How to assign 2 variables simultaneously in Rust? - rust

Question

I'm currently teaching myself Rust with a roguelike tutorial, and I'm attempting to get a key press to move a character diagonally which would mean player_x -=1, player_y -= 1 for up left.
No matter which way I try to arrange the code, I keep getting error messages from the compiler. I couldn't find any example of this anywhere in the documentation or on GitHub.
Key { code: Escape, .. } => return true, // exit game
// movement keys
Key { code: Up, .. } => *player_y -= 1,
Key { code: Down, .. } => *player_y += 1,
Key { code: Left, .. } => *player_x -= 1,
Key { code: Right, .. } => *player_x += 1,
Key { printable: 'k', .. } => *player_y -= 1,
Key { printable: 'k', .. } => *player_x -= 1,
_ => {}
You can kind of see what I'm trying to do here, but this throws an error message saying the pattern is unreachable, how would I fix this code?
Here's the full code, it's fairly small, not sure what would be alone necessary to compile:
extern crate tcod;
extern crate input;
use tcod::console::*;
use tcod::colors;
// actual size of the window
const SCREEN_WIDTH: i32 = 80;
const SCREEN_HEIGHT: i32 = 50;
const LIMIT_FPS: i32 = 20; // 20 frames-per-second maximum
fn handle_keys(root: &mut Root, player_x: &mut i32, player_y: &mut i32) -> bool {
use tcod::input::Key;
use tcod::input::KeyCode::*;
let key = root.wait_for_keypress(true);
match key {
Key { code: Enter, alt: true, .. } => {
// Alt+Enter: toggle fullscreen
let fullscreen = root.is_fullscreen();
root.set_fullscreen(!fullscreen);
}
Key { code: Escape, .. } => return true, // exit game
// movement keys
Key { code: Up, .. } => *player_y -= 1,
Key { code: Down, .. } => *player_y += 1,
Key { code: Left, .. } => *player_x -= 1,
Key { code: Right, .. } => *player_x += 1,
Key { printable: 'k', ..} => *player_y -= 1,
Key { printable: 'k', ..} => *player_x -= 1,
_ => {},
}
false
}
fn main() {
let mut root = Root::initializer()
.font("terminal8x8_gs_tc.png", FontLayout::Tcod)
.font_type(FontType::Greyscale)
.size(SCREEN_WIDTH, SCREEN_HEIGHT)
.title("Rust/libtcod tutorial")
.init();
tcod::system::set_fps(LIMIT_FPS);
let mut player_x = SCREEN_WIDTH / 2;
let mut player_y = SCREEN_HEIGHT / 2;
while !root.window_closed() {
root.set_default_foreground(colors::WHITE);
root.put_char(player_x, player_y, '#', BackgroundFlag::None);
root.flush();
root.put_char(player_x, player_y, ' ', BackgroundFlag::None);
// handle keys and exit game if needed
let exit = handle_keys(&mut root, &mut player_x, &mut player_y);
if exit {
break
}
}
}

Answer 1

How to assign 2 variables simultaneously in Rust?
You cannot. You can bind two variables at once:
let (a, b) = (1, 2);
If you aren't trying to create new bindings, you need to have two assignment statements:
let mut a = 1;
let mut b = 2;
a = 3;
b = 4;
In your case, for a match statement, you need to introduce a block:
let key = 42;
let mut a = 1;
let mut b = 2;
match key {
0 => {
a += 1;
b -= 1;
}
_ => {
a -= 10;
b *= 100;
}
}
You could also have the match expression evaluate to a tuple, which you then create new bindings for and apply them afterwards:
let key = 42;
let mut x = 1;
let mut y = 2;
let (d_x, d_y) = match key {
0 => (1, -1),
_ => (10, 10),
};
x += d_x;
y += d_y;
I strongly recommend reading The Rust Programming Language instead of trying to learn Rust by intuition or trial and error. It has an entire chapter on the match statement.
See also:
How to swap two variables?

Answer 2

The second printable: 'k' is unreachable, since the first will match instead. What you want is doing both assignments in the same arm of the match, like this:
Key { printable: 'k', .. } => {
*player_y -= 1;
*player_x -= 1;
}