Loop execution flow - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 1 to 5 using a for loop.

C

for(int i = 1; i [1] 5; i++) {
    printf("%d\n", i);
}

Drag options to blanks, or click blank then click option'

A>

B<

C<=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will stop the loop before printing 5.

Using '>' or '>=' will cause the loop to never run.

The loop should continue as long as i is less than or equal to 5 to print numbers 1 through 5.

2fill in blank

medium

Complete the code to sum numbers from 1 to 10 using a while loop.

C

int sum = 0;
int i = 1;
while(i [1] 10) {
    sum += i;
    i++;
}

Drag options to blanks, or click blank then click option'

A>

B<=

C>=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will exclude 10 from the sum.

Using '>' or '>=' will cause the loop to never run.

The loop should run while i is less than or equal to 10 to include 10 in the sum.

3fill in blank

hard

Fix the error in the for loop condition to print even numbers from 2 to 10.

C

for(int num = 2; num [1] 10; num += 2) {
    printf("%d\n", num);
}

Drag options to blanks, or click blank then click option'

A<=

B>=

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will exclude 10 from printing.

Using '>' or '>=' will cause the loop to never run.

The loop should continue while num is less than or equal to 10 to include 10.

4fill in blank

hard

Fill both blanks to create a for loop that prints numbers from 10 down to 1.

C

for(int i = [1]; i [2] 1; i--) {
    printf("%d\n", i);
}

Drag options to blanks, or click blank then click option'

A10

B<=

C>=

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at 1 instead of 10.

Using '<=' instead of '>=' in the condition.

The loop starts at 10 and continues while i is greater than or equal to 1, decreasing by 1 each time.

5fill in blank

hard

Fill all three blanks to create a for loop that prints squares of numbers from 1 to 5.

C

for(int [1] = 1; [2] [3] 5; [1]++) {
    printf("%d\n", [1] * [1]);
}

Drag options to blanks, or click blank then click option'

Ai

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in initialization and condition.

Using '<' instead of '<=' to include 5.

The loop variable is i. It starts at 1 and runs while i is less than or equal to 5.

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