GATE-CS-2014-(Set-1) | Question 22

Problem Statement

Consider the C code fragment given below:

void f(int n)
{
    if(n<=0) return;
    printf("%d",n);
    f(n-2);
    printf("%d",n);
}

What is the output of the following function for input 4?

void main()
{
    f(4);
}

Options

A. 41324
B. 43124
C. 41342
D. 43142

Solution

The function f(int n) prints the integers in the following order for an input of n:

1. If n is less than or equal to 0, return from the function.
2. Print n.
3. Call the function f(n-2).
4. Print n.

So, if the input is 4, then the order of printing would be:

1. 4 - Step 2
2. 2 - Step 2 (from f(2) - Step 3)
3. 2 - Step 4 (from f(2) - Step 3)
4. 4 - Step 4 (from f(2) - Step 3)
5. 1 - Step 2 (from f(1) - Step 3)
6. 1 - Step 4 (from f(1) - Step 3)
7. 3 - Step 4 (from f(3) - Step 3)
8. 3 - Step 2 (from f(3) - Step 3)
9. 4 - Step 4
10. 4 - Step 2

Therefore, the output would be 2412434.

Hence, the correct answer is option A.

Code Implementation

void f(int n)
{
    if(n<=0) return;
    printf("%d",n);
    f(n-2);
    printf("%d",n);
}

void main()
{
    f(4);
}