CODEWITHSUNDEEP
rustlapack
xgb84j
xgb84j

Reputation: 559

What do letters enclosed by two single quotes next to a function argument mean?

I tried using LAPACK bindings for Rust when I came over some syntax that I could not find anything about.

The example code from https://github.com/stainless-steel/lapack:

let n = 3;

let mut a = vec![3.0, 1.0, 1.0, 1.0, 3.0, 1.0, 1.0, 1.0, 3.0];
let mut w = vec![0.0; n];
let mut work = vec![0.0; 4 * n];
let lwork = 4 * n as isize;
let mut info = 0;

lapack::dsyev(b'V', b'U', n, &mut a, n, &mut w, &mut work, lwork, &mut info);

for (one, another) in w.iter().zip(&[2.0, 2.0, 5.0]) {
    assert!((one - another).abs() < 1e-14);
}

What does b'V' and b'U' mean?

Upvotes: 1

Views: 879

Answers (2)

Levans
Levans

Reputation: 15002

It creates a u8 value with the ASCII value of the char between quote.

For ASCII literals, it's the same as writing 'V' as u8.

Also, the b prefix on a double quoted string will create a byte array containing the UTF8 content of the string.

let s: &[u8; 11] = b"Hello world";

Upvotes: 4

Shepmaster
Shepmaster

Reputation: 430991

b'A' means to create a byte literal. Specifically, it will be a u8 containing the ASCII value of the character:

fn main() {
    let what = b'a';
    println!("{}", what);
    // let () = what;
}

The commented line shows you how to find the type.

b"hello" is similar, but produces a reference to an array of u8, a byte string:

fn main() {
    let what = b"hello";
    println!("{:?}", what);
    // let () = what;
}

Things like this are documented in the Syntax Index which is currently only available in the nightly version of the docs.

Upvotes: 4

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