibility/raw_identifiers.html"><strong aria-hidden="true">23.1.</strong> Raw identifiers</a></li></ol></li><li class="chapter-item "><a href="../../meta.html"><strong aria-hidden="true">24.</strong> Meta</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../meta/doc.html"><strong aria-hidden="true">24.1.</strong> Documentation</a></li><li class="chapter-item "><a href="../../meta/playpen.html"><strong aria-hidden="true">24.2.</strong> Playpen</a></li></ol></li></ol> |
| </div> |
| <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div> |
| </nav> |
| |
| <div id="page-wrapper" class="page-wrapper"> |
| |
| <div class="page"> |
| |
| <div id="menu-bar-hover-placeholder"></div> |
| <div id="menu-bar" class="menu-bar sticky bordered"> |
| <div class="left-buttons"> |
| <button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar"> |
| <i class="fa fa-bars"></i> |
| </button> |
| <button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list"> |
| <i class="fa fa-paint-brush"></i> |
| </button> |
| <ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu"> |
| <li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li> |
| </ul> |
| <button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> |
| <i class="fa fa-search"></i> |
| </button> |
| </div> |
| |
| <h1 class="menu-title">Rust By Example</h1> |
| |
| <div class="right-buttons"> |
| <a href="../../print.html" title="Print this book" aria-label="Print this book"> |
| <i id="print-button" class="fa fa-print"></i> |
| </a> |
| </div> |
| </div> |
| |
| <div id="search-wrapper" class="hidden"> |
| <form id="searchbar-outer" class="searchbar-outer"> |
| <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> |
| </form> |
| <div id="searchresults-outer" class="searchresults-outer hidden"> |
| <div id="searchresults-header" class="searchresults-header"></div> |
| <ul id="searchresults"> |
| </ul> |
| </div> |
| </div> |
| <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> |
| <script type="text/javascript"> |
| document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); |
| document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); |
| Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { |
| link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); |
| }); |
| </script> |
| |
| <div id="content" class="content"> |
| <main> |
| <h1 id="testcase-map-reduce"><a class="header" href="#testcase-map-reduce">Testcase: map-reduce</a></h1> |
| <p>Rust makes it very easy to parallelise data processing, without many of the headaches traditionally associated with such an attempt.</p> |
| <p>The standard library provides great threading primitives out of the box. |
| These, combined with Rust's concept of Ownership and aliasing rules, automatically prevent |
| data races.</p> |
| <p>The aliasing rules (one writable reference XOR many readable references) automatically prevent |
| you from manipulating state that is visible to other threads. (Where synchronisation is needed, |
| there are synchronisation |
| primitives like <code>Mutex</code>es or <code>Channel</code>s.)</p> |
| <p>In this example, we will calculate the sum of all digits in a block of numbers. |
| We will do this by parcelling out chunks of the block into different threads. Each thread will sum |
| its tiny block of digits, and subsequently we will sum the intermediate sums produced by each |
| thread.</p> |
| <p>Note that, although we're passing references across thread boundaries, Rust understands that we're |
| only passing read-only references, and that thus no unsafety or data races can occur. Because |
| we're <code>move</code>-ing the data segments into the thread, Rust will also ensure the data is kept alive |
| until the threads exit, so no dangling pointers occur.</p> |
| <pre><pre class="playground"><code class="language-rust editable">use std::thread; |
| |
| // This is the `main` thread |
| fn main() { |
| |
| // This is our data to process. |
| // We will calculate the sum of all digits via a threaded map-reduce algorithm. |
| // Each whitespace separated chunk will be handled in a different thread. |
| // |
| // TODO: see what happens to the output if you insert spaces! |
| let data = "86967897737416471853297327050364959 |
| 11861322575564723963297542624962850 |
| 70856234701860851907960690014725639 |
| 38397966707106094172783238747669219 |
| 52380795257888236525459303330302837 |
| 58495327135744041048897885734297812 |
| 69920216438980873548808413720956532 |
| 16278424637452589860345374828574668"; |
| |
| // Make a vector to hold the child-threads which we will spawn. |
| let mut children = vec![]; |
| |
| /************************************************************************* |
| * "Map" phase |
| * |
| * Divide our data into segments, and apply initial processing |
| ************************************************************************/ |
| |
| // split our data into segments for individual calculation |
| // each chunk will be a reference (&str) into the actual data |
| let chunked_data = data.split_whitespace(); |
| |
| // Iterate over the data segments. |
| // .enumerate() adds the current loop index to whatever is iterated |
| // the resulting tuple "(index, element)" is then immediately |
| // "destructured" into two variables, "i" and "data_segment" with a |
| // "destructuring assignment" |
| for (i, data_segment) in chunked_data.enumerate() { |
| println!("data segment {} is \"{}\"", i, data_segment); |
| |
| // Process each data segment in a separate thread |
| // |
| // spawn() returns a handle to the new thread, |
| // which we MUST keep to access the returned value |
| // |
| // 'move || -> u32' is syntax for a closure that: |
| // * takes no arguments ('||') |
| // * takes ownership of its captured variables ('move') and |
| // * returns an unsigned 32-bit integer ('-> u32') |
| // |
| // Rust is smart enough to infer the '-> u32' from |
| // the closure itself so we could have left that out. |
| // |
| // TODO: try removing the 'move' and see what happens |
| children.push(thread::spawn(move || -> u32 { |
| // Calculate the intermediate sum of this segment: |
| let result = data_segment |
| // iterate over the characters of our segment.. |
| .chars() |
| // .. convert text-characters to their number value.. |
| .map(|c| c.to_digit(10).expect("should be a digit")) |
| // .. and sum the resulting iterator of numbers |
| .sum(); |
| |
| // println! locks stdout, so no text-interleaving occurs |
| println!("processed segment {}, result={}", i, result); |
| |
| // "return" not needed, because Rust is an "expression language", the |
| // last evaluated expression in each block is automatically its value. |
| result |
| |
| })); |
| } |
| |
| |
| /************************************************************************* |
| * "Reduce" phase |
| * |
| * Collect our intermediate results, and combine them into a final result |
| ************************************************************************/ |
| |
| // combine each thread's intermediate results into a single final sum. |
| // |
| // we use the "turbofish" ::<> to provide sum() with a type hint. |
| // |
| // TODO: try without the turbofish, by instead explicitly |
| // specifying the type of final_result |
| let final_result = children.into_iter().map(|c| c.join().unwrap()).sum::<u32>(); |
| |
| println!("Final sum result: {}", final_result); |
| } |
| |
| |
| </code></pre></pre> |
| <h3 id="assignments"><a class="header" href="#assignments">Assignments</a></h3> |
| <p>It is not wise to let our number of threads depend on user inputted data. |
| What if the user decides to insert a lot of spaces? Do we <em>really</em> want to spawn 2,000 threads? |
| Modify the program so that the data is always chunked into a limited number of chunks, |
| defined by a static constant at the beginning of the program.</p> |
| <h3 id="see-also"><a class="header" href="#see-also">See also:</a></h3> |
| <ul> |
| <li><a href="../threads.html">Threads</a></li> |
| <li><a href="../../std/vec.html">vectors</a> and <a href="../../trait/iter.html">iterators</a></li> |
| <li><a href="../../fn/closures.html">closures</a>, <a href="../../scope/move.html">move</a> semantics and <a href="https://doc.rust-lang.org/book/ch13-01-closures.html#closures-can-capture-their-environment"><code>move</code> closures</a></li> |
| <li><a href="https://doc.rust-lang.org/book/ch18-03-pattern-syntax.html#destructuring-to-break-apart-values">destructuring</a> assignments</li> |
| <li><a href="https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.collect">turbofish notation</a> to help type inference</li> |
| <li><a href="../../error/option_unwrap.html">unwrap vs. expect</a></li> |
| <li><a href="https://doc.rust-lang.org/book/loops.html#enumerate">enumerate</a></li> |
| </ul> |
| |
| </main> |
| |
| <nav class="nav-wrapper" aria-label="Page navigation"> |
| <!-- Mobile navigation buttons --> |
| <a rel="prev" href="../../std_misc/threads.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> |
| <i class="fa fa-angle-left"></i> |
| </a> |
| <a rel="next" href="../../std_misc/channels.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> |
| <i class="fa fa-angle-right"></i> |
| </a> |
| <div style="clear: both"></div> |
| </nav> |
| </div> |
| </div> |
| |
| <nav class="nav-wide-wrapper" aria-label="Page navigation"> |
| <a rel="prev" href="../../std_misc/threads.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> |
| <i class="fa fa-angle-left"></i> |
| </a> |
| <a rel="next" href="../../std_misc/channels.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> |
| <i class="fa fa-angle-right"></i> |
| </a> |
| </nav> |
| |
| </div> |
| |
| <script type="text/javascript"> |
| window.playground_copyable = true; |
| </script> |
| <script src="../../ace.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../editor.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../mode-rust.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../theme-dawn.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../theme-tomorrow_night.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../mark.min.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../searcher.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../clipboard.min.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../highlight.js" type="text/javascript" charset="utf-8"></script> |
| <script src="../../book.js" type="text/javascript" charset="utf-8"></script> |
| |
| <!-- Custom JS scripts --> |
| </body> |
| </html> |
