This question already has answers here:
What is the easiest way to pad a string with 0 to the left?
(2 answers)
Closed 2 years ago.
I'm new to Rust and completely lost in the standard library. I do not want to add a crate just for padding.
How do I pad left a number with zeroes up to a certain width?
Lets say I have this code and I need the pad_left implementation.
fn main() {
let i = 5.to_string();
let padded_i = pad_left(i, 2);
}
The Rust Docs has an example of how you could format a number as a string with leading zeroes up to a certain width (5 in this example):
format!("Hello {:05}!", 5); // returns, "Hello 00005!"
format!("Hello {:05}!", -5); // returns, "Hello -0005!"
If you need to print the number, then you can replace the format! macro with print! or println! instead.
This question already has answers here:
In Rust, what's the difference between "shadowing" and "mutability"?
(1 answer)
Proper way to return a new string in Rust
(2 answers)
Return local String as a slice (&str)
(7 answers)
Why can't I return an &str value generated from a String?
(1 answer)
How do I make format! return a &str from a conditional expression?
(3 answers)
Closed 2 years ago.
How is the following made to work (safely)? There are serious downsides to using 'static.
fn whatever() {
let mut myslice = "goodbye"; //variable that lives, but not necessarily the whole program!
print!("{}", myslice);
{
let mystring = "hello".to_string();
myslice = &*mystring;
}
print!("{}", myslice);
}
The second print should produce 'hello'.
I encounter this problem commonly in lots of forms.
The open bracket could represent multiple things, like calling a function or using an if statement.
E.g.'If there are problems with the value in myslice and things are not working properly. {'
Working out the replacement, (which proved in the above example to be 'hello') is frequently no easy or quick matter, and involves code not to be touched unless it was proved there was a problem. As is normal in Rust, there are many alternatives to &*mystring (&mystring[..], : &str on the left, &*mystring, mystring.as_str(), etc.) but none explicitly manipulate the perfectly available, mutable and live long enough variable as if I had typed let found = "hello".to_string; myslice = &found;' outside the curly brackets. I have tried.clone()` in various places. Why does Rust make such a meal of this simple request? Obviously I am prepared to pay the minuscule processor time to actually do the
request.
I would like a general solution. However, it seems the above problem is explicitly with the type 'String'. e.g. let found = "hello"; let myslice = found; seems to work, even inside the brackets. (found is now &str - it does not seemed ever 'borrowed'.) Is the problem directly or indirectly tied up with not knowing length at compile time? Unfortunately and frequently this is not in my control, I have to use what crates decide to give.
I have a String, and I want to make a new String, with every character in the first one doubled. So "abc" would become "aabbcc" and so on.
The best I've come up with is:
let mut result = String::new();
for c in original_string.chars() {
result.push(c);
result.push(c);
}
result
This works fine. but is there a more succinct (or more idiomatic) way to do this?
In JavaScript I would probably write something like:
original.split('').map(c => c+c).join('')
Or in Ruby:
(original.chars.map { |c| c+c }).join('')
Since Rust also has functional elements, I was wondering if there is a similarly succinct solution.
I would use std::iter::repeat to repeat every char value from the input. This creates an infinite iterator, but for your case we only need to iterate 2 times, so we can use take to limit our iterator, then flatten all the iterators that hold the doubled chars.
use std::iter;
fn main() {
let input = "abc"; //"abc".to_string();
let output = input
.chars()
.flat_map(|c| iter::repeat(c).take(2))
.collect::<String>();
println!("{:?}", output);
}
Playground
Note: To double we are using take(2) but you can use any usize to increase the repetition.
Personally, I would just do exactly what you're doing. Its intent is clear (more clear than the functional approaches you presented from JavaScript or Ruby, in my opinion) and it is efficient. The only thing I would change is perhaps reserve space for the characters, since you know exactly how much space you will need.
let mut result = String::with_capacity(original_string.len() * 2);
However, if you are really in love with this style, you could use flat_map
let result: String = original_string.chars()
.flat_map(|c| std::iter::repeat(c).take(2))
.collect();
This question already has answers here:
Why does my string not match when reading user input from stdin?
(3 answers)
Closed 6 years ago.
I use String::from("string") to get a String
let dog = String::from("dog")
and
dog == String::from("dog")
returns false. Even in pattern matching.
match dog.as_ref() {
"dog" => println!("Dog is a dog"), //no output
_ => println!("Dog is not a dog")
}
What is wrong?
Example
use std::io;
fn main() {
let mut sure = String::from("");
println!("Hello, world!");
println!("Are you sure(Y/N)");
io::stdin().read_line(&mut sure).expect("Failed");
println!("sure {}", sure );
let surely = {sure == String::from("Y")};
println!("surely {} ", surely ); //this line output is "surely false"
if surely {
dog_loop("HA");
}
}
As a general rule, when comparing Strings in Rust, it's better to turn the string into a &str for comparison against a string literal, rather than converting the string literal into a String. The reason for this is that the latter requires object creation (allocating for a String), while the first doesn't, and so it's more efficient.
The specific problem you are seeing here comes from the fact that your input does not have excess whitespace stripped. After the line
io::stdin().read_line(&mut sure).expect("Failed");
The value of sure is not "Y" as you might expect, but is actually "Y\n" on Unix, or "Y\r\n" on Windows. You can compare this directly by modifying your comparison as so:
let surely = {sure.as_str() == "Y\n"};
println!("surely {} ", surely );
And you will see it return "surely true". However, this makes your code platform-dependent. Preferably, use the string method String.trim(), which will remove the trailing whitespace.
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
}