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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" class="active"><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"><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="#accepting-command-line-arguments" id="accepting-command-line-arguments">Accepting Command Line Arguments</a></h2>
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<p>Let’s create a new project with, as always, <code>cargo new</code>. We’ll call our project
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<code>minigrep</code> to distinguish it from the <code>grep</code> tool that you might already have
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on your system.</p>
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<pre><code class="language-text">$ cargo new minigrep
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Created binary (application) `minigrep` project
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$ cd minigrep
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</code></pre>
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<p>The first task is to make <code>minigrep</code> accept its two command line arguments: the
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filename and a string to search for. That is, we want to be able to run our
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program with <code>cargo run</code>, a string to search for, and a path to a file to
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search in, like so:</p>
|
||
<pre><code class="language-text">$ cargo run searchstring example-filename.txt
|
||
</code></pre>
|
||
<p>Right now, the program generated by <code>cargo new</code> cannot process arguments we
|
||
give it. Some existing libraries on <a href="https://crates.io/">crates.io</a> can help
|
||
with writing a program that accepts command line arguments, but because you’re
|
||
just learning this concept, let’s implement this capability ourselves.</p>
|
||
<h3><a class="header" href="#reading-the-argument-values" id="reading-the-argument-values">Reading the Argument Values</a></h3>
|
||
<p>To enable <code>minigrep</code> to read the values of command line arguments we pass to
|
||
it, we’ll need a function provided in Rust’s standard library, which is
|
||
<code>std::env::args</code>. This function returns an iterator of the command line
|
||
arguments that were given to <code>minigrep</code>. We’ll cover iterators fully in
|
||
<a href="ch13-00-functional-features.html">Chapter 13</a><!-- ignore -->. For now, you only need to know two details
|
||
about iterators: iterators produce a series of values, and we can call the
|
||
<code>collect</code> method on an iterator to turn it into a collection, such as a vector,
|
||
containing all the elements the iterator produces.</p>
|
||
<p>Use the code in Listing 12-1 to allow your <code>minigrep</code> program to read any
|
||
command line arguments passed to it and then collect the values into a vector.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">use std::env;
|
||
|
||
fn main() {
|
||
let args: Vec<String> = env::args().collect();
|
||
println!("{:?}", args);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 12-1: Collecting the command line arguments into
|
||
a vector and printing them</span></p>
|
||
<p>First, we bring the <code>std::env</code> module into scope with a <code>use</code> statement so we
|
||
can use its <code>args</code> function. Notice that the <code>std::env::args</code> function is
|
||
nested in two levels of modules. As we discussed in <a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html#creating-idiomatic-use-paths">Chapter
|
||
7</a><!-- ignore -->, in cases where the desired function is
|
||
nested in more than one module, it’s conventional to bring the parent module
|
||
into scope rather than the function. By doing so, we can easily use other
|
||
functions from <code>std::env</code>. It’s also less ambiguous than adding <code>use std::env::args</code> and then calling the function with just <code>args</code>, because <code>args</code>
|
||
might easily be mistaken for a function that’s defined in the current module.</p>
|
||
<blockquote>
|
||
<h3><a class="header" href="#the-args-function-and-invalid-unicode" id="the-args-function-and-invalid-unicode">The <code>args</code> Function and Invalid Unicode</a></h3>
|
||
<p>Note that <code>std::env::args</code> will panic if any argument contains invalid
|
||
Unicode. If your program needs to accept arguments containing invalid
|
||
Unicode, use <code>std::env::args_os</code> instead. That function returns an iterator
|
||
that produces <code>OsString</code> values instead of <code>String</code> values. We’ve chosen to
|
||
use <code>std::env::args</code> here for simplicity, because <code>OsString</code> values differ
|
||
per platform and are more complex to work with than <code>String</code> values.</p>
|
||
</blockquote>
|
||
<p>On the first line of <code>main</code>, we call <code>env::args</code>, and we immediately use
|
||
<code>collect</code> to turn the iterator into a vector containing all the values produced
|
||
by the iterator. We can use the <code>collect</code> function to create many kinds of
|
||
collections, so we explicitly annotate the type of <code>args</code> to specify that we
|
||
want a vector of strings. Although we very rarely need to annotate types in
|
||
Rust, <code>collect</code> is one function you do often need to annotate because Rust
|
||
isn’t able to infer the kind of collection you want.</p>
|
||
<p>Finally, we print the vector using the debug formatter, <code>:?</code>. Let’s try running
|
||
the code first with no arguments and then with two arguments:</p>
|
||
<pre><code class="language-text">$ cargo run
|
||
--snip--
|
||
["target/debug/minigrep"]
|
||
|
||
$ cargo run needle haystack
|
||
--snip--
|
||
["target/debug/minigrep", "needle", "haystack"]
|
||
</code></pre>
|
||
<p>Notice that the first value in the vector is <code>"target/debug/minigrep"</code>, which
|
||
is the name of our binary. This matches the behavior of the arguments list in
|
||
C, letting programs use the name by which they were invoked in their execution.
|
||
It’s often convenient to have access to the program name in case you want to
|
||
print it in messages or change behavior of the program based on what command
|
||
line alias was used to invoke the program. But for the purposes of this
|
||
chapter, we’ll ignore it and save only the two arguments we need.</p>
|
||
<h3><a class="header" href="#saving-the-argument-values-in-variables" id="saving-the-argument-values-in-variables">Saving the Argument Values in Variables</a></h3>
|
||
<p>Printing the value of the vector of arguments illustrated that the program is
|
||
able to access the values specified as command line arguments. Now we need to
|
||
save the values of the two arguments in variables so we can use the values
|
||
throughout the rest of the program. We do that in Listing 12-2.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust should_panic">use std::env;
|
||
|
||
fn main() {
|
||
let args: Vec<String> = env::args().collect();
|
||
|
||
let query = &args[1];
|
||
let filename = &args[2];
|
||
|
||
println!("Searching for {}", query);
|
||
println!("In file {}", filename);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 12-2: Creating variables to hold the query
|
||
argument and filename argument</span></p>
|
||
<p>As we saw when we printed the vector, the program’s name takes up the first
|
||
value in the vector at <code>args[0]</code>, so we’re starting at index <code>1</code>. The first
|
||
argument <code>minigrep</code> takes is the string we’re searching for, so we put a
|
||
reference to the first argument in the variable <code>query</code>. The second argument
|
||
will be the filename, so we put a reference to the second argument in the
|
||
variable <code>filename</code>.</p>
|
||
<p>We temporarily print the values of these variables to prove that the code is
|
||
working as we intend. Let’s run this program again with the arguments <code>test</code>
|
||
and <code>sample.txt</code>:</p>
|
||
<pre><code class="language-text">$ cargo run test sample.txt
|
||
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
|
||
Running `target/debug/minigrep test sample.txt`
|
||
Searching for test
|
||
In file sample.txt
|
||
</code></pre>
|
||
<p>Great, the program is working! The values of the arguments we need are being
|
||
saved into the right variables. Later we’ll add some error handling to deal
|
||
with certain potential erroneous situations, such as when the user provides no
|
||
arguments; for now, we’ll ignore that situation and work on adding file-reading
|
||
capabilities instead.</p>
|
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