control-flow

Iterate through code with control flow

• Iterate through code with control flow
  • if/else expressions
  • Loop forever with loop
  • Loop until a criteria is met with while loops
  • Iterate with for loops

if/else expressions

We can create conditional branches in the execution of our code by using the if and else keywords, similar to what other languages do.

The form of an if expression is a condition expression followed by a consequent block, any number of else if conditions and blocks, and an optional trailing else block. The condition expressions must have type bool.

if 1 == 2 {
    println!("whoops, mathematics broke");
} else {
    println!("everything's fine!");
}

In the preceding example, the condition of if is the expression 1 == 2, which evaluates into a boolean type with the value false.

You can have multiple conditions by combining if and else in an else if expression. For example:

let number = 6;

if number % 4 == 0 {
    println!("number is divisible by 4");
} else if number % 3 == 0 {
    println!("number is divisible by 3");
} else if number % 2 == 0 {
    println!("number is divisible by 2");
} else {
    println!("number is not divisible by 4, 3, or 2");
}

This program has four possible paths it can take. After running it, you should see the following output:

    number is divisible by 3

If a condition expression evaluates to true, the consequent block is executed. Any subsequent else if or else block is skipped. If a condition expression evaluates to false, the consequent block is skipped. Any subsequent else if condition is evaluated. If all if and else if conditions evaluate to false, then any else block is executed.

Unlike in most languages, if blocks can also act as expressions. Remember that all branches must return the same type for our code to compile.

let formal = true;
let greeting = if formal {
    "Good evening."
} else {
    "Hello, friend!"
};
println!(greeting) // prints "Good evening."

In the preceding example, we can see that the result of the if expression is stored in the greeting variable.

Loop forever with loop

A loop expression denotes an infinite loop. It repeats execution of its body continuously:

loop {
    println!("I loop forever");
}

Unlike the other kinds of loops in Rust like while and for, loop can be used in expressions that return values via break.

let mut i = 1;
let something = loop {
    i *= 2;
    if i > 100 {
        break i;
    }
};
assert_eq!(something, 128);

Every break a loop has must have the same type. When it's not explicitly giving something, break; returns () (an empty tuple).

Loop until a criteria is met with while loops

A while expression loops until a predicate is false.

A while loop begins by evaluating the boolean loop conditional expression. If the loop conditional expression evaluates to true, the loop body block executes. Control then returns to the loop conditional expression. If the loop conditional expression evaluates to false, the while expression completes.

The following code loops until the predicate evaluates to true:

let mut counter = 0;

while counter < 10 {
    println!("hello");
    counter = counter + 1;
}

Iterate with for loops

A for expression extracts values from an iterator. It loops until the iterator is empty.

In Rust, an iterator is any type that can iterate over values. Some values can be iterated over directly and others can produce iterators by calling methods like .iter().

let a = [10, 20, 30, 40, 50];

for element in a.iter() {
    println!("the value is: {}", element);
}

The preceding code iterates through each element in the array and binds it to the element variable. The println! macro then prints each of those values in sequence.

Another easy way to create an iterator is to use the range notation a..b. This notation yields values from a (inclusive) to b (exclusive) in steps of one.

for item in 0..5 {
    println!("{}", item * 2);
}

The preceding code iterates through the numbers 0, 1, 2, 3, and 4 and binds it to the item variable for each cycle of this loop.