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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* * * * *

C++ Loops: Predict Output Questions (For Beginners)

C++ Looping Statements

Predict Output Questions For Beginners

(while, do-while and for loop)

Part A: While Loop Predict Output Questions

Q1

#include <iostream>

using namespace std;

int main() {

    int i = 1;

    while(i <= 5) {

        cout << i << " ";

        i++;

    }

    return 0;

}

Output:

1 2 3 4 5

Q2

#include <iostream>

using namespace std;

int main() {

    int i = 5;

    while(i > 0) {

        cout << i << " ";

        i -= 2;

    }

    return 0;

}

Output:

5 3 1

Q3

#include <iostream>

using namespace std;

int main() {

    int i = 1, sum = 0;

    while(i <= 4) {

        sum += i;

        i++;

    }

    cout << sum;

    return 0;

}

Output:

10

Q4

#include <iostream>

using namespace std;

int main() {

    int i = 0;

    while(i < 3) {

        cout << i;

        i++;

    }

    cout << i;

    return 0;

}

Output:

0123

Q5

#include <iostream>

using namespace std;

int main() {

    int i = 1;

    while(i < 10) {

        cout << i << " ";

        i *= 2;

    }

    return 0;

}

Output:

1 2 4 8

Part B: Do-While Loop Predict Output

Q6

#include <iostream>

using namespace std;

int main() {

    int i = 1;

    do {

        cout << i << " ";

        i++;

    } while(i <= 3);

    return 0;

}

Output:

1 2 3

Q7

#include <iostream>

using namespace std;

int main() {

    int i = 5;

    do {

        cout << i << " ";

        i--;

    } while(i > 2);

    return 0;

}

Output:

5 4 3

Q8

#include <iostream>

using namespace std;

int main() {

    int i = 0;

    do {

        cout << i;

        i++;

    } while(i < 0);

    return 0;

}

Output:

0

Q9

#include <iostream>

using namespace std;

int main() {

    int i = 1, product = 1;

    do {

        product *= i;

        i++;

    } while(i <= 4);

    cout << product;

    return 0;

}

Output:

24

Q10

#include <iostream>

using namespace std;

int main() {

    int i = 2;

    do {

        cout << i << " ";

        i += 3;

    } while(i < 10);

    return 0;

}

Output:

2 5 8

Part C: For Loop Predict Output Questions

Q11

#include <iostream>

using namespace std;

int main() {

    for(int i = 1; i <= 4; i++)

        cout << i * 2 << " ";

    return 0;

}

Output:

2 4 6 8

Q12

#include <iostream>

using namespace std;

int main() {

    int sum = 0;

    for(int i = 1; i <= 3; i++)

        sum += i;

    cout << sum;

    return 0;

}

Output:

6

Q13

#include <iostream>

using namespace std;

int main() {

    for(int i = 5; i > 0; i -= 2)

        cout << i << " ";

    return 0;

}

Output:

5 3 1

Q14

#include <iostream>

using namespace std;

int main() {

    int i;

    for(i = 0; i < 3; i++)

        cout << i;

    cout << i;

    return 0;

}

Output:

0123

Q15

#include <iostream>

using namespace std;

int main() {

    for(int i = 1; i <= 3; i++) {

        if(i == 2)

            continue;

        cout << i << " ";

    }

    return 0;

}

Output:

1 3

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* * * * *

C++ Tricky Decision Making: Predict Output Questions (Logical Errors)

C++ Decision-making Statements

Tricky Predict Output Questions (Logical Errors)

(if, if-else, nested if, else if ladder, switch, etc.)

Answers are given at the bottom of this page.

Q1 - Assignment Instead of Comparison

#include <iostream>

using namespace std;

int main() {

    int x = 5;

    if (x = 0)

        cout << "True";

    else

        cout << "False";

    return 0;

}

Q2 - Missing Braces

#include <iostream>

using namespace std;

int main() {

    int x = 10;

    if (x > 5)

        cout << "A";

        cout << "B";

    return 0;

}

Q3 - Dangling Else Problem

#include <iostream>

using namespace std;

int main() {

    int x = 5;

    if (x > 2)

        if (x < 4)

            cout << "A";

        else

            cout << "B";

    return 0;

}

Q4 - Switch Without Break

#include <iostream>

using namespace std;

int main() {

    int x = 2;

    switch(x) {

        case 1: cout << "One";

        case 2: cout << "Two";

        case 3: cout << "Three";

        default: cout << "Default";

    }

    return 0;

}

Q5 - Logical AND Mistake

#include <iostream>

using namespace std;

int main() {

    int x = 5;

    if (x > 2 && x < 5)

        cout << "Yes";

    else

        cout << "No";

    return 0;

}

Q6 - Semicolon After If

#include <iostream>

using namespace std;

int main() {

    int x = 10;

    if (x > 5);

        cout << "Hello";

    return 0;

}

Q7 - Character Comparison

#include <iostream>

using namespace std;

int main() {

    char ch = 'A';

    if (ch == 65)

        cout << "Match";

    else

        cout << "No Match";

    return 0;

}

Q8 - Multiple If Instead of Else If

#include <iostream>

using namespace std;

int main() {

    int marks = 85;

    if (marks > 50)

        cout << "Pass ";

    if (marks > 75)

        cout << "Distinction";

    return 0;

}

Q9 - Not Equal Confusion

#include <iostream>

using namespace std;

int main() {

    int x = 5;

    if (!x == 5)

        cout << "True";

    else

        cout << "False";

    return 0;

}

Q10 - Boolean Value Check

#include <iostream>

using namespace std;

int main() {

    bool flag = 2;

    if (flag)

        cout << "ON";

    else

        cout << "OFF";

    return 0;

}

* * * * *

Answer Key (Tricky Outputs)

Que.| Output    | Explanation

Q1   False      x = 0 assigns 0 → condition false

Q2   AB         Only first cout inside if; second always executes

Q3   B          else belongs to inner if

Q4   TwoThreeDefault        No break, so fall-through occurs

Q5   No         x < 5 is false (5 is not less than 5)

Q6   Hello      Semicolon ends if; cout always executes

Q7   Match      ASCII value of 'A' is 65

Q8   Pass Distinction   Both if conditions are true

Q9   False      !x → !5 → false (0); 0 == 5 false

Q10  ON         Any non-zero value converts to true

C++ Decision Making Statements: Predict Output Questions (For Beginners)

C++ Decision-making Statements

Predict Output Questions For Beginners 

(if, if-else, nested if, else if ladder, switch, etc.)