C Programming Day 5
Introduction
Welcome to Day 5 of the C Programming Full Course.
In the previous lesson, we learned how to make decisions using conditional statements such as if, if-else, nested if, else-if, and switch.
But what happens when we need to execute the same block of code repeatedly?
For example:
- Print numbers from 1 to 100
- Display a multiplication table
- Calculate the sum of numbers
- Process multiple student records
- Generate patterns
- Repeat a menu until the user exits
- Read multiple values from the user
Writing the same statement hundreds of times would be inefficient.
This is where loops in C become extremely useful.
A loop allows a program to execute a block of code repeatedly as long as a specified condition is satisfied.
In this lesson, you'll learn:
- What loops are
- Why loops are important
- for loop
- while loop
- do-while loop
- Nested loops
- break statement
- continue statement
- Infinite loops
- Practical programs
- Common mistakes
- Interview questions
- Practice exercises
What is a Loop in C?
A loop is a programming structure that repeatedly executes a block of statements while a condition is true.
Simple Example
Suppose you want to print:
five times.
Without a loop:
printf("Hello C Programming\n");
printf("Hello C Programming\n");
printf("Hello C Programming\n");
With a loop:
{
printf("Hello C Programming\n");
}
The loop makes the program shorter, cleaner, and easier to maintain.
Why Are Loops Important in C?
Loops are one of the most important concepts in programming.
They are used for:
- Repeating calculations
- Processing arrays
- Reading multiple inputs
- Searching data
- Generating patterns
- Creating menus
- Mathematical calculations
- File processing
- Game development
- Data processing
- Algorithm implementation
Almost every significant C program uses loops in some form.
Types of Loops in C
C provides three primary looping statements:
Loop Description
for Best when the number of iterations is known
while Best when repetition depends on a condition
do-while Executes the loop body at least once
The basic structure is:
↓
Check Condition
↙ ↘
True False
↓ ↓
Execute Code End
↓
Update
↓
Check Again
for Loop in C
The for loop is commonly used when you know approximately how many times you want to execute a block of code.
Syntax
{
// statements
}
It has three main parts:
Initialization
Runs once before the loop begins.
Condition
Checked before each iteration.
Update
Changes the loop variable.
Example 1: Print Numbers from 1 to 10
#include <stdio.h>
{
int i;
for(i = 1; i <= 10; i++)
{
printf("%d\n", i);
}
return 0;
}
Output
2
3
4
5
6
7
8
9
10
How the for Loop Works
The execution happens in this order:
↓
i <= 10?
↓
Print i
↓
i++
↓
Check condition again
When i becomes 11, the condition becomes false and the loop stops.
Example 2: Print Even Numbers
#include <stdio.h>
{
int i;
for(i = 2; i <= 20; i += 2)
{
printf("%d ", i);
}
return 0;
}
Output
Example 3: Print Odd Numbers
#include <stdio.h>
{
int i;
for(i = 1; i <= 20; i += 2)
{
printf("%d ", i);
}
return 0;
}
Example 4: Countdown
#include <stdio.h>
{
int i;
for(i = 10; i >= 1; i--)
{
printf("%d\n", i);
}
printf("Start!");
return 0;
}
Example 5: Multiplication Table
#include <stdio.h>
{
int n, i;
printf("Enter a number: ");
scanf("%d", &n);
for(i = 1; i <= 10; i++)
{
printf("%d x %d = %d\n", n, i, n * i);
}
return 0;
}
If the user enters 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
while Loop in C
The while loop executes a block of code as long as a condition is true.
Syntax
{
// statements
}
The condition is checked before the loop body executes.
Therefore, a while loop is called a pre-test loop.
Example 1: Print Numbers from 1 to 10
#include <stdio.h>
{
int i = 1;
while(i <= 10)
{
printf("%d\n", i);
i++;
}
return 0;
}
How the while Loop Works
↓
Check condition
↓
Condition true?
YES
↓
Execute statements
↓
Update i
↓
Check condition again
NO
↓
End
Example 2: Sum of Numbers
#include <stdio.h>
{
int i = 1;
int sum = 0;
while(i <= 10)
{
sum = sum + i;
i++;
}
printf("Sum = %d", sum);
return 0;
}
Output
Example 3: User-Controlled Loop
#include <stdio.h>
{
int number;
printf("Enter a number: ");
scanf("%d", &number);
while(number != 0)
{
printf("You entered: %d\n", number);
printf("Enter another number (0 to stop): ");
scanf("%d", &number);
}
printf("Program ended.");
return 0;
}
This type of loop is useful when you don't know beforehand how many times the loop needs to run.
do-while Loop in C
The do-while loop is similar to the while loop, but there is one important difference.
The body of a do-while loop executes at least once, even if the condition is initially false.
Syntax
{
// statements
}
while(condition);
Notice the semicolon after the condition:
Example
#include <stdio.h>
{
int i = 1;
do
{
printf("%d\n", i);
i++;
}
while(i <= 10);
return 0;
}
Important Difference Between for, while & do-while
Consider:
while(i <= 10)
{
printf("%d", i);
}
The body executes zero times.
But:
do
{
printf("%d", i);
}
while(i <= 10);
The body executes one time.
Output:
That's the key difference between while and do-while.
Example: Menu-Driven Program
A do-while loop is very useful for menus.
#include <stdio.h>
{
int choice;
do
{
printf("\n===== MENU =====\n");
printf("1. Add\n");
printf("2. Subtract\n");
printf("3. Exit\n");
printf("Enter choice: ");
scanf("%d", &choice);
switch(choice)
{
case 1:
printf("Addition selected.");
break;
case 2:
printf("Subtraction selected.");
break;
case 3:
printf("Exiting...");
break;
default:
printf("Invalid choice.");
}
}
while(choice != 3);
return 0;
}
This is a practical example of combining:
- do-while
- switch
- break
- User input
for vs while vs do-while
Condition Before execution Before execution After execution
Minimum executions 0 0 1
Best use Known iterations Unknown iterations Menu/input systems
Initialization Usually inside loop Usually before loop Usually before loop
Update Usually inside loop expression Inside body Inside body
When Should You Use Each Loop?
Use for when:
You know how many times the loop should run.
Example:
Use while when:
The number of iterations depends on a condition.
Example:
Use do-while when:
The code needs to execute at least once.
Example:
{
displayMenu();
}
while(choice != 0);
Nested Loops
A loop inside another loop is called a nested loop.
Example:
{
for(j = 1; j <= 3; j++)
{
printf("* ");
}
printf("\n");
}
Output
* * *
* * *
The outer loop controls the rows, while the inner loop controls the columns.
Example: Multiplication Table Grid
#include <stdio.h>
{
int i, j;
for(i = 1; i <= 5; i++)
{
for(j = 1; j <= 5; j++)
{
printf("%d\t", i * j);
}
printf("\n");
}
return 0;
}
Example: Star Pattern
#include <stdio.h>
{
int i, j;
for(i = 1; i <= 5; i++)
{
for(j = 1; j <= i; j++)
{
printf("* ");
}
printf("\n");
}
return 0;
}
Output
* *
* * *
* * * *
* * * * *
break Statement
The break statement immediately terminates a loop.
Example
#include <stdio.h>
{
int i;
for(i = 1; i <= 10; i++)
{
if(i == 6)
{
break;
}
printf("%d ", i);
}
return 0;
}
Output
When i becomes 6, break terminates the loop.
continue Statement
The continue statement skips the remaining statements in the current iteration and moves to the next iteration.
Example
#include <stdio.h>
{
int i;
for(i = 1; i <= 10; i++)
{
if(i == 5)
{
continue;
}
printf("%d ", i);
}
return 0;
}
Output
The number 5 is skipped.
break vs continue
break continue
Terminates the loop Skips current iteration
Exits the loop completely Continues with next iteration
Used to stop execution Used to skip specific cases
Infinite Loop
An infinite loop is a loop that never ends because its condition always remains true.
Example:
{
printf("Hello\n");
}
Another example:
for(;;)
{
printf("Running...");
}
Infinite loops can be useful in certain applications, such as servers and embedded systems, but they can also be caused accidentally.
Common Cause
while(i <= 10)
{
printf("%d", i);
}
The value of i never changes.
Therefore, the condition remains true forever.
Correct version:
while(i <= 10)
{
printf("%d", i);
i++;
}
Real-World Programs Using Loops
Program 1: Calculate Factorial
The factorial of 5 is:
C Program
#include <stdio.h>
{
int n, i;
long long factorial = 1;
printf("Enter a number: ");
scanf("%d", &n);
for(i = 1; i <= n; i++)
{
factorial = factorial * i;
}
printf("Factorial = %lld", factorial);
return 0;
}
Program 2: Reverse a Number
#include <stdio.h>
{
int number;
int reverse = 0;
int remainder;
printf("Enter a number: ");
scanf("%d", &number);
while(number != 0)
{
remainder = number % 10;
reverse = reverse * 10 + remainder;
number = number / 10;
}
printf("Reverse = %d", reverse);
return 0;
}
Program 3: Count Digits
#include <stdio.h>
{
int number;
int count = 0;
printf("Enter a number: ");
scanf("%d", &number);
while(number != 0)
{
number = number / 10;
count++;
}
printf("Number of digits = %d", count);
return 0;
}
Program 4: Sum of Digits
#include <stdio.h>
{
int number;
int sum = 0;
int digit;
printf("Enter a number: ");
scanf("%d", &number);
while(number != 0)
{
digit = number % 10;
sum = sum + digit;
number = number / 10;
}
printf("Sum of digits = %d", sum);
return 0;
}
Program 5: Check Prime Number
#include <stdio.h>
{
int n, i;
int isPrime = 1;
printf("Enter a number: ");
scanf("%d", &n);
if(n <= 1)
{
isPrime = 0;
}
else
{
for(i = 2; i * i <= n; i++)
{
if(n % i == 0)
{
isPrime = 0;
break;
}
}
}
if(isPrime)
printf("Prime Number");
else
printf("Not a Prime Number");
return 0;
}
Program 6: Fibonacci Series
The Fibonacci sequence begins:
C Program
#include <stdio.h>
{
int n;
int a = 0, b = 1, next;
int i;
printf("Enter number of terms: ");
scanf("%d", &n);
for(i = 1; i <= n; i++)
{
printf("%d ", a);
next = a + b;
a = b;
b = next;
}
return 0;
}
Program 7: Password Retry System
#include <stdio.h>
{
int password;
int attempts = 0;
while(attempts < 3)
{
printf("Enter password: ");
scanf("%d", &password);
if(password == 1234)
{
printf("Login Successful!");
break;
}
attempts++;
printf("Incorrect password.\n");
}
if(attempts == 3)
{
printf("Account temporarily locked.");
}
return 0;
}
This demonstrates how loops can be used in a simple authentication workflow.
Common Mistakes When Using Loops
1. Forgetting to Update the Loop Variable
Incorrect:
while(i <= 10)
{
printf("%d", i);
}
This creates an infinite loop.
Correct:
int i = 1;
while(i <= 10)
{
printf("%d", i);
i++;
}
2. Using the Wrong Condition
Incorrect:
This condition is false immediately.
Correct:
3. Off-by-One Errors
Be careful with:
and:
The first runs from 0 through 9 if starting at 0, while the second includes 10.
4. Forgetting the Semicolon in do-while
Correct:
{
printf("Hello");
}
while(condition);
The semicolon after while(condition) is required.
Best Practices for Loops
Follow these practices when writing loops:
1. Choose the appropriate loop
Use for, while, or do-while based on the problem.
2. Keep the loop condition clear
Avoid unnecessarily complicated conditions.
3. Update the loop variable correctly
Make sure the loop can eventually terminate.
4. Avoid unnecessary nested loops
Nested loops can increase execution time significantly.
5. Use meaningful variable names
Instead of:
consider:
when appropriate.
6. Test boundary cases
Test values such as:
1
maximum expected value
negative values
for, while and do-while: Quick Revision
↓
Known number of repetitions
WHILE
↓
Condition-controlled repetition
DO-WHILE
↓
Execute at least once
Remember:
for = known iterations
while = condition first
do-while = execute first
Interview Questions on Loops in C
1. What is a loop in C?
A loop repeatedly executes a block of statements while a specified condition is satisfied.
2. How many basic types of loops are available in C?
There are three:
- for
- while
- do-while
3. Which loop executes at least once?
The do-while loop.
4. What is a nested loop?
A loop placed inside another loop is called a nested loop.
5. What does break do?
It immediately terminates the nearest enclosing loop or switch.
6. What does continue do?
It skips the remaining statements of the current iteration and proceeds to the next iteration.
7. What is an infinite loop?
A loop that continues indefinitely because its termination condition never becomes false.
8. Which loop should I use when the number of iterations is known?
Generally, use a for loop.
Practice Problems
Try solving these programs yourself without looking at the solutions.
Beginner
- Print numbers from 1 to 100.
- Print numbers from 100 to 1.
- Print all even numbers between 1 and 100.
- Print all odd numbers between 1 and 100.
- Print a multiplication table.
- Calculate the sum of numbers from 1 to N.
- Calculate the factorial of a number.
Intermediate
- Reverse a number.
- Count the digits of a number.
- Find the sum of digits.
- Check whether a number is palindrome.
- Check whether a number is prime.
- Print prime numbers between 1 and 100.
- Generate the Fibonacci series.
- Find the greatest common divisor of two numbers.
Advanced
- Create a menu-driven calculator.
- Create an ATM menu using do-while and switch.
- Create a password retry system.
- Print different star patterns using nested loops.
- Create a number guessing game using loops.
Summary
In this lesson, you learned one of the most important concepts in C programming: loops.
You learned how to:
- Repeat code using for
- Repeat code using while
- Guarantee at least one execution with do-while
- Create nested loops
- Stop loops using break
- Skip iterations using continue
- Understand infinite loops
- Build practical programs using loops
- Generate number and star patterns
Loops are fundamental to programming because they allow us to process large amounts of data and repeat operations efficiently.
In the next lesson, we'll move to Arrays in C, where you'll learn how to store and process multiple values using a single variable structure.
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