Loop execution flow

Introduction

Loops help repeat actions many times without writing the same code again and again.

When you want to print numbers from 1 to 10.

When you need to check each item in a list one by one.

When you want to keep asking a user for input until they give a valid answer.

When you want to sum all values in an array.

When you want to repeat a task a fixed number of times.

Syntax

C

for (initialization; condition; update) {
    // code to repeat
}

while (condition) {
    // code to repeat
}

do {
    // code to repeat
} while (condition);

Initialization runs once at the start.

Condition is checked before each loop cycle (except in do-while).

Examples

This prints numbers 0 to 4 using a for loop.

C

for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

This prints "Hello" three times using a while loop.

C

int i = 0;
while (i < 3) {
    printf("Hello\n");
    i++;
}

This prints "World" two times using a do-while loop.

C

int i = 0;
do {
    printf("World\n");
    i++;
} while (i < 2);

Sample Program

This program shows how three types of loops work in C. It counts up with a for loop, counts down with a while loop, and shows do-while runs at least once.

C

#include <stdio.h>

int main() {
    int count = 1;
    printf("Counting from 1 to 5:\n");
    
    for (int i = 1; i <= 5; i++) {
        printf("%d\n", i);
    }

    printf("\nUsing while loop to count down from 3:\n");
    int j = 3;
    while (j > 0) {
        printf("%d\n", j);
        j--;
    }

    printf("\nUsing do-while loop to print once even if condition false:\n");
    int k = 0;
    do {
        printf("This prints once even if k is %d\n", k);
        k--;
    } while (k > 0);

    return 0;
}

Important Notes

Loops run the code inside repeatedly as long as the condition is true.

For loops are great when you know how many times to repeat.

While loops are good when you repeat until something changes.

Do-while loops always run the code at least once.

Summary

Loops repeat code to save writing the same steps many times.

For, while, and do-while loops have different ways to control repetition.

Understanding loop flow helps you control how many times code runs.

Practice

1. Which loop in C always executes its body at least once, regardless of the condition?

easy

A. do-while loop

B. for loop

C. while loop

D. if statement

Solution

1. Step 1: Understand loop types

   The do-while loop checks its condition after running the loop body once.

2. Step 2: Compare with other loops

   The for and while loops check the condition before running the body, so they may not run at all.

3. Final Answer:

   do-while loop -> Option A

4. Quick Check:

   do-while runs body once before condition check [OK]

Hint: do-while runs body first, then checks condition [OK]

Common Mistakes:

• Thinking for or while loops always run once
• Confusing if statement with loops
• Assuming do-while checks condition first

2. Which of the following is the correct syntax for a for loop in C that counts from 0 to 4?

easy

A. for (i < 5; i = 0; i++) {}

B. for i = 0 to 4 {}

C. for (int i = 0; i < 5; i++) {}

D. for (int i = 0; i <= 5; i++) {}

Solution

1. Step 1: Check for loop syntax

   The correct syntax is for (initialization; condition; increment). for (int i = 0; i < 5; i++) {} matches this.

2. Step 2: Validate loop range

   for (int i = 0; i < 5; i++) {} counts i from 0 up to but not including 5, so 0 to 4.

3. Final Answer:

   for (int i = 0; i < 5; i++) {} -> Option C

4. Quick Check:

   Standard for loop syntax with i from 0 to 4 [OK]

Hint: for loops use (init; condition; increment) syntax [OK]

Common Mistakes:

• Using 'for i = 0 to 4' which is not C syntax
• Swapping condition and initialization
• Using <= 5 to count 0 to 4 (includes 5)

3. What is the output of this C code?

int i = 0;
while (i < 3) {
    printf("%d ", i);
    i++;
}

medium

A. 0 1 2

B. 1 2 3

C. 0 1 2 3

D. 3 2 1 0

Solution

1. Step 1: Analyze loop condition and variable

   Variable i starts at 0 and increments by 1 each loop while i < 3.

2. Step 2: Trace output values

   Loop runs for i = 0, 1, 2 printing each value followed by a space.

3. Final Answer:

   0 1 2 -> Option A

4. Quick Check:

   while loop prints 0,1,2 then stops [OK]

Hint: Check loop condition carefully to count printed values [OK]

Common Mistakes:

• Including 3 in output by mistake
• Starting count from 1 instead of 0
• Printing after increment instead of before

4. Identify the error in this loop code:

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

medium

A. Incorrect printf format specifier

B. Missing increment of i inside the loop

C. Condition should be i <= 5

D. Variable i should be declared inside the loop

Solution

1. Step 1: Check loop body for changes to i

   The variable i is never incremented inside the loop, so i stays 0 forever.

2. Step 2: Understand loop consequence

   Without increment, the condition i < 5 is always true, causing an infinite loop.

3. Final Answer:

   Missing increment of i inside the loop -> Option B

4. Quick Check:

   Infinite loop due to no i++ [OK]

Hint: Always update loop variable to avoid infinite loops [OK]

Common Mistakes:

• Thinking condition is wrong instead of missing increment
• Assuming printf format is incorrect
• Declaring i inside loop is optional, not error

5. Consider this nested loop:

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= i; j++) {
        printf("%d", j);
    }
    printf("\n");
}

What is the output?

hard

A. 1
22
333

B. 1
2
3

C. 123
123
123

D. 1
12
123

Solution

1. Step 1: Understand outer loop iterations

   The outer loop runs i from 1 to 3, inclusive, so 3 times.

2. Step 2: Trace inner loop for each i

   For each i, inner loop runs j from 1 to i, printing j values without spaces.

3. Step 3: Combine outputs

   Lines printed are:
   i=1: "1"
   i=2: "12"
   i=3: "123"
   Each followed by newline.

4. Final Answer:

   1 12 123 -> Option D

5. Quick Check:

   Nested loops print increasing sequences per line [OK]

Hint: Inner loop runs up to outer loop counter each time [OK]

Common Mistakes:

• Confusing inner loop range with outer loop
• Printing j values incorrectly
• Missing newline after each inner loop