Introduction
In the previous chapter, we learned about the while loop in C. In this chapter, we will focus on the do-while loop. The do-while loop is another fundamental control flow structure that allows you to execute a block of code repeatedly based on a given condition. Unlike the while loop, the do-while loop guarantees that the loop body will be executed at least once.
What is a do-while Loop?
A do-while loop is used to repeatedly execute a block of code as long as the specified condition is true. The condition is evaluated after the execution of the loop body, ensuring that the loop body is executed at least once.
Syntax
The basic syntax of a do-while loop in C is as follows:
do {
// Code to be executed repeatedly
} while (condition);
condition: This expression is evaluated after each iteration of the loop. If it evaluates to true, the loop body is executed again. If it evaluates to false, the loop terminates.
Example: Simple do-while Loop
Let’s look at a simple example to understand how the do-while loop works.
Example: Printing Numbers from 1 to 5
#include <stdio.h>
int main() {
int i = 1; // Initializing loop counter
do {
printf("%d\n", i); // Printing the value of i
i++; // Updating the loop counter
} while (i <= 5); // Condition
return 0; // Returning 0 to indicate successful execution
}
Output:
1
2
3
4
5
Detailed Steps
- Initialization: The loop counter
iis initialized to 1. - Execution: The
printfstatement prints the value ofi. - Update: The loop counter
iis incremented by 1. - Condition: The condition
i <= 5is checked. Since it is true, the loop body is executed again. - Repeat: Steps 2-4 are repeated until the condition
i <= 5becomes false.
Infinite do-while Loop
A do-while loop can run indefinitely if the condition always evaluates to true. This is known as an infinite loop.
Example: Infinite do-while Loop
#include <stdio.h>
int main() {
int i = 1;
do {
printf("This is an infinite loop.\n");
if (i == 5) {
break; // To break the loop, you can use a break statement
}
i++;
} while (1); // Infinite loop
return 0; // Returning 0 to indicate successful execution
}
Output:
This is an infinite loop.
This is an infinite loop.
This is an infinite loop.
This is an infinite loop.
This is an infinite loop.
Using break and continue in do-while Loops
Break Statement
The break statement is used to exit the loop prematurely, regardless of the condition.
Example: Using break in do-while Loop
#include <stdio.h>
int main() {
int i = 1;
do {
if (i == 3) {
break; // Exit the loop when i is 3
}
printf("%d\n", i);
i++;
} while (i <= 5);
return 0; // Returning 0 to indicate successful execution
}
Output:
1
2
Continue Statement
The continue statement skips the current iteration of the loop and proceeds to the next iteration.
Example: Using continue in do-while Loop
#include <stdio.h>
int main() {
int i = 1;
do {
if (i == 3) {
i++;
continue; // Skip the iteration when i is 3
}
printf("%d\n", i);
i++;
} while (i <= 5);
return 0; // Returning 0 to indicate successful execution
}
Output:
1
2
4
5
Nested do-while Loops
You can use nested do-while loops to handle more complex scenarios, such as iterating over a 2D array or performing matrix operations.
Example: Nested do-while Loop
#include <stdio.h>
int main() {
int i = 1;
do {
int j = 1;
do {
printf("i = %d, j = %d\n", i, j); // Printing the values of i and j
j++;
} while (j <= 2); // Inner loop condition
i++;
} while (i <= 3); // Outer loop condition
return 0; // Returning 0 to indicate successful execution
}
Output:
i = 1, j = 1
i = 1, j = 2
i = 2, j = 1
i = 2, j = 2
i = 3, j = 1
i = 3, j = 2
Simple C Programs to Demonstrate do-while Loop
Program 1: Sum of First N Natural Numbers
Example:
#include <stdio.h>
int main() {
int n, sum = 0, i = 1;
printf("Enter a positive integer: ");
scanf("%d", &n);
do {
sum += i; // Adding i to sum
i++; // Updating loop counter
} while (i <= n);
printf("Sum of first %d natural numbers is: %d\n", n, sum);
return 0; // Returning 0 to indicate successful execution
}
Output:
Enter a positive integer: 5
Sum of first 5 natural numbers is: 15
Program 2: Factorial of a Number
Example:
#include <stdio.h>
int main() {
int n, factorial = 1, i = 1;
printf("Enter a positive integer: ");
scanf("%d", &n);
do {
factorial *= i; // Multiplying i to factorial
i++; // Updating loop counter
} while (i <= n);
printf("Factorial of %d is: %d\n", n, factorial);
return 0; // Returning 0 to indicate successful execution
}
Output:
Enter a positive integer: 5
Factorial of 5 is: 120
Program 3: Multiplication Table
Example:
#include <stdio.h>
int main() {
int n, i = 1;
printf("Enter an integer: ");
scanf("%d", &n);
do {
printf("%d x %d = %d\n", n, i, n * i); // Printing multiplication table
i++; // Updating loop counter
} while (i <= 10);
return 0; // Returning 0 to indicate successful execution
}
Output:
Enter an integer: 5
5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50
Program 4: Printing an Array
Example:
#include <stdio.h>
int main() {
int arr[] = {10, 20, 30, 40, 50};
int n = sizeof(arr) / sizeof(arr[0]);
int i = 0;
do {
printf("Element %d: %d\n", i, arr[i]); // Printing each element of the array
i++; // Updating loop counter
} while (i < n);
return 0; // Returning 0 to indicate successful execution
}
Output:
Element 0: 10
Element 1: 20
Element 2: 30
Element 3: 40
Element 4: 50
Conclusion
The do-while loop is a powerful and flexible control flow statement in C. It allows you to execute a block of code repeatedly based on a given condition, with the guarantee that the loop body will be executed at least once. By understanding and using the do-while loop, you can write more efficient and concise code for scenarios where the loop body needs to be executed at least once. You can also use nested do-while loops and control statements like break and continue to handle more complex scenarios and achieve greater control over your program’s execution flow.