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## 读取文件
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> [ch12-02-reading-a-file.md](https://github.com/rust-lang/book/blob/master/second-edition/src/ch12-02-reading-a-file.md)
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> <br>
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> commit c49e5ee8859f8eb8f8867cbeafbdf5b802aa5894
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现在有了一些包含我们需要的信息的变量了，让我们试着使用他们。下一步目标是打开需要搜索的文件。为此，我需要一个文件。在项目的根目录创建一个文件`poem.txt`，并写入一些艾米莉·狄金森（Emily Dickinson）的诗：
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<span class="filename">Filename: poem.txt</span>
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```
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I'm nobody! Who are you?
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Are you nobody, too?
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Then there's a pair of us — don't tell!
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They'd banish us, you know.
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How dreary to be somebody!
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How public, like a frog
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To tell your name the livelong day
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To an admiring bog!
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```
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<!-- Public domain Emily Dickinson poem. This will work best with something
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short, but that has multiple lines and some repetition. We could search through
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code; that gets a bit meta and possibly confusing... Changes to this are most
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welcome. /Carol -->
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创建完这个文件后，让我们编辑 *src/main.rs* 并增加如列表 12-3 所示用来打开文件的代码：
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<span class="filename">Filename: src/main.rs</span>
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```rust,ignore
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use std::env;
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use std::fs::File;
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use std::io::prelude::*;
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fn main() {
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let args: Vec<String> = env::args().collect();
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let search = &args[1];
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let filename = &args[2];
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println!("Searching for {}", search);
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println!("In file {}", filename);
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let mut f = File::open(filename).expect("file not found");
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let mut contents = String::new();
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f.read_to_string(&mut contents).expect("something went wrong reading the file");
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println!("With text:\n{}", contents);
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}
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```
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<span class="caption">Listing 12-3: Read the contents of the file specified by
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the second argument</span>
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<!-- Will add ghosting and wingdings in libreoffice /Carol -->
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这里增加了一些新内容。首先，需要更多的`use`语句来引入标准库中的相关部分：我们需要`std::fs::File`来处理文件，而`std::io::prelude::*`则包含许多对于 I/O 包括文件 I/O 有帮助的 trait。类似于 Rust 有一个通用的 prelude 来自动引入特定内容，`std::io`也有其自己的 prelude 来引入处理 I/O 时需要的内容。不同于默认的 prelude，必须显式`use`位于`std::io`中的 prelude。
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在`main`中，我们增加了三点内容：第一，我们获取了文件的句柄并使用`File::open`函数与第二个参数中指定的文件名来打开这个文件。第二，我们在变量`contents`中创建了一个空的可变的`String`，接着对文件句柄调用`read_to_string`并以`contents`字符串作为参数，`contents`是`read_to_string`将会放置它读取到的数据地方。最后，我们打印出了整个文件的内容，这是一个确认目前为止的程序能够工作的方法。
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尝试运行这些代码，随意指定第一个参数（因为还未实现搜索功能的部分）而将 *poem.txt* 文件将作为第二个参数：
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```
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$ cargo run the poem.txt
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Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
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Running `target\debug\greprs.exe the poem.txt`
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Searching for the
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In file poem.txt
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With text:
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I'm nobody! Who are you?
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Are you nobody, too?
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Then there's a pair of us — don't tell!
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They'd banish us, you know.
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How dreary to be somebody!
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How public, like a frog
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To tell your name the livelong day
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To an admiring bog!
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```
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好的！我们的代码可以工作了！然而，它还有一些瑕疵。因为程序还很小，这些瑕疵并不是什么大问题，不过随着程序功能的丰富，将会越来越难以用简单的方法修复他们。现在就让我们开始重构而不是等待之后处理。重构在只有少量代码时会显得容易得多。
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