github_mirrors/rust-book-cn
1
0
Fork 0
mirror of https://github.com/rust-lang-cn/book-cn.git synced 2025-01-25 00:18:46 +08:00
Code Issues Packages Projects Releases Wiki Activity

Merge pull request #157 from jonathandturner/first-third-try

Browse Source 
Split up example for better readability
This commit is contained in:
Carol Nichols 2016-07-29 20:00:33 -04:00 committed by GitHub
commit ae14cc407c
1 changed files with 22 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

43
src/ch03-03-slices.md
View File

@ -37,38 +37,39 @@ fn first_word(s: &String) -> usize {
Let’s break that down a bit:
```rust
fn first_word(s: &String) -> usize {
// Since we need to go through the String element by element and
// check if a value is a space, we will convert our String to an
// array of bytes using the `.as_bytes()` method.
let bytes = s.as_bytes();
```
// We will be discussing iterators in more detail in Chapter XX, but for
// now, know that `iter()` is a method that returns each element in a
// collection, and `enumerate()` modifies the result of `iter()` and returns
// a tuple instead. The first element of the tuple is the index, and the
// second element is a reference to the element itself. This is a bit
// nicer than calculating the index ourselves.
//
// Since it’s a tuple, we can use patterns, just like elsewhere in Rust.
// So we match against the tuple with i for the index and &byte for
// the byte itself.
Since we need to go through the String element by element and
check if a value is a space, we will convert our String to an
array of bytes using the `.as_bytes()` method.
```rust
for (i, &byte) in bytes.iter().enumerate() {
```
// 32 is the value of a space in UTF-8
We will be discussing iterators in more detail in Chapter XX, but for
now, know that `iter()` is a method that returns each element in a
collection, and `enumerate()` modifies the result of `iter()` and returns
a tuple instead. The first element of the tuple is the index, and the
second element is a reference to the element itself. This is a bit
nicer than calculating the index ourselves.
Since it’s a tuple, we can use patterns, just like elsewhere in Rust.
So we match against the tuple with i for the index and &byte for
the byte itself.
```rust
if byte == 32 {
// We found a space! Return this position.
return i;
}
}
// If we got here, we didn’t find a space, so the whole string must be a
// word. Return the length.
s.len()
}
```
We search for the value 32, which represents a space in UTF-8. If we find one, we return the
position. Otherwise, we return the length of the string, using `s.len()`.
This works, but there’s a problem. We’re returning a `usize` on its own, but
it’s only a meaningful number in the context of the `&String`. In other
words, because it’s a separate value from the `String`, there’s no guarantee