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% if
Rust’s take on `if` is not particularly complex, but it’s much more like the
`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` 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`, there is one choice that leads down two paths:
```rust
let x = 5;
if x == 5 {
println!("x is five!");
}
```
If we changed the value of `x` to something else, this line would not print.
More specifically, if the expression after the `if` evaluates to `true`, then
the block is executed. If it’s `false`, then it is not.
If you want something to happen in the `false` case, use an `else`:
```rust
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`:
```rust
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:
```rust
let x = 5;
let y = if x == 5 {
10
} else {
15
}; // y: i32
```
Which we can (and probably should) write like this:
```rust
let x = 5;
let y = if x == 5 { 10 } else { 15 }; // y: i32
```
This works because `if` is an expression. The value of the expression is the
value of the last expression in whichever branch was chosen. An `if` without an
`else` always results in `()` as the value.
|
