GATE 2013 COMPUTER SCIENCE & INFORMATION TECH. – CS – C Programming | Q. 42

int f(int &x, int c) {
    c = c - 1;
    if (c == 0) return 1;
    x = x + 1;
    return f(x, c) * x;
}

int f(int &x, int c) {
    c = c - 1;
    if (c == 0) return 1;
    x = x + 1;
    return f(x, c) * x;
}

int p = 5;
f(p, p);

c = 5 - 1
c = 4

x = x + 1
x = 6

p = 6

return f(x, 4) * x;

p = 6

c = 3;
x = x + 1;

p = 7

f(x, 3) * x

c = 2;
x = x + 1;

p = 8

f(x, 2) * x

c = 1;
x = x + 1;

p = 9

f(x, 1) * x

c = 0;

return 1;

1 × 9 = 9

9 × 9 = 81

81 × 9 = 729

729 × 9 = 6561

🟢 Call by Value – Give a Copy

Imagine a teacher gives a student a photocopy of the marks record. The student changes the photocopy from 80 to 100.

The teacher’s original record is still 80.

Simple rule:
“I give you a copy. You can change your copy, but my original does not change.”

🔵 Call by Reference – Access the Original

Now imagine that the teacher gives the student access to the original marks record. If the student changes the marks from 80 to 100, the original record also becomes 100.

Simple rule:
“I give you access to the original. If you change it, my original changes too.”