When writing a computer program, we often want it to execute different actions for different scenarios and conditions. 

In Rust, this is where the `if` expression comes in. Let's have a look at what the `if` expression is.

# Syntax

The form of an `if` expression is a condition, followed by a block of code; there can be an else expression with this block of code.
See the syntax below for a basic `if` expression in Rust:


```
if expression {
    /* block of code in case expression
       is TRUE */
} else {
    /* block of code in case expression
       is FALSE */
}
```

# Code

Let's consider a simple example where we have to determine which number is greater out of the integer variables `a` and `b`. The code snippet below illustrates how this can be done using an `if` expression in Rust:

fn main() {
  let a = 50;
  let b = 20;

  if a > b {
    // condition is TRUE
    println!("a is greater than b")
  } else {
    // condition is FALSE
    println!("a is less than b")
  }
}

The code above follows the process flow below:

If expression in Rust

Rust's if expression executes different actions based on conditions using if and optional else blocks.