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 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 body of the loop. Therefore, the loop executes at least once.

In general, the syntax of a DO-WHILE loop has the following form,

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

// rest of the code

Equivalence with WHILE loop

In fact, both loops are practically equivalent (in fact, it is possible to easily convert one to the other).

Choosing between both 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 that is executed within the loop.

For example, imagine you have a function that calculateThings(), and it returns a result. If the result meets a condition, for example, is less than 10, you need to recalculate it.

That with a WHILE loop looks like this:

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

// rest of the code

As we see, the statement I have above the WHILE is the same as the one I have in the body of the loop. 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 clean:

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

// rest of the code

DO-WHILE loop examples in different languages

As we mentioned, the DO-WHILE and WHILE loops are close siblings. So we don’t expect major differences in syntax 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”).