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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> Paths for Referring to an Item in the Module Tree</a></li><li class="expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> Bringing Paths Into Scope with the use Keyword</a></li><li class="expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> Separating Modules into Different Files</a></li></ol></li><li class="expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li class="expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li class="expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Storing UTF-8 Encoded Text with Strings</a></li><li class="expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Storing Keys with Associated Values in Hash Maps</a></li></ol></li><li class="expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li class="expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li class="expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li class="expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li class="expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li class="expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li class="expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li class="expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li class="expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li class="expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li class="expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li class="expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li class="expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li class="expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li class="expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li class="expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li class="expanded "><a href="ch12-04-testing-the-librarys-functionality.html" class="active"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li class="expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li class="expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li class="expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li class="expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li class="expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li class="expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li class="expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li class="expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li class="expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li class="expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li class="expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li class="expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li class="expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li class="expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li class="expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Using Box<T> to Point to Data on the Heap</a></li><li class="expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> Treating Smart Pointers Like Regular References with the Deref Trait</a></li><li class="expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> Running Code on Cleanup with the Drop Trait</a></li><li class="expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li class="expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li class="expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Reference Cycles Can Leak Memory</a></li></ol></li><li class="expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li class="expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li class="expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Using Message Passing to Transfer Data Between Threads</a></li><li class="expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li class="expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency with the Sync and Send Traits</a></li></ol></li><li class="expanded "><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li class="expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li class="expanded "><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects That Allow for Values of Different Types</a></li><li class="expanded "><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li class="expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns Can Be Used</a></li><li class="expanded "><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li class="expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li class="expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li class="expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li class="expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> Advanced Traits</a></li><li class="expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> Advanced Types</a></li><li class="expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> Advanced Functions and Closures</a></li><li class="expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> Macros</a></li></ol></li><li class="expanded "><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li class="expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> Building a Single-Threaded Web Server</a></li><li class="expanded "><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning Our Single-Threaded Server into a Multithreaded Server</a></li><li class="expanded "><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li class="expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li class="expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li class="expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li class="expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li class="expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li class="expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li class="expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li class="expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<h2><a class="header" href="#developing-the-librarys-functionality-with-test-driven-development" id="developing-the-librarys-functionality-with-test-driven-development">Developing the Library’s Functionality with Test-Driven Development</a></h2>
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<p>Now that we’ve extracted the logic into <em>src/lib.rs</em> and left the argument
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collecting and error handling in <em>src/main.rs</em>, it’s much easier to write tests
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for the core functionality of our code. We can call functions directly with
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various arguments and check return values without having to call our binary
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from the command line. Feel free to write some tests for the functionality in
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the <code>Config::new</code> and <code>run</code> functions on your own.</p>
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<p>In this section, we’ll add the searching logic to the <code>minigrep</code> program by
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using the Test-driven development (TDD) process. This software development
|
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technique follows these steps:</p>
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||
<ol>
|
||
<li>Write a test that fails and run it to make sure it fails for the reason you
|
||
expect.</li>
|
||
<li>Write or modify just enough code to make the new test pass.</li>
|
||
<li>Refactor the code you just added or changed and make sure the tests
|
||
continue to pass.</li>
|
||
<li>Repeat from step 1!</li>
|
||
</ol>
|
||
<p>This process is just one of many ways to write software, but TDD can help drive
|
||
code design as well. Writing the test before you write the code that makes the
|
||
test pass helps to maintain high test coverage throughout the process.</p>
|
||
<p>We’ll test drive the implementation of the functionality that will actually do
|
||
the searching for the query string in the file contents and produce a list of
|
||
lines that match the query. We’ll add this functionality in a function called
|
||
<code>search</code>.</p>
|
||
<h3><a class="header" href="#writing-a-failing-test" id="writing-a-failing-test">Writing a Failing Test</a></h3>
|
||
<p>Because we don’t need them anymore, let’s remove the <code>println!</code> statements from
|
||
<em>src/lib.rs</em> and <em>src/main.rs</em> that we used to check the program’s behavior.
|
||
Then, in <em>src/lib.rs</em>, we’ll add a <code>tests</code> module with a test function, as we
|
||
did in <a href="ch11-01-writing-tests.html#the-anatomy-of-a-test-function">Chapter 11</a><!-- ignore -->. The test function specifies
|
||
the behavior we want the <code>search</code> function to have: it will take a query and
|
||
the text to search for the query in, and it will return only the lines from the
|
||
text that contain the query. Listing 12-15 shows this test, which won’t compile
|
||
yet.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
||
</span><span class="boring"> vec![]
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>#[cfg(test)]
|
||
mod tests {
|
||
use super::*;
|
||
|
||
#[test]
|
||
fn one_result() {
|
||
let query = "duct";
|
||
let contents = "\
|
||
Rust:
|
||
safe, fast, productive.
|
||
Pick three.";
|
||
|
||
assert_eq!(
|
||
vec!["safe, fast, productive."],
|
||
search(query, contents)
|
||
);
|
||
}
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 12-15: Creating a failing test for the <code>search</code>
|
||
function we wish we had</span></p>
|
||
<p>This test searches for the string <code>"duct"</code>. The text we’re searching is three
|
||
lines, only one of which contains <code>"duct"</code>. We assert that the value returned
|
||
from the <code>search</code> function contains only the line we expect.</p>
|
||
<p>We aren’t able to run this test and watch it fail because the test doesn’t even
|
||
compile: the <code>search</code> function doesn’t exist yet! So now we’ll add just enough
|
||
code to get the test to compile and run by adding a definition of the <code>search</code>
|
||
function that always returns an empty vector, as shown in Listing 12-16. Then
|
||
the test should compile and fail because an empty vector doesn’t match a vector
|
||
containing the line <code>"safe, fast, productive."</code></p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
||
vec![]
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 12-16: Defining just enough of the <code>search</code>
|
||
function so our test will compile</span></p>
|
||
<p>Notice that we need an explicit lifetime <code>'a</code> defined in the signature of
|
||
<code>search</code> and used with the <code>contents</code> argument and the return value. Recall in
|
||
<a href="ch10-03-lifetime-syntax.html">Chapter 10</a><!-- ignore --> that the lifetime parameters
|
||
specify which argument lifetime is connected to the lifetime of the return
|
||
value. In this case, we indicate that the returned vector should contain string
|
||
slices that reference slices of the argument <code>contents</code> (rather than the
|
||
argument <code>query</code>).</p>
|
||
<p>In other words, we tell Rust that the data returned by the <code>search</code> function
|
||
will live as long as the data passed into the <code>search</code> function in the
|
||
<code>contents</code> argument. This is important! The data referenced <em>by</em> a slice needs
|
||
to be valid for the reference to be valid; if the compiler assumes we’re making
|
||
string slices of <code>query</code> rather than <code>contents</code>, it will do its safety checking
|
||
incorrectly.</p>
|
||
<p>If we forget the lifetime annotations and try to compile this function, we’ll
|
||
get this error:</p>
|
||
<pre><code class="language-text">error[E0106]: missing lifetime specifier
|
||
--> src/lib.rs:5:51
|
||
|
|
||
5 | pub fn search(query: &str, contents: &str) -> Vec<&str> {
|
||
| ^ expected lifetime
|
||
parameter
|
||
|
|
||
= help: this function's return type contains a borrowed value, but the
|
||
signature does not say whether it is borrowed from `query` or `contents`
|
||
</code></pre>
|
||
<p>Rust can’t possibly know which of the two arguments we need, so we need to tell
|
||
it. Because <code>contents</code> is the argument that contains all of our text and we
|
||
want to return the parts of that text that match, we know <code>contents</code> is the
|
||
argument that should be connected to the return value using the lifetime syntax.</p>
|
||
<p>Other programming languages don’t require you to connect arguments to return
|
||
values in the signature. Although this might seem strange, it will get easier
|
||
over time. You might want to compare this example with the <a href="ch10-03-lifetime-syntax.html#validating-references-with-lifetimes">“Validating
|
||
References with Lifetimes”</a><!-- ignore
|
||
--> section in Chapter 10.</p>
|
||
<p>Now let’s run the test:</p>
|
||
<pre><code class="language-text">$ cargo test
|
||
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
||
--warnings--
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.43 secs
|
||
Running target/debug/deps/minigrep-abcabcabc
|
||
|
||
running 1 test
|
||
test tests::one_result ... FAILED
|
||
|
||
failures:
|
||
|
||
---- tests::one_result stdout ----
|
||
thread 'tests::one_result' panicked at 'assertion failed: `(left ==
|
||
right)`
|
||
left: `["safe, fast, productive."]`,
|
||
right: `[]`)', src/lib.rs:48:8
|
||
note: Run with `RUST_BACKTRACE=1` for a backtrace.
|
||
|
||
|
||
failures:
|
||
tests::one_result
|
||
|
||
test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
|
||
|
||
error: test failed, to rerun pass '--lib'
|
||
</code></pre>
|
||
<p>Great, the test fails, exactly as we expected. Let’s get the test to pass!</p>
|
||
<h3><a class="header" href="#writing-code-to-pass-the-test" id="writing-code-to-pass-the-test">Writing Code to Pass the Test</a></h3>
|
||
<p>Currently, our test is failing because we always return an empty vector. To fix
|
||
that and implement <code>search</code>, our program needs to follow these steps:</p>
|
||
<ul>
|
||
<li>Iterate through each line of the contents.</li>
|
||
<li>Check whether the line contains our query string.</li>
|
||
<li>If it does, add it to the list of values we’re returning.</li>
|
||
<li>If it doesn’t, do nothing.</li>
|
||
<li>Return the list of results that match.</li>
|
||
</ul>
|
||
<p>Let’s work through each step, starting with iterating through lines.</p>
|
||
<h4><a class="header" href="#iterating-through-lines-with-the-lines-method" id="iterating-through-lines-with-the-lines-method">Iterating Through Lines with the <code>lines</code> Method</a></h4>
|
||
<p>Rust has a helpful method to handle line-by-line iteration of strings,
|
||
conveniently named <code>lines</code>, that works as shown in Listing 12-17. Note this
|
||
won’t compile yet.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
||
for line in contents.lines() {
|
||
// do something with line
|
||
}
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 12-17: Iterating through each line in <code>contents</code>
|
||
</span></p>
|
||
<p>The <code>lines</code> method returns an iterator. We’ll talk about iterators in depth in
|
||
<a href="ch13-02-iterators.html">Chapter 13</a><!-- ignore -->, but recall that you saw this way of using an
|
||
iterator in <a href="ch03-05-control-flow.html#looping-through-a-collection-with-for">Listing 3-5</a><!-- ignore -->, where we used a <code>for</code> loop
|
||
with an iterator to run some code on each item in a collection.</p>
|
||
<h4><a class="header" href="#searching-each-line-for-the-query" id="searching-each-line-for-the-query">Searching Each Line for the Query</a></h4>
|
||
<p>Next, we’ll check whether the current line contains our query string.
|
||
Fortunately, strings have a helpful method named <code>contains</code> that does this for
|
||
us! Add a call to the <code>contains</code> method in the <code>search</code> function, as shown in
|
||
Listing 12-18. Note this still won’t compile yet.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
||
for line in contents.lines() {
|
||
if line.contains(query) {
|
||
// do something with line
|
||
}
|
||
}
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 12-18: Adding functionality to see whether the
|
||
line contains the string in <code>query</code></span></p>
|
||
<h4><a class="header" href="#storing-matching-lines" id="storing-matching-lines">Storing Matching Lines</a></h4>
|
||
<p>We also need a way to store the lines that contain our query string. For that,
|
||
we can make a mutable vector before the <code>for</code> loop and call the <code>push</code> method
|
||
to store a <code>line</code> in the vector. After the <code>for</code> loop, we return the vector, as
|
||
shown in Listing 12-19.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
||
let mut results = Vec::new();
|
||
|
||
for line in contents.lines() {
|
||
if line.contains(query) {
|
||
results.push(line);
|
||
}
|
||
}
|
||
|
||
results
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 12-19: Storing the lines that match so we can
|
||
return them</span></p>
|
||
<p>Now the <code>search</code> function should return only the lines that contain <code>query</code>,
|
||
and our test should pass. Let’s run the test:</p>
|
||
<pre><code class="language-text">$ cargo test
|
||
--snip--
|
||
running 1 test
|
||
test tests::one_result ... ok
|
||
|
||
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
|
||
</code></pre>
|
||
<p>Our test passed, so we know it works!</p>
|
||
<p>At this point, we could consider opportunities for refactoring the
|
||
implementation of the search function while keeping the tests passing to
|
||
maintain the same functionality. The code in the search function isn’t too bad,
|
||
but it doesn’t take advantage of some useful features of iterators. We’ll
|
||
return to this example in <a href="ch13-02-iterators.html">Chapter 13</a><!-- ignore -->, where we’ll
|
||
explore iterators in detail, and look at how to improve it.</p>
|
||
<h4><a class="header" href="#using-the-search-function-in-the-run-function" id="using-the-search-function-in-the-run-function">Using the <code>search</code> Function in the <code>run</code> Function</a></h4>
|
||
<p>Now that the <code>search</code> function is working and tested, we need to call <code>search</code>
|
||
from our <code>run</code> function. We need to pass the <code>config.query</code> value and the
|
||
<code>contents</code> that <code>run</code> reads from the file to the <code>search</code> function. Then <code>run</code>
|
||
will print each line returned from <code>search</code>:</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore">pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
||
let contents = fs::read_to_string(config.filename)?;
|
||
|
||
for line in search(&config.query, &contents) {
|
||
println!("{}", line);
|
||
}
|
||
|
||
Ok(())
|
||
}
|
||
</code></pre>
|
||
<p>We’re still using a <code>for</code> loop to return each line from <code>search</code> and print it.</p>
|
||
<p>Now the entire program should work! Let’s try it out, first with a word that
|
||
should return exactly one line from the Emily Dickinson poem, “frog”:</p>
|
||
<pre><code class="language-text">$ cargo run frog poem.txt
|
||
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.38 secs
|
||
Running `target/debug/minigrep frog poem.txt`
|
||
How public, like a frog
|
||
</code></pre>
|
||
<p>Cool! Now let’s try a word that will match multiple lines, like “body”:</p>
|
||
<pre><code class="language-text">$ cargo run body poem.txt
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
|
||
Running `target/debug/minigrep body poem.txt`
|
||
I’m nobody! Who are you?
|
||
Are you nobody, too?
|
||
How dreary to be somebody!
|
||
</code></pre>
|
||
<p>And finally, let’s make sure that we don’t get any lines when we search for a
|
||
word that isn’t anywhere in the poem, such as “monomorphization”:</p>
|
||
<pre><code class="language-text">$ cargo run monomorphization poem.txt
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
|
||
Running `target/debug/minigrep monomorphization poem.txt`
|
||
</code></pre>
|
||
<p>Excellent! We’ve built our own mini version of a classic tool and learned a lot
|
||
about how to structure applications. We’ve also learned a bit about file input
|
||
and output, lifetimes, testing, and command line parsing.</p>
|
||
<p>To round out this project, we’ll briefly demonstrate how to work with
|
||
environment variables and how to print to standard error, both of which are
|
||
useful when you’re writing command line programs.</p>
|
||
|
||
</main>
|
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