100 exercises to learn Rust

book/src/02_basic_calculator/06_while.md

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+# 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:
+
+```rust
+while <condition> {
+    // code to execute
+}
+```
+
+For example, we might want to sum the numbers from 1 to 5:
+
+```rust
+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 `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:
+
+```text
+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:
+
+```rust
+// `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.
+
+## References
+
+- The exercise for this section is located in `exercises/02_basic_calculator/06_while`
+
+## Further reading
+
+- [`while` loop documentation](https://doc.rust-lang.org/std/keyword.while.html)