Loops, part 1: while

Your implementation of factorial has been forced to use recursion.
This may feel natural to you, especially if you're coming from a functional programming background. Or it may feel strange, if you're used to more imperative languages like C or Python.

Let's see how you can implement the same functionality using a loop instead.

The while loop

A while loop is a way to execute a block of code as long as a condition is true.
Here's the general syntax:

while <condition> {
    // code to execute
}

For example, we might want to sum the numbers from 1 to 5:

let sum = 0;
let i = 1;
// "while i is less than or equal to 5"
while i <= 5 {
    // `+=` is a shorthand for `sum = sum + i`
    sum += i;
    i += 1;
}

This will keep adding 1 to i and i to sum until i is no longer less than or equal to 5.

The mut keyword

The example above won't compile as is. You'll get an error like:

error[E0384]: cannot assign twice to immutable variable `sum`
 --> src/main.rs:7:9
  |
2 |     let sum = 0;
  |         ---
  |         |
  |         first assignment to `sum`
  |         help: consider making this binding mutable: `mut sum`
...
7 |         sum += i;
  |         ^^^^^^^^ cannot assign twice to immutable variable

error[E0384]: cannot assign twice to immutable variable `i`
 --> src/main.rs:8:9
  |
3 |     let i = 1;
  |         -
  |         |
  |         first assignment to `i`
  |         help: consider making this binding mutable: `mut i`
...
8 |         i += 1;
  |         ^^^^^^ cannot assign twice to immutable variable

This is because variables in Rust are immutable by default.
You can't change their value once it has been assigned.

If you want to allow modifications, you have to declare the variable as mutable using the mut keyword:

// `sum` and `i` are mutable now!
let mut sum = 0;
let mut i = 1;

while i <= 5 {
    sum += i;
    i += 1;
}

This will compile and run without errors.

Further reading

Exercise

The exercise for this section is located in 02_basic_calculator/06_while