if expressions

You use if expressions exactly like if statements in other languages:

1. ```
   fn main() {
   ```
2. ```
       let x = 10;
   ```
3. ```
       if x == 0 {
   ```
4. ```
           println!("zero!");
   ```
5. ```
       } else if x < 100 {
   ```
6. ```
           println!("biggish");
   ```
7. ```
       } else {
   ```
8. ```
           println!("huge");
   ```
9. ```
       }
   ```
10. ```
    }
    ```

In addition, you can use if as an expression. The last expression of each block becomes the value of the if expression:

1. ```
   fn main() {
   ```
2. ```
       let x = 10;
   ```
3. ```
       let size = if x < 20 { "small" } else { "large" };
   ```
4. ```
       println!("number size: {}", size);
   ```
5. ```
   }
   ```

This slide should take about 4 minutes.

Because if is an expression and must have a particular type, both of its branch blocks must have the same type. Show what happens if you add ; after "small" in the second example.

An if expression should be used in the same way as the other expressions. For example, when it is used in a let statement, the statement must be terminated with a ; as well. Remove the ; before println! to see the compiler error.