Rust’s take on if`ifis not particularly complex, but it’s much more like the` is not particularly complex, but it’s much more like the
if`if` you’ll find in a dynamically typed language than in a more traditional
systems language. So let’s talk about it, to make sure you grasp the nuances.
if`if` is a specific form of a more general concept, the ‘branch’. The name comes
from a branch in a tree: a decision point, where depending on a choice,
multiple paths can be taken.
In the case of if`if`, there is one choice that leads down two paths:
let x = 5; if x == 5 { println!("x is five!"); }
If we changed the value of x`xto something else, this line would not print. More specifically, if the expression after the` to something else, this line would not print.
More specifically, if the expression after the if`ifevaluates to` evaluates to true`true, then the block is executed. If it’s`, then
the block is executed. If it’s false`false`, then it is not.
If you want something to happen in the false`falsecase, use an` case, use an else`else`:
let x = 5; if x == 5 { println!("x is five!"); } else { println!("x is not five :("); }
If there is more than one case, use an else if`else if`:
let x = 5; if x == 5 { println!("x is five!"); } else if x == 6 { println!("x is six!"); } else { println!("x is not five or six :("); }
This is all pretty standard. However, you can also do this:
fn main() { let x = 5; let y = if x == 5 { 10 } else { 15 }; // y: i32 }let x = 5; let y = if x == 5 { 10 } else { 15 }; // y: i32
Which we can (and probably should) write like this:
fn main() { let x = 5; let y = if x == 5 { 10 } else { 15 }; // y: i32 }let x = 5; let y = if x == 5 { 10 } else { 15 }; // y: i32
This works because if`ifis an expression. The value of the expression is the value of the last expression in whichever branch was chosen. An` is an expression. The value of the expression is the
value of the last expression in whichever branch was chosen. An if`ifwithout an` without an
else`elsealways results in` always results in ()`()` as the value.