The DOWHILE loop

The DO-WHILE loop is a control structure that repeats a block of code at least once, and then checks a condition to decide whether to continue or exit the loop.

In natural language, the DO-WHILE loop means,

Do this 🡆 while this is true

The DO-WHILE loop is the “sibling” of the WHILE loop, and its operation is very similar.

The difference is that in the DO-WHILE loop the condition is checked after executing the loop body. Therefore, the loop executes at least once.

In general, the syntax of a DO-WHILE loop is as follows,

do
{
    // Block of code to execute while the condition is true
}
while(condition)

// rest of the code

Equivalence with WHILE Loop

Actually both loops are practically equivalent (in fact, it’s easy to convert one into the other).

Choosing between them depends on how your program works. It makes sense to use a DO-WHILE when the condition depends on a process, which is the same process executed inside the loop.

For example, imagine you have a function calculateThings(), and it returns a result. If the result meets a condition, for example it’s less than 10, you need to calculate it again.

That with a WHILE loop looks like this:

result = calculateThings()
while(result < 10)
{
    result = calculateThings()
}

// rest of the code

As we see, the instruction above the WHILE is the same as the one inside the loop body. This situation “smells like DO-WHILE”

Indeed, if you change the loop to a DO-WHILE, it looks like this, which is more concise and cleaner:

do
{
    result = calculateThings()
}
while(result < 10)

// rest of the code

Examples of DO-WHILE Loop in Different Languages

As we said, the DO-WHILE and WHILE loops are close siblings. So we don’t expect major syntax differences between them.

int counter = 0;
do
{
	// do something
    counter++;
} while (counter < 10);

let counter = 0;
do
{
    console.log("Iteration", counter);
    counter++;
} while (counter < 3);

$counter = 0;
do {
    // do something
    $counter++;
} while ($counter < 10);

counter = 0
# do things
while counter < 10:
    # do things
    counter += 1

Internal operation Advanced

The internal operation of a DO-WHILE loop is very similar to that of a WHILE loop. The difference is that the conditional jump is located at the end of the body of the loop, instead of at the beginning.

In this case, the program flow directly enters the body of the loop, so it will always execute once.

At the end it encounters a conditional jump. If it is met, the control flow returns to the start of the body of the loop. Otherwise, it continues with the program (“and so be it”).