Lesson 6.2. While loop in C

Lesson 6.1 Summary Lesson 6.3

The while loop is similar to the do..while loop except that the condition is tested first (before the block of instructions). This implies - unlike the do..while - that the instruction block will not necessarily be executed at least once. In other words, it is possible to jump the loop if the condition is false from the start.

Syntax

The while loop repeats a sequence of statements as long as the condition is true, but the condition is evaluated first :

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

Here is the flowchart for the while loop:

With the while loop, the body of the loop may not be executed if the test is false at the begininng.

There is no semicolon at the end of the while, unless the instruction block is empty: while(1);.

Chacun des paramètres est optionnel, on peut écrire for(;;);, ce qui est équivalent à while(1);

Example

Here is an example of a while loop that asks the user to enter a negative number:

int n;
// First request of a number
printf ("Enter a number: ");
scanf ("%d", &n);

// If the number is positive, ask for the number again
while (n>=0) {
  printf ("Enter a negative number: ");
  scanf ("%d", &n);
}

// Display the valid number
printf ("You entered %d.\n", n);

If on the first request, the user enters a negative number, we do not enter the loop and the execution goes directly to the display.

Try online

Exercise 1

Write a program that asks the user to enter a percentage. If the percentage is not between 0 and 100, the program asks for the percentage again but then specifies that it must be between 0 and 100 according to the following example:

Enter a percentage: -3.2
Enter a percentage between 0 and 100: 125.6
Enter a percentage between 0 and 100: 25.2
The discount is 25.20%.

float dicount;

// Request a percentage
// COMPLETE HERE

// Check if the percentage is between 0 and 100
// COMPLETE HERE

// Display the percentage
// COMPLETE HERE

Do the exercise

Solution

Exercise 2

Write a program with a while loop that computes the following sequence:

$$ U_n = \dfrac{1}{U_{n-1}+1} $$

with $ U_0 = 1 $.

The user enters the index of the element he wants to calculate and the program displays this element:

Enter the index n: 0
U(0) = 1.000000

Enter the index n: 3
U(3) = 0.600000

Enter the index n: 100
U(100) = 0.618034

Here is the code to complete:

int i=0, n;
float U=1;

// Ask for the index
printf ("Enter the index n: ");
scanf ("%d", &n);

// Compute element Un
// COMPLETE HERE

// Display the result
printf ("U(%d) = %.6f\n", n, U);

Do the exercise

Solution

Exercise 3

Write a program with a while' loop that finds and displays the largest divisor of a numberNentered by the user. The program does not consider the 1 andNwhich are always divisors ofN`.

If the number is 0 or 1, the program displays:

Enter a strictly positive integer n: 0
0 has no divisor.

Enter a strictly positive integer n: 1
1 has no divisor.

Si le programme trouve un diviseur, il affiche ce diviseur :

Enter a strictly positive integer n: 15
5 is the greatest divisor of 15.

If N is prime (it has no divisor other than itself and 1), the program displays:

Enter a strictly positive integer n: 11
11 is prime.

Do the exercise

Solution

Quiz

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

What does the following code display?

int x=2;
while(x<8) {
    printf ("%d ", x++);
}

Nothing

2 3 4 5 6 7

3 4 5 6 7 8

2 3 4 5 6 7 8

Check

What does the following code display?

int x=2;
while(x>8) {
    printf ("%d ", x++);
}

Nothing

2 3 4 5 6 7

3 4 5 6 7 8

2 3 4 5 6 7 8

Check

What does the following code display?

int a=0, b;
while(a<4) {
    b=0;
    while (b<2) printf ("%d", b++);
    a++;
}

0123

01234

00112233

01010101

Check

What does the following code display?

int a=0, b;
while(a<4) {
    b=0;
    while (b<a) printf ("%d", b++);
    a++;
}

001012

01010101

00101213

010120123

Check Bravo! The first loop counts from 0 to 3, so it executes 4 times the second loop which counts from 0 to a-1.

Which codes create an infinite loop?

while(0);

while(1);

while(0) {}

while(1) {}

Check

What does the following code display?

int x=0;
while (++x<5);
{
    printf ("%d", x);
}

1234

01234

12345

Check Bravo! A classic beginner's mistake: there is a semicolon at the end of the while. The instruction block is empty.

Lesson 6.1 Summary Lesson 6.3

Lesson 6.2. While loop in C

Syntax

Example

Exercise 1

Exercise 2

Exercise 3

Quiz

See also