Tricky C++ Loops: Predict Output Questions

 C++ Loops - Tricky Predict Output Questions

1. Post-Increment in Condition

#include <iostream>

using namespace std;

int main() {

    int i = 0;

    while(i++ < 3)

        cout << i << " ";

    return 0;

}

Output:

1 2 3

Explanation:

• i++ uses old value for comparison, then increments.
• Values checked: 0,1,2 ; hence prints incremented values 1,2,3.

2. Pre-Increment in Condition

int main() {

    int i = 0;

    while(++i < 3)

        cout << i << " ";

}

Output:

1 2

Explanation:

• ++i increments first.
• Loop runs for i = 1, 2 only.

3. Missing Braces Trap

int main() {

    int i = 1;

    while(i <= 3)

        cout << i++ << " ";

        cout << "Done";

}

Output:

1 2 3 Done

Explanation:

• Only first statement belongs to while.
• cout << "Done" executes once after loop.

4. Decrement in Condition

int main() {

    int i = 3;

    while(i--)

        cout << i << " ";

}

Output:

2 1 0

Explanation:

• Condition checks old value.
• After decrement, printed values: 2,1,0.

5. Continue Before Print

int main() {

    for(int i = 1; i <= 5; i++) {

        if(i % 2 == 0)

            continue;

        cout << i << " ";

    }

}

Output:

1 3 5

Explanation: Even numbers skipped using continue.

6. Break Before Increment

int main() {

    int i = 1;

    while(i <= 5) {

        if(i == 4)

            break;

        cout << i << " ";

        i++;

    }

}

Output:

1 2 3

Explanation: Loop stops when i == 4.

7. Infinite Loop Trap

int main() {

    for(int i = 1; i < 3; )

        cout << i << " ";

}

Output:

1 1 1 1 ... (Infinite)

Explanation: i never increments.

8. Printing in For Update Section

int main() {

    for(int i = 0; i < 3; cout << i++ << " ");

}

Output:

0 1 2

Explanation: Printing happens in update section of for loop.

9. Double Increment Trap

int main() {

    int i = 1;

    do {

        cout << i++ << " ";

    } while(i++ < 4);

}

Output:

1 3

Explanation: i increments twice each iteration.

10. Continue in Do-While

int main() {

    int i = 0;

    do {

        i++;

        if(i == 2)

            continue;

        cout << i << " ";

    } while(i < 3);

}

Output:

1 3

Explanation: When i=2, continue skips printing.

11. Nested Loop with Break

int main() {

    for(int i = 1; i <= 3; i++) {

        for(int j = 1; j <= 3; j++) {

            if(j == 2)

                break;

            cout << i << j << " ";

        }

    }

}

Output:

11 21 31

Explanation: Inner loop stops when j==2.

12. Logical Trap with Semicolon

int main() {

    int i = 0;

    while(i < 3);

    {

        cout << i;

    }

}

Output:

Infinite Loop

Explanation: Semicolon ends while loop → empty infinite loop.

13. Scope of Variable

int main() {

    int i;

    for(i = 0; i < 3; i++)

        cout << i;

    cout << i;

}

Output:

0123

Explanation: After loop, i becomes 3.

14. Decrement by 2

int main() {

    for(int i = 5; i > 0; i -= 2)

        cout << i << " ";

}

Output:

5 3 1

Explanation: i decreases by 2 each iteration.

15. Nested Continue Trap

int main() {

    for(int i = 1; i <= 2; i++) {

        for(int j = 1; j <= 3; j++) {

            if(j == 2)

                continue;

            cout << i << j << " ";

        }

    }

}

Output:

11 13 21 23

Explanation: j=2 skipped for each i.

16. Logical Trap

#include <iostream>

using namespace std;

int main() {

    int i = 1;

    while(i < 4)

        cout << i++;

        cout << "Done";

    return 0;

}

Output:

123Done

Explanation: Only first cout is inside loop (no braces!).

17. For loop body

#include <iostream>

using namespace std;

int main() {

    for(int i = 0; i < 3; cout << i++ << " ");

    return 0;

}

Output:

0 1 2

Explanation: Printing happens in update section!

18. No Braces Trap

#include <iostream>

using namespace std;

int main() {

    for(int i = 1; i <= 3; i++)

        cout << i << " ";

        cout << "End";

    return 0;

}

Output:

1 2 3 End

Explanation: Only first statement is inside loop.

19. Infinite Loop Trap

#include <iostream>

using namespace std;

int main() {

    for(int i = 1; i < 5; ) {

        cout << i << " ";

    }

    return 0;

}

Output:

1 1 1 1 1 ... (Infinite Loop)

Explanation: i never changes!

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