100 exercises to learn Rust

book/src/02_basic_calculator/03_if_else.md

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+# Control flow, part 1
+
+All our programs so far have been pretty straightforward.  
+A sequence of instructions is executed from top to bottom, and that's it.
+
+It's time to introduce some **branching**.
+
+## `if` expressions
+
+The `if` keyword is used to execute a block of code only if a condition is true.
+
+Here's a simple example:
+
+```rust
+let number = 3;
+if number < 5 {
+    println!("`number` is smaller than 5");
+}
+```
+
+This program will print `number is smaller than 5` because the condition `number < 5` is true.
+
+Like most programming languages, Rust supports an optional `else` branch to execute a block of code when the condition in an
+`if` expression is false.  
+For example:
+
+```rust
+let number = 3;
+
+if number < 5 {
+    println!("`number` is smaller than 5");
+} else {
+    println!("`number` is greater than or equal to 5");
+}
+```
+
+## Booleans
+
+The condition in an `if` expression must be of type `bool`, a **boolean**.  
+Booleans, just like integers, are a primitive type in Rust.
+
+A boolean can have one of two values: `true` or `false`.
+
+### No truthy or falsy values
+
+If the condition in an `if` expression is not a boolean, you'll get a compilation error.
+
+For example, the following code will not compile:
+
+```rust
+let number = 3;
+if number {
+    println!("`number` is not zero");
+}
+```
+
+You'll get the following compilation error:
+
+```text
+error[E0308]: mismatched types
+ --> src/main.rs:3:8
+  |
+3 |     if number {
+  |        ^^^^^^ expected `bool`, found integer
+```
+
+This follows from Rust's philosophy around type coercion: there's no automatic conversion from non-boolean types to booleans.
+Rust doesn't have the concept of **truthy** or **falsy** values, like JavaScript or Python.  
+You have to be explicit about the condition you want to check.
+
+### Comparison operators
+
+It's quite common to use comparison operators to build conditions for `if` expressions.  
+Here are the comparison operators available in Rust when working with integers:
+
+- `==`: equal to
+- `!=`: not equal to
+- `<`: less than
+- `>`: greater than
+- `<=`: less than or equal to
+- `>=`: greater than or equal to
+
+## References
+
+- The exercise for this section is located in `exercises/02_basic_calculator/03_if_else`