How to initialize empty str?

[rm-rf-etc]

Is it possible to initialize an empty str of a known length? I'm trying to write a parser. The output of each parsed message will be a 2x10 table, where each row has 2 str's of a known length, and I want to push characters into each str until it's full.

Should I use [char; N] instead?

Since I want to push characters in one at a time, should I implement my own struct to hold an index for the next insert?

[cole-miller]

It sounds like you should be using String here.

let mut s = String::new(); // starts empty; you could use `String::with_capacity`
                           // here since you know the full size in advance
s.push('~');

Vec<u8> would be another possibility, if all your characters are ASCII and you need indexing.

[steffahn]

For a string with a short compile-time-known upper bound in length, you could use the arrayvec crateʼs ArrayString<CAP> type. This avoids heap allocations. Note that the capacity CAP describes the number of bytes in UTF-8; non-ascii chars can take up to 4 bytes. It basically stores the string in a contained [MaybeUninit<u8>; CAP].

[rm-rf-etc]

Ya, I have a working implementation using String, but I want to rewrite it to avoid heap allocation.

[cole-miller]

In that case @steffahn's suggestion is the way to go.

[bradleyharden]

heapless::String is also an option.

[alice]

You can also just use an [u8; N] and use str::from_utf8 at the end.

[rm-rf-etc]

Alice, if I use [u8; N], do I have to implement push myself? I assume that means storing an index value so it knows where to insert the pushed character.

[alice]

Yes

[Vincent_0]

You can also use iterator if you don't want to implement push yourself.

let input = vec![1,2,3,4,5];
let mut output = [0;5];
input.iter().zip(output.iter_mut())
    for_each(|(i,o)| *o = *i);
assert_eq!(output, [1,2,3,4,5]);

But if you do so you should be sure that input size is equal to output size otherwise you can miss one input element:

// 5 is not read
let input = vec![1,2,3,4,5];
let mut output = [0;4];
input.iter().zip(output.iter_mut())
    .for_each(|(i,o)| *o = *i);
assert_eq!(output, [1,2,3,4]);

or have some output values that are not properly set.

// Number of input values is too small
let input = vec![1,2,3,4,5];
let mut output = [0;6];
input.iter().zip(output.iter_mut())
   .for_each(|(i,o)| *o = *i);
assert_eq!(output, [1,2,3,4,5,0]);