Lesson 6.1. do..while loop in C

Lesson 5.6 Summary Lesson 6.2

In C, it is possible to automatically repeat blocks of instructions until a condition is true (or false). This type of repetition is called a loop. In C, there are 3 loops:

do..while loop
while loop
for loop

In this chapter, we will study each of these loops starting with the do..while.

Syntax

The do..while loop repeats a block of instructions while the condition is true :

do {
    // ...
    Instructions block
    // ...
}
while (test);

Here is the flowchart for the do..while loop:

The instruction block is also called the body of the loop. It is the part between the curly braces. One execution of the loop is called an iteration.

With the do..while loop, the body of the loop is necessarily executed once, even if the test is false at the first iteration.

With the do..while loop, the body of the loop is required to be executed at least once, even if the test is false on the first iteration.

Example

Here is an example of a do..while loop that asks the user to enter a number as long as it is not strictly negative.

int n;
do {
  // Body of the loop: ask a number to the user
  printf ("Enter a negative integer: ");
  scanf ("%d", &n);
}
while (n>=0);
printf ("You entered %d.\n", n);

The loop will be repeated while n is positive or zero.

Try online

Exercise 1

Write a program that asks the user to enter a grade until the grade is not between 0 and 20.

  float grade;

  // Entering the grade between 0 and 20
  // COMPLETE HERE

  // Display the grade
  printf ("The grade is %.1f.\n", grade);

Here is the expected output:

Enter the grade between 0 and 20: -5
Enter the grade between 0 and 20: 22
Enter the grade between 0 and 20: 16.4
The grade is 16.4.

Do the exercise

Solution

Exercise 2

Write a program with a do..while loop that counts from 3 to 22 according to the following pattern:

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Do the exercise

Solution

Exercise 3

With a do..while loop, display the following sequence:

$$ U_{n+1} = U_{n} * 2 $$

with $ U_0 = 1 $

Display the sequence until (U_n\) is less than or equal to 1024 according to the following model:

1 2 4 8 16 32 64 128 256 512 1024

Do the exercise

Solution

Quiz

For the analysis of the programs below, it is best to have a paper and pencil.

In programming, what can a loop be used for?

To travel in time thanks to the time loops.

To repeat a block of instructions indefinitely.

To repeat a block of instructions until a given condition is reached.

To block a program until a condition is true.

Check Bravo! On embedded systems it is common to block a program ( while(1); ) or wait for an external event with a loop.

What are the valid loops in C?

for loop

do..while loop

while loop

do..until loop

Check

What does the following code display?

int x=7;
do {
    printf ("%d ", ++x);
} while (x<10);

8 9

7 8 9

8 9 10

7 8 9 10

Check Bravo! The pre-increment is done before the display and in the last iteration, x is 9 before increment.

What does the following code display?

int x=7;
do {
    printf ("%d ", x--);
} while (x>=5);

6 5

7 6 5

6 5 4

7 6 5 4

Check Bravo! The post-increment is done after the display and in the last iteration, x is 5 before the printf.

What does the following code display?

int x=2;
do { 
    x=x+1; 
} while (x<=10);
printf ("%d", x);

This is an infinite loop.

Check Bravo! While x is less or equal to 10, we go on. So we exit the loop at 11.

What must the user enter to exit the loop?

int x;
do{ 
    scanf ("%d", &x);
} while (x<0 || x>100);

This is an infinite loop.

A number between 0 and 100 inclusive.

A number between 0 and 100 excluded.

A number smaller than 0 or greater than 100.

Check Bravo! While x is less than 0 or greater than 100, we loop.

What must the user enter to exit the loop?

int x;
do{ 
    scanf ("%d", &x);
} while (x<0 && x>100);

This is an infinite loop.

Any number.

A number between 0 and 100 excluded.

A number between 0 and 100 inclusive.

Check

Lesson 5.6 Summary Lesson 6.2

Lesson 6.1. do..while loop in C

Syntax

Example

Exercise 1

Exercise 2

Exercise 3

Quiz

See also