GATE CS 2025 Set 2 Q.33: Data Types, Operators and Control Flow

int x=126,y=105;
do {
     if(x>y) x=x-y;
     else y=y-x;
} while(x!=y);

printf("%d",x);

The output of the given C code segment is ________. (Answer in integer)

Your answer

Official answer and solution

Solution

Recognize the loop as Euclid's subtraction algorithm for $\gcd(126, 105)$ .

The do-while loop repeatedly subtracts the smaller variable from the larger variable until x == y, which computes $\gcd(126, 105)$ :

Start: $(x, y) = (126, 105)$
Iteration 1: $x = 126 - 105 = 21 \implies (21, 105)$
Iterations 2 to 5: $y$ decreases by $21$ four times ( $105 \rightarrow 84 \rightarrow 63 \rightarrow 42 \rightarrow 21$ ).
Now $x = y = 21$ , the loop terminates, and printf outputs 21.

Final answer

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

Paper: GATE 2025 Computer Science and Information Technology - Session 2 (CS2)
Question: 33 of 65
Section: Computer Science and Information Technology

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