I am trying to write a palindrome program in Rust.
Even when the input is a palindrome word, my attempt is not showing a palindrome:
use std::io;
fn main(){
println!("enter a word to know if palindrome or not");
let mut inp=String::new();
io::stdin().read_line(&mut inp).expect("needed a string");
let arr:Vec<_>=inp.chars().collect();
let mut new_st=String::new();
for i in 0..arr.len(){
new_st.push(arr[arr.len()-1-i]);
}
if inp.eq(&new_st[1..]) {
println!("Palindrome");
}
else{
println!("not a palindrome..");
}
println!("{}",&new_st[1..]);
}
Output:
enter a word to know if palindrome or not
amma
not a palindrome..
amma
The problem is that the .read_line() function adds a \n to the end of inp string. You should remove it from the string or better yet use the .trim() method on the String to strip out any newline or whitespace characters.
inp = inp.trim().to_string();
Just some more improvement on your code, you should leverage Rust's iterators to reverse the String faster rather than manually doing it.
You can write this:-
let rev = inp.chars().rev().collect::<String>();
This iterates over all the characters in the inp string and reverses them in order. Finally it collects them into a String that is stored in the rev variable.
Also, you should use == rather than using the .eq() operator, it's just much more clearer.
See this playground link for complete code
I am attempting to write a lexer for fun, however something keeps bothering me.
let mut chars: Vec<char> = Vec::new();
let mut contents = String::new();
let mut tokens: Vec<&String> = Vec::new();
let mut append = String::new();
//--snip--
for _char in chars {
append += &_char.to_string();
append = append.trim().to_string();
if append.contains("print") {
println!("print found at: \n{}", append);
append = "".to_string();
}
}
Any time I want to do something as simple as append a &str to a String I have to convert it using .to_string, String::from(), .to_owned, etc.
Is there something I am doing wrong, so that I don't have to constantly do this, or is this the primary way of appending?
If you're trying to do something with a type, check the documentation. From the documentation for String:
push: "Appends the given char to the end of this String."
push_str: "Appends a given string slice onto the end of this String."
It's important to understand the differences between String and &str, and why different methods accept and return each of them.
A &str or &mut str are usually preferred in function arguments and return types. That's because they are just pointers to data so nothing needs to be copied or moved when they are passed around.
A String is returned when a function needs to do some new allocation, while &str and &mut str are slices into an existing String. Even though &mut str is mutable, you can't mutate it in a way that increases its length because that would require additional allocation.
The trim function is able to return a &str slice because that doesn't involve mutating the original string - a trimmed string is just a substring, which a slice perfectly describes. But sometimes that isn't possible; for example, a function that pads a string with an extra character would have to return a String because it would be allocating new memory.
You can reduce the number of type conversions in your code by choosing different methods:
for c in chars {
append.push(c); // append += &_char.to_string();
append = append.trim().to_string();
if append.contains("print") {
println!("print found at: \n{}", append);
append.clear(); // append = "".to_string();
}
}
There isn't anything like a trim_in_place method for String, so the way you have done it is probably the only way.
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.
Is there a function to do something like this:
fn string_to_unicode_char(s: &str) -> Option<char> {
// ...
}
fn main() {
let s = r"\u{00AA}"; // note the raw string literal!
string_to_unicode_char(s).unwrap();
}
Note that r"\u{00AA}" uses a raw string i. e. it isn't a Unicode sequence but 8 separate symbols, as \ u { 0 0 A A }.
I need to interpret/convert/parse this string and return a char if all is good, None otherwise. I don't have any experience with Unicode, so any ideas are welcome.
I believe the function you are looking for is char::from_u32:
fn string_to_unicode_char(s: &str) -> Option<char> {
// Do something more appropriate to find the actual number
let number = &s[3..7];
u32::from_str_radix(number, 16)
.ok()
.and_then(std::char::from_u32)
}
fn main() {
let s = r"\u{00AA}"; // note the raw string literal!
let ch = string_to_unicode_char(s);
assert_eq!(ch, Some('\u{00AA}'));
}
I indeed completely misunderstood your question; my old answer can be seen in the edit logs
Is there a builtin function to parse a string containing a Rust unicode escape into the corresponding unicode character?
AFAIK, no, there is not a builtin function to do that.
The answer to "how to do it yourself" is a bit broad, as there are many ways to do it (and it's not clear whether you also want to parse standard escapes, such as "\n").
Use a regex
Do simple, naive manual parsing
Embed it into a bigger lexer (the function in the Rust compiler parsing such unicode escapes)
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
}