I got this code as an exercise in my school:
#include <stdio.h>
main()
{
int unknown(int a, int b)
{
if (b == 1)
return a;
else
return a + unknown(a, b - 1);
}
printf("Value = %i", unknown(3, 4));
}
The outcome is "Value=12". I can't seem to understand why. AFAIK it should be a=3, b=4 and then 3+4, right?
The key to this is that unless b==1, unknown() calls unknown() - this is called recursion.
For brevity, I'm going to call the function f instead of unknown:
Some languages present functions like this in a clearer way with pattern matching; the equivalent in an imaginary pattern matching language might be:
f(x,1) := x
f(x,y) := x + f(x,y-1)
And so...
f(3,4) = 3 + f(3, 4-1)
= 3 + f(3, 3)
= 3 + ( 3 + f(3, 3-1))
= 3 + ( 3 + f(3, 2))
= 3 + ( 3 + ( 3 + f(3, 2 - 1)))
= 3 + ( 3 + ( 3 + f(3, 1)))
= 3 + ( 3 + ( 3 + (3)))
= 12
I guess your homework is to decide what a better name for the function is than "unknown". Once you've decided, note that recursion is not the best way to implement that function unless your language has specific support for a feature called tail recursion optimisation (this might be a topic you want to shelve for later).
Also, others have noted that nested functions are not allowed in C -- even though your particular compiler might handle them. That means that although your program does this:
int function1() {
int function2(int x) {
...
}
int x = function2(3);
}
... a standard C compiler would not allow it. The normal way is:
int function1() {
int x = function2(3);
}
int function2(int x) {
...
}
Each time a is added to a for b times
a = 3
b = 4
So, four times a gets added which means 4 * 3 = 12
Edit: a added to a, b times. -Dabo
Its the multiplication using recursive addition . Output should be 12. Finally function will return to main
3 + 3 + 3 + 3 = 12
Note: Although you used nested function here and its working, but it is not allowed by C standard. Its a compiler extension.
