GATE CSE 2025 Session 2 Q33: Output of a subtraction loop on 126 and 105

Consider the following C code segment:

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)

Try it: numerical answer: type the value

Your answer

Numerical answer. No negative marking.

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Answer: 21 (official GATE 2025 final answer key)

Concept used: Repeated subtraction is Euclid's algorithm for the GCD.

First step: The loop stops when $x = y = \gcd(126, 105)$ ; factor $126 = 2 \cdot 3^2 \cdot 7$ and $105 = 3 \cdot 5 \cdot 7$ .

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Source: official GATE 2025 Session 2 CS question paper and final answer key, organised by IIT Roorkee. Question paper · Answer key