What I am trying to do is to slice a string that has Nordic letters but it throws this error:
'byte index 1 is not a char boundary; it is inside 'å' (bytes 0..2) of å'
fn main() {
let str = "äåö".to_string();
println!("{}", &str[1..]);
}
fn main() {
let str = "äåö".to_string();
let slice_position = str.char_indices().nth(1).unwrap().0;
println!("{}", &str[slice_position..]);
}
åö
The problem here is that str's indexing is in bytes, but it is UTF-8 encoded and ä takes more than one byte in UTF-8. So slicing at 1 actually cuts off half a character, which is a runtime error in Rust.
The reason str behaves this way is because you can't actually determine the position of the n-th character without iterating over the entire string. UTF-8 has variable-length characters, meaning, the position of a character depends on the previous characters.
Related
Say I have a struct Foo that owns a string:
struct Foo {
owned_string: String
}
I want to implement some methods on this struct that return substrings from the owned String. For efficiency reasons, I don't want to allocate any new memory for this, I just want the return values to point to the original String.
Let's say I know the substring I want, it's characters 10 through 15.
I can't just slice it like self.owned_string[10..16], since that would give me bytes, not characters.
I can take the characters and collect them into a new String object, like self.owned_string.chars().skip(9).take(6).collect::<String>(), but that creates a new String object. String objects own their strings (AFAIK), so presumably new memory was allocated for this, which is not what I want.
How do I create string slices that reference a substring of a String object, but using character positions? (Without allocating any new memory)
You can use char_indices() then slice the string according to the positions the iterator gives you:
let mut iter = s.char_indices();
let (start, _) = iter.nth(10).unwrap();
let (end, _) = iter.nth(5).unwrap();
let slice = &s[start..end];
However, note that as mentioned in the documentation of chars():
It’s important to remember that char represents a Unicode Scalar Value, and might not match your idea of what a ‘character’ is. Iteration over grapheme clusters may be what you actually want. This functionality is not provided by Rust’s standard library, check crates.io instead.
#ChayimFriedman's answer is of course correct, I just wanted to contribute a more telling example:
fn print_string(s: &str) {
println!("String: {}", s);
}
fn main() {
let s: String = "🤣😄😁😆😅".to_string();
let mut iter = s.char_indices();
// Retrieve the position of the char at pos 1
let (start, _) = iter.nth(1).unwrap();
// Now the next char will be at position `2`. Which would be
// equivalent of querying `.next()` or `.nth(0)`.
// So if we query for `nth(2)` we query 3 characters; meaning
// the position of character 4.
let (end, _) = iter.nth(2).unwrap();
// Gives you a &str, which is exactly what you want.
// A reference to a substring, zero allocations, zero overhead.
let substring = &s[start..end];
print_string(&s);
print_string(substring);
}
String: 🤣😄😁😆😅
String: 😄😁😆
I've done it with smileys because smileys are definitely multi-byte unicode characters.
As #ChayimFriedman already noted, the reason why we have to iterate through the char_indices is because unicode characters are variably sized. They can be anywhere from 1 to 8 bytes long, so the only way to find out where the character boundaries are is to actually read the string up to the character we desire.
In Python, this would be final_char = mystring[-1]. How can I do the same in Rust?
I have tried
mystring[mystring.len() - 1]
but I get the error the type 'str' cannot be indexed by 'usize'
That is how you get the last char (which may not be what you think of as a "character"):
mystring.chars().last().unwrap();
Use unwrap only if you are sure that there is at least one char in your string.
Warning: About the general case (do the same thing as mystring[-n] in Python): UTF-8 strings are not to be used through indexing, because indexing is not a O(1) operation (a string in Rust is not an array). Please read this for more information.
However, if you want to index from the end like in Python, you must do this in Rust:
mystring.chars().rev().nth(n - 1) // Python: mystring[-n]
and check if there is such a character.
If you miss the simplicity of Python syntax, you can write your own extension:
trait StrExt {
fn from_end(&self, n: usize) -> char;
}
impl<'a> StrExt for &'a str {
fn from_end(&self, n: usize) -> char {
self.chars().rev().nth(n).expect("Index out of range in 'from_end'")
}
}
fn main() {
println!("{}", "foobar".from_end(2)) // prints 'b'
}
One option is to use slices. Here's an example:
let len = my_str.len();
let final_str = &my_str[len-1..];
This returns a string slice from position len-1 through the end of the string. That is to say, the last byte of your string. If your string consists of only ASCII values, then you'll get the final character of your string.
The reason why this only works with ASCII values is because they only ever require one byte of storage. Anything else, and Rust is likely to panic at runtime. This is what happens when you try to slice out one byte from a 2-byte character.
For a more detailed explanation, please see the strings section of the Rust book.
As #Boiethios mentioned
let last_ch = mystring.chars().last().unwrap();
Or
let last_ch = codes.chars().rev().nth(0).unwrap();
I would rather have (how hard is that!?)
let last_ch = codes.chars(-1); // Not implemented as rustc 1.56.1
Based on the Rust book, the String::len method returns the number of bytes composing the string, which may not correspond to the length in characters.
For example if we consider the following string in Japanese, len() would return 30, which is the number of bytes and not the number of characters, which would be 10:
let s = String::from("ラウトは難しいです！");
s.len() // returns 30.
The only way I have found to get the number of characters is using the following function:
s.chars().count()
which returns 10, and is the correct number of characters.
Is there any method on String that returns the characters count, aside from the one I am using above?
Is there any method on String that returns the characters count, aside from the one I am using above?
No. Using s.chars().count() is correct. Note that this is an O(N) operation (because UTF-8 is complex) while getting the number of bytes is an O(1) operation.
You can see all the methods on str for yourself.
As pointed out in the comments, a char is a specific concept:
It's important to remember that char represents a Unicode Scalar Value, and may not match your idea of what a 'character' is. Iteration over grapheme clusters may be what you actually want.
One such example is with precomposed characters:
fn main() {
println!("{}", "é".chars().count()); // 2
println!("{}", "é".chars().count()); // 1
}
You may prefer to use graphemes from the unicode-segmentation crate instead:
use unicode_segmentation::UnicodeSegmentation; // 1.6.0
fn main() {
println!("{}", "é".graphemes(true).count()); // 1
println!("{}", "é".graphemes(true).count()); // 1
}
I'd like to capitalize the first letter of a &str. It's a simple problem and I hope for a simple solution. Intuition tells me to do something like this:
let mut s = "foobar";
s[0] = s[0].to_uppercase();
But &strs can't be indexed like this. The only way I've been able to do it seems overly convoluted. I convert the &str to an iterator, convert the iterator to a vector, upper case the first item in the vector, which creates an iterator, which I index into, creating an Option, which I unwrap to give me the upper-cased first letter. Then I convert the vector into an iterator, which I convert into a String, which I convert to a &str.
let s1 = "foobar";
let mut v: Vec<char> = s1.chars().collect();
v[0] = v[0].to_uppercase().nth(0).unwrap();
let s2: String = v.into_iter().collect();
let s3 = &s2;
Is there an easier way than this, and if so, what? If not, why is Rust designed this way?
Similar question
Why is it so convoluted?
Let's break it down, line-by-line
let s1 = "foobar";
We've created a literal string that is encoded in UTF-8. UTF-8 allows us to encode the 1,114,112 code points of Unicode in a manner that's pretty compact if you come from a region of the world that types in mostly characters found in ASCII, a standard created in 1963. UTF-8 is a variable length encoding, which means that a single code point might take from 1 to 4 bytes. The shorter encodings are reserved for ASCII, but many Kanji take 3 bytes in UTF-8.
let mut v: Vec<char> = s1.chars().collect();
This creates a vector of characters. A character is a 32-bit number that directly maps to a code point. If we started with ASCII-only text, we've quadrupled our memory requirements. If we had a bunch of characters from the astral plane, then maybe we haven't used that much more.
v[0] = v[0].to_uppercase().nth(0).unwrap();
This grabs the first code point and requests that it be converted to an uppercase variant. Unfortunately for those of us who grew up speaking English, there's not always a simple one-to-one mapping of a "small letter" to a "big letter". Side note: we call them upper- and lower-case because one box of letters was above the other box of letters back in the day.
This code will panic when a code point has no corresponding uppercase variant. I'm not sure if those exist, actually. It could also semantically fail when a code point has an uppercase variant that has multiple characters, such as the German ß. Note that ß may never actually be capitalized in The Real World, this is the just example I can always remember and search for. As of 2017-06-29, in fact, the official rules of German spelling have been updated so that both "ẞ" and "SS" are valid capitalizations!
let s2: String = v.into_iter().collect();
Here we convert the characters back into UTF-8 and require a new allocation to store them in, as the original variable was stored in constant memory so as to not take up memory at run time.
let s3 = &s2;
And now we take a reference to that String.
It's a simple problem
Unfortunately, this is not true. Perhaps we should endeavor to convert the world to Esperanto?
I presume char::to_uppercase already properly handles Unicode.
Yes, I certainly hope so. Unfortunately, Unicode isn't enough in all cases.
Thanks to huon for pointing out the Turkish I, where both the upper (İ) and lower case (i) versions have a dot. That is, there is no one proper capitalization of the letter i; it depends on the locale of the the source text as well.
why the need for all data type conversions?
Because the data types you are working with are important when you are worried about correctness and performance. A char is 32-bits and a string is UTF-8 encoded. They are different things.
indexing could return a multi-byte, Unicode character
There may be some mismatched terminology here. A char is a multi-byte Unicode character.
Slicing a string is possible if you go byte-by-byte, but the standard library will panic if you are not on a character boundary.
One of the reasons that indexing a string to get a character was never implemented is because so many people misuse strings as arrays of ASCII characters. Indexing a string to set a character could never be efficient - you'd have to be able to replace 1-4 bytes with a value that is also 1-4 bytes, causing the rest of the string to bounce around quite a lot.
to_uppercase could return an upper case character
As mentioned above, ß is a single character that, when capitalized, becomes two characters.
Solutions
See also trentcl's answer which only uppercases ASCII characters.
Original
If I had to write the code, it'd look like:
fn some_kind_of_uppercase_first_letter(s: &str) -> String {
let mut c = s.chars();
match c.next() {
None => String::new(),
Some(f) => f.to_uppercase().chain(c).collect(),
}
}
fn main() {
println!("{}", some_kind_of_uppercase_first_letter("joe"));
println!("{}", some_kind_of_uppercase_first_letter("jill"));
println!("{}", some_kind_of_uppercase_first_letter("von Hagen"));
println!("{}", some_kind_of_uppercase_first_letter("ß"));
}
But I'd probably search for uppercase or unicode on crates.io and let someone smarter than me handle it.
Improved
Speaking of "someone smarter than me", Veedrac points out that it's probably more efficient to convert the iterator back into a slice after the first capital codepoints are accessed. This allows for a memcpy of the rest of the bytes.
fn some_kind_of_uppercase_first_letter(s: &str) -> String {
let mut c = s.chars();
match c.next() {
None => String::new(),
Some(f) => f.to_uppercase().collect::<String>() + c.as_str(),
}
}
Is there an easier way than this, and if so, what? If not, why is Rust designed this way?
Well, yes and no. Your code is, as the other answer pointed out, not correct, and will panic if you give it something like བོད་སྐད་ལ་. So doing this with Rust's standard library is even harder than you initially thought.
However, Rust is designed to encourage code reuse and make bringing in libraries easy. So the idiomatic way to capitalize a string is actually quite palatable:
extern crate inflector;
use inflector::Inflector;
let capitalized = "some string".to_title_case();
It's not especially convoluted if you are able to limit your input to ASCII-only strings.
Since Rust 1.23, str has a make_ascii_uppercase method (in older Rust versions, it was available through the AsciiExt trait). This means you can uppercase ASCII-only string slices with relative ease:
fn make_ascii_titlecase(s: &mut str) {
if let Some(r) = s.get_mut(0..1) {
r.make_ascii_uppercase();
}
}
This will turn "taylor" into "Taylor", but it won't turn "édouard" into "Édouard". (playground)
Use with caution.
I did it this way:
fn str_cap(s: &str) -> String {
format!("{}{}", (&s[..1].to_string()).to_uppercase(), &s[1..])
}
If it is not an ASCII string:
fn str_cap(s: &str) -> String {
format!("{}{}", s.chars().next().unwrap().to_uppercase(),
s.chars().skip(1).collect::<String>())
}
The OP's approach taken further:
replace the first character with its uppercase representation
let mut s = "foobar".to_string();
let r = s.remove(0).to_uppercase().to_string() + &s;
or
let r = format!("{}{s}", s.remove(0).to_uppercase());
println!("{r}");
works with Unicode characters as well eg. "😎foobar"
The first guaranteed to be an ASCII character, can changed to a capital letter in place:
let mut s = "foobar".to_string();
if !s.is_empty() {
s[0..1].make_ascii_uppercase(); // Foobar
}
Panics with a non ASCII character in first position!
Since the method to_uppercase() returns a new string, you should be able to just add the remainder of the string like so.
this was tested in rust version 1.57+ but is likely to work in any version that supports slice.
fn uppercase_first_letter(s: &str) -> String {
s[0..1].to_uppercase() + &s[1..]
}
Here's a version that is a bit slower than #Shepmaster's improved version, but also more idiomatic:
fn capitalize_first(s: &str) -> String {
let mut chars = s.chars();
chars
.next()
.map(|first_letter| first_letter.to_uppercase())
.into_iter()
.flatten()
.chain(chars)
.collect()
}
This is how I solved this problem, notice I had to check if self is not ascii before transforming to uppercase.
trait TitleCase {
fn title(&self) -> String;
}
impl TitleCase for &str {
fn title(&self) -> String {
if !self.is_ascii() || self.is_empty() {
return String::from(*self);
}
let (head, tail) = self.split_at(1);
head.to_uppercase() + tail
}
}
pub fn main() {
println!("{}", "bruno".title());
println!("{}", "b".title());
println!("{}", "🦀".title());
println!("{}", "ß".title());
println!("{}", "".title());
println!("{}", "བོད་སྐད་ལ".title());
}
Output
Bruno
B
🦀
ß
བོད་སྐད་ལ
Inspired by get_mut examples I code something like this:
fn make_capital(in_str : &str) -> String {
let mut v = String::from(in_str);
v.get_mut(0..1).map(|s| { s.make_ascii_uppercase(); &*s });
v
}
I have a String (in particular a SHA1 hex digest) that I would like to split into two substrings - the first two characters and the rest of of the string. Is there a clean way to do this in Rust?
If you know that your string only contains ASCII characters (as in case with sha digests), you can use slices directly:
let s = "13e3f28a65a42bf6258cbd1d883d1ce3dac8f085";
let first = &s[..2]; // "13"
let rest = &s[2..]; // "e3f28a65a42bf6258cbd1d883d1ce3dac8f085"
It won't work correctly if your string contains non-ASCII characters because slicing uses byte offsets, and if any index used in slicing points into the middle of a code point representation, your program will panic.
There's a split_at method since Rust 1.4, use it like this:
let s = "13e3f28a65a42bf6258cbd1d883d1ce3dac8f085";
let (first, last) = s.split_at(2);
assert_eq!("13", first);
assert_eq!("e3f28a65a42bf6258cbd1d883d1ce3dac8f085", last);
Note that the index is a byte position and must lie on a character boundary. In this case this works because you know that your input string is ASCII.
If you are expecting two Strings instead of slices, you can use the chars() method and some Iterator methods to obtain them.
let text = "abcdefg".to_string();
let start: String = text.chars().take(2).collect();
let end: String = text.chars().skip(2).collect();
If you don't want to do heap allocations, you can use slices instead:
let start: &str = text.slice_chars(0, 2);
let end: &str = text.slice_chars(2, text.char_len());
Note that the slices version requires you to use unstable rust (nightly builds, not the beta)
Here is a way to efficiently split a String into two Strings, in case you have this owned string data case. The allocation of the input string is retained in the first piece by just using truncation.
/// Split a **String** at a particular index
///
/// **Panic** if **byte_index** is not a character boundary
fn split_string(mut s: String, byte_index: usize) -> (String, String)
{
let tail = s[byte_index..].into();
s.truncate(byte_index);
(s, tail)
}
Note: The .into() method is from the generic conversion trait Into and in this case it converts &str into String.