Loop execution flow - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested for loops with break

What is the output of the following C code?

C

#include <stdio.h>

int main() {
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            if (j == 2) {
                break;
            }
            printf("%d%d ", i, j);
        }
    }
    return 0;
}

A 11 12 21 22 31 32

B 11 21 31

C 11 12 13 21 22 23 31 32 33

D 12 22 32

💡 Hint

Remember that break exits the innermost loop immediately.

The inner loop prints values until j == 2, then breaks. So only j == 1 prints for each i. The output is 11 21 31 .

❓ Predict Output

intermediate

Output of while loop with continue

What is the output of this C program?

C

#include <stdio.h>

int main() {
    int i = 0;
    while (i < 5) {
        i++;
        if (i == 3) {
            continue;
        }
        printf("%d ", i);
    }
    return 0;
}

A 2 3 4 5

B 1 2 3 4 5

C 1 2 4

D 1 2 4 5

💡 Hint

Think about what continue does inside a loop.

When i == 3, continue skips the printf for that iteration. So 3 is not printed. The output is 1 2 4 5 .

❓ Predict Output

advanced

Output of for loop with multiple increments

What is the output of this C code?

C

#include <stdio.h>

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

A 0 2 4

B 0 1 2 3 4

C 0 1 3

D 1 3 5

💡 Hint

Notice the extra i++ inside the loop body.

The loop increments i twice each iteration: once in the for statement and once inside the loop. So i takes values 0, 2, 4 and prints them.

❓ Predict Output

advanced

Output of do-while loop with break

What is the output of this C program?

C

#include <stdio.h>

int main() {
    int i = 0;
    do {
        if (i == 2) {
            break;
        }
        printf("%d ", i);
        i++;
    } while (i < 5);
    return 0;
}

A 0 1 2

B 0 1 2 3 4

C 0 1

D 1 2

💡 Hint

Remember that break exits the loop immediately.

The loop prints i values 0 and 1. When i == 2, it breaks before printing. So output is 0 1 .

🧠 Conceptual

expert

Number of iterations in nested loops with variable increments

Consider this C code snippet:

int count = 0;
for (int i = 1; i <= 4; i += 2) {
    for (int j = 0; j < i; j++) {
        count++;
    }
}

What is the final value of count after the loops finish?

A 4

B 10

C 8

D 12

💡 Hint

Calculate how many times the inner loop runs for each i value.

The outer loop runs for i = 1 (inner loop: 1 iteration) and i = 3 (inner loop: 3 iterations). Total: 1 + 3 = 4. Final value of count is 4.

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