C While Loop

Loops

Loops can execute a block of code as long as a specified condition is reached.

Loops are handy because they save time, reduce errors, and they make code more readable.

While Loop

The while loop loops through a block of code as long as a specified condition is true:

Syntax

while (condition) {
  // code block to be executed
}

In the example below, the code in the loop will run, over and over again, as long as a variable (i) is less than 5:

Example

int i = 0;

while (i < 5) {
  printf("%d\n", i);
  i++;
}

Note: Do not forget to increase the variable used in the condition (i++), otherwise the loop will never end!

The Do/While Loop

The do/while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.

Syntax

do {
  // code block to be executed
}
while (condition);

The example below uses a do/while loop. The loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:

Example

int i = 0;

do {
  printf("%d\n", i);
  i++;
}
while (i < 5);

Do not forget to increase the variable used in the condition, otherwise the loop will never end!

Real-Life Examples

To demonstrate a practical example of the while loop, we have created a simple "countdown" program:

Example

int countdown = 3;

while (countdown > 0) {
  printf("%d\n", countdown);
  countdown--;
}

printf("Happy New Year!!\n");

In this example, we create a program that only print even numbers between 0 and 10 (inclusive):

Example

int i = 0;

while (i <= 10) {
  printf("%d\n", i);
  i += 2;
}

In this example we use a while loop to reverse some numbers:

Example

// A variable with some specific numbers
int numbers = 12345;

// A variable to store the reversed number
int revNumbers = 0;

// Reverse and reorder the numbers
while (numbers) {
  // Get the last number of 'numbers' and add it to 'revNumber'
  revNumbers = revNumbers * 10 + numbers % 10;
  // Remove the last number of 'numbers'
  numbers /= 10;
}

To demonstrate a practical example of the while loop combined with an if else statement, let's say we play a game of Yatzy:

Example

Print "Yatzy!" If the dice number is 6:

int dice = 1;

while (dice <= 6) {
  if (dice < 6) {
    printf("No Yatzy\n");
  } else {
    printf("Yatzy!\n");
  }
  dice = dice + 1;
}

If the loop passes the values ranging from 1 to 5, it prints "No Yatzy". Whenever it passes the value 6, it prints "Yatzy!".