GATE CS 2017 Set 2 Q.43 (CS Q.43): Pointers, Arrays and Strings

Consider the following snippet of a C program. Assume that swap(&x, &y) exchanges the contents of x and y.

int main() {
    int array[] = {3, 5, 1, 4, 6, 2};
    int done = 0;
    int i;

    while (done == 0) {
        done = 1;
        for (i=0; i<=4; i++) {
            if (array[i] < array[i+1]) {
                swap(&array[i], &array[i+1]);
                done = 0;
            }
        }
        for (i=5; i>=1; i--) {
            if (array[i] > array[i-1]) {
                swap(&array[i], &array[i-1]);
                done = 0;
            }
        }
    }
    printf("%d", array[3]);
}

The output of the program is

Your answer

Official answer and solution

Official answer: 3
From: Official GATE 2017 answer key

Solution

Trace the execution of the bidirectional bubble sort (cocktail shaker sort) on the given integer array.

The array initially is array = {3, 5, 1, 4, 6, 2}. Notice the conditions in the two loops:

Forward loop: if (array[i] < array[i+1]) swap(...) sorts elements in descending order (largest first).
Backward loop: if (array[i] > array[i-1]) swap(...) also bubbles larger elements toward earlier indices.

Trace iteration by iteration:

Pass 1 - Forward (i from 0 to 4):
- i=0: array[0]=3 < array[1]=5 $\implies$ swap $\to$ {5, 3, 1, 4, 6, 2}
- i=1: array[1]=3 > array[2]=1 (no swap)
- i=2: array[2]=1 < array[3]=4 $\implies$ swap $\to$ {5, 3, 4, 1, 6, 2}
- i=3: array[3]=1 < array[4]=6 $\implies$ swap $\to$ {5, 3, 4, 6, 1, 2}
- i=4: array[4]=1 < array[5]=2 $\implies$ swap $\to$ {5, 3, 4, 6, 2, 1}
Pass 1 - Backward (i from 5 down to 1):
- i=5: array[5]=1 < array[4]=2 (no swap)
- i=4: array[4]=2 < array[3]=6 (no swap)
- i=3: array[3]=6 > array[2]=4 $\implies$ swap $\to$ {5, 3, 6, 4, 2, 1}
- i=2: array[2]=6 > array[1]=3 $\implies$ swap $\to$ {5, 6, 3, 4, 2, 1}
- i=1: array[1]=6 > array[0]=5 $\implies$ swap $\to$ {6, 5, 3, 4, 2, 1}
Pass 2 - Forward (i from 0 to 4):
- Swaps 3 and 4 $\to$ {6, 5, 4, 3, 2, 1}
Pass 2 - Backward (i from 5 down to 1):
- No elements out of order $\to$ {6, 5, 4, 3, 2, 1}
Pass 3: Neither loop makes a swap, done remains 1, while loop terminates.

The final array sorted in descending order is:
array = {6, 5, 4, 3, 2, 1}
Index 3 contains:
array[3] = 3

Final answer

Common trap

Assuming standard ascending sort order; the comparison array[i] < array[i+1] produces descending order.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2017 Computer Science and Information Technology (CS) Session 2
Question: 43 of 65, printed as CS Q.43
Section: Computer Science and Information Technology

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