Loops

Now I will introduce you the concept of loops. A loop can repeat code. There are several kind of loops but we will start with the simplest loop - The while loop.

While Loops

The while loop is a head-controlled loop. That means that the body will only be executed if the condition is true, otherwise the body will be skipped.

Here is the syntactic structure:

<while_loop> ::= "while" <condition> "{" <body> "}"

Here is a simple example:

func main() {
    let mut number = 42;

    while number > 0 { //only execute if number is greater than zero
        print(number); //prints the number

        number = number + 1; //increses number by 1
    }
}

Do-While Loops

The do-while loop is a foot-controlled loop. The body will always be executed and stops if the condition is false.

The struture:

<do-while> ::= "do" <statements> "while" <condition> ";"

A simple example:

func main() {
    let mut input : string = none;
    do {
        input = Console::ReadLine();

        print("echoed $input");
    } while input != "exit";
}

This sample expects an userinput and write it back only if the user types not exit.

For-Loop

A for loop can be used to iterate over arrays/iterators. To repeat code for a given number of iterations you can use ranges.

The definition of a for-loop:

<for_loop> ::= "for" <name> "in" <iterable> "{" <body> "}"

A simple example to repeat code:

for x in 1..12 {
    print("index: " + x);
}

You can also use ranges to reverse the direction of the loop:

for x in 12..1 {
    print("index: " + x);
}

Exercices

  1. Write a program to calculate the factorial of a given number with a while loop.

  2. Would it be possible to solve this problem with a for loop?

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