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6744822454
* order the Cat and Dog correctly in the vec * make example editable * patterns: capture vs. constant * clarify notes on indexing a string
43 lines
2.1 KiB
Markdown
43 lines
2.1 KiB
Markdown
# String
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[`String`][1] is the standard heap-allocated growable UTF-8 string buffer:
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```rust,editable
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fn main() {
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let mut s1 = String::new();
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s1.push_str("Hello");
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println!("s1: len = {}, capacity = {}", s1.len(), s1.capacity());
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let mut s2 = String::with_capacity(s1.len() + 1);
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s2.push_str(&s1);
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s2.push('!');
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println!("s2: len = {}, capacity = {}", s2.len(), s2.capacity());
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let s3 = String::from("🇨🇭");
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println!("s3: len = {}, number of chars = {}", s3.len(),
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s3.chars().count());
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}
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```
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`String` implements [`Deref<Target = str>`][2], which means that you can call all
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`str` methods on a `String`.
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[1]: https://doc.rust-lang.org/std/string/struct.String.html
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[2]: https://doc.rust-lang.org/std/string/struct.String.html#deref-methods-str
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<details>
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* `String::new` returns a new empty string, use `String::with_capacity` when you know how much data you want to push to the string.
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* `String::len` returns the size of the `String` in bytes (which can be different from its length in characters).
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* `String::chars` returns an iterator over the actual characters. Note that a `char` can be different from what a human will consider a "character" due to [grapheme clusters](https://docs.rs/unicode-segmentation/latest/unicode_segmentation/struct.Graphemes.html).
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* When people refer to strings they could either be talking about `&str` or `String`.
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* When a type implements `Deref<Target = T>`, the compiler will let you transparently call methods from `T`.
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* `String` implements `Deref<Target = str>` which transparently gives it access to `str`'s methods.
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* Write and compare `let s3 = s1.deref();` and `let s3 = &*s1`;.
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* `String` is implemented as a wrapper around a vector of bytes, many of the operations you see supported on vectors are also supported on `String`, but with some extra guarantees.
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* Compare the different ways to index a `String`:
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* To a character by using `s3.chars().nth(i).unwrap()` where `i` is in-bound, out-of-bounds.
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* To a substring by using `s3[0..4]`, where that slice is on character boundaries or not.
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</details>
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