Challenge 16

Rust Quiz

What is the output of this Rust program?


fn main() {
    let (.., x, y) = (0, 1, ..);

    println!("{:}", b"066"[y][x]);
}

A. The program does not compile

B. The program prints 54

C. The program is guaranteed to output: []

Solution

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This question demonstrates two different meanings of ..

In expression position, .. is the syntax for constructing various types of ranges. Here the expression (0, 1, ..) is a tuple with three elements, the third one having type RangeFull.

On the other hand in a pattern, .. is used to mean “any number of elements”. So the pattern (.., x, y) matches a tuple with 2 or more elements, binding the second-last one to x and the last one to y.

Coming out of the first line of main, we have x = 1 and y = (..). Thus the value printed is going to be b“066“[..][1].

The expression b“066“ is a byte-string literal of type &‘static [u8; 3] containing the three ASCII bytes b’0’, b’6’, b’6’.

When we slice the byte-string with RangeFull we get a dynamically sized slice [u8] of length 3. Next we access element 1 of the slice, which is the byte b’6’ of type u8. When printed, we see the decimal representation of the byte value of the ASCII digit 6, which is the number 54.

You can play with the code on Rust Playground.

Attribution: Challenge courtesy of dtolnay, including the insightful explanation!

Rust Bytes Challenges

Challenge 16

Rust Quiz

Solution