src/doc/book/if.md

   1 % if
   2
   3 Rust’s take on `if` is not particularly complex, but it’s much more like the
   4 `if` you’ll find in a dynamically typed language than in a more traditional
   5 systems language. So let’s talk about it, to make sure you grasp the nuances.
   6
   7 `if` is a specific form of a more general concept, the ‘branch’, whose name comes
   8 from a branch in a tree: a decision point, where depending on a choice,
   9 multiple paths can be taken.
  10
  11 In the case of `if`, there is one choice that leads down two paths:
  12
  13 ```rust
  14 let x = 5;
  15
  16 if x == 5 {
  17     println!("x is five!");
  18 }
  19 ```
  20
  21 If we changed the value of `x` to something else, this line would not print.
  22 More specifically, if the expression after the `if` evaluates to `true`, then
  23 the block is executed. If it’s `false`, then it is not.
  24
  25 If you want something to happen in the `false` case, use an `else`:
  26
  27 ```rust
  28 let x = 5;
  29
  30 if x == 5 {
  31     println!("x is five!");
  32 } else {
  33     println!("x is not five :(");
  34 }
  35 ```
  36
  37 If there is more than one case, use an `else if`:
  38
  39 ```rust
  40 let x = 5;
  41
  42 if x == 5 {
  43     println!("x is five!");
  44 } else if x == 6 {
  45     println!("x is six!");
  46 } else {
  47     println!("x is not five or six :(");
  48 }
  49 ```
  50
  51 This is all pretty standard. However, you can also do this:
  52
  53 ```rust
  54 let x = 5;
  55
  56 let y = if x == 5 {
  57     10
  58 } else {
  59     15
  60 }; // y: i32
  61 ```
  62
  63 Which we can (and probably should) write like this:
  64
  65 ```rust
  66 let x = 5;
  67
  68 let y = if x == 5 { 10 } else { 15 }; // y: i32
  69 ```
  70
  71 This works because `if` is an expression. The value of the expression is the
  72 value of the last expression in whichever branch was chosen. An `if` without an
  73 `else` always results in `()` as the value.