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<ol class="chapter"><li class="affix"><a href="foreword.html">Foreword</a></li><li class="affix"><a href="ch00-00-introduction.html">Introduction</a></li><li><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slices</a></li></ol></li><li><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><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li><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><a href="ch07-00-modules.html"><strong aria-hidden="true">7.</strong> Modules</a></li><li><ol class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong aria-hidden="true">7.1.</strong> mod and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong aria-hidden="true">7.2.</strong> Controlling Visibility with pub</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong aria-hidden="true">7.3.</strong> Referring to Names in Different Modules</a></li></ol></li><li><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Hash Maps</a></li></ol></li><li><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li><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><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Testing</a></li><li><ol class="section"><li><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li><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><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li><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><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><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li><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><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><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li><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><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li><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><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li><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><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Box<T> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> The Deref Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> The Drop Trait Runs Code on Cleanup</a></li><li><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ol></li><li><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency: Sync and Send</a></li></ol></li><li><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><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li><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><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><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns Match the Structure of Values</a></li><li><ol class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> All the Pattern Syntax</a></li></ol></li><li><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong aria-hidden="true">19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.5.</strong> Advanced Functions & Closures</a></li></ol></li><li><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><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> A Single Threaded Web Server</a></li><li><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><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><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li><a href="appendix-04-macros.html"><strong aria-hidden="true">21.4.</strong> D - Macros</a></li><li><a href="appendix-05-translation.html"><strong aria-hidden="true">21.5.</strong> E - Translations</a></li><li><a href="appendix-06-newest-features.html"><strong aria-hidden="true">21.6.</strong> F - Newest Features</a></li><li><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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<a class="header" href="ch12-04-testing-the-librarys-functionality.html#developing-the-librarys-functionality-with-test-driven-development" id="developing-the-librarys-functionality-with-test-driven-development"><h2>Developing the Library’s Functionality with Test-Driven Development</h2></a>
<p>Now that we’ve extracted the logic into <em>src/lib.rs</em> and left the argument
collecting and error handling in <em>src/main.rs</em>, it’s much easier to write tests
for the core functionality of our code. We can call functions directly with
various arguments and check return values without having to call our binary
from the command line. Feel free to write some tests for the functionality in
the <code>Config::new</code> and <code>run</code> functions on your own.</p>
<p>In this section, we’ll add the searching logic to the <code>minigrep</code> program by
using the Test-driven development (TDD) process. This software development
technique follows these steps:</p>
<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>
<a class="header" href="ch12-04-testing-the-librarys-functionality.html#writing-a-failing-test" id="writing-a-failing-test"><h3>Writing a Failing Test</h3></a>
<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>test</code> module with a test function, as we
did in Chapter 11. 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">
# #![allow(unused_variables)]
#fn main() {
# fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
# vec![]
# }
#
#[cfg(test)]
mod test {
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)
);
}
}
#}</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">
# #![allow(unused_variables)]
#fn main() {
pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
vec![]
}
#}</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
Chapter 10 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 “Validating
References with Lifetimes” 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 test::one_result ... FAILED
failures:
---- test::one_result stdout ----
thread 'test::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:
test::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>
<a class="header" href="ch12-04-testing-the-librarys-functionality.html#writing-code-to-pass-the-test" id="writing-code-to-pass-the-test"><h3>Writing Code to Pass the Test</h3></a>
<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>
<a class="header" href="ch12-04-testing-the-librarys-functionality.html#iterating-through-lines-with-the-lines-method" id="iterating-through-lines-with-the-lines-method"><h4>Iterating Through Lines with the <code>lines</code> Method</h4></a>
<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
Chapter 13, but recall that you saw this way of using an iterator in Listing
3-5, where we used a <code>for</code> loop with an iterator to run some code on each item
in a collection.</p>
<a class="header" href="ch12-04-testing-the-librarys-functionality.html#searching-each-line-for-the-query" id="searching-each-line-for-the-query"><h4>Searching Each Line for the Query</h4></a>
<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>
<a class="header" href="ch12-04-testing-the-librarys-functionality.html#storing-matching-lines" id="storing-matching-lines"><h4>Storing Matching Lines</h4></a>
<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 test::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 Chapter 13, where we’ll explore iterators in detail,
and look at how to improve it.</p>
<a class="header" href="ch12-04-testing-the-librarys-functionality.html#using-the-search-function-in-the-run-function" id="using-the-search-function-in-the-run-function"><h4>Using the <code>search</code> Function in the <code>run</code> Function</h4></a>
<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<Error>> {
let mut f = File::open(config.filename)?;
let mut contents = String::new();
f.read_to_string(&mut contents)?;
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>
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