Magic in C recursion? who can explain? - c

Question

Can somebody explain to me how this recursion works?
I'm getting stuck on how the result is 20 when the recursion leaves only on if a == b and it's a = 6 and b = 6
int main()
{
printf("%d \n",mistero(2, 6));
return 0;
}
mistero( a, b)
{
if( a == b ){
return a;
}
return a + mistero(a+1, b);
}

Answer 1

mistero(2, 6) =
2 + mistero(3, 6) =
2 + 3 + mistero(4, 6) =
2 + 3 + 4 + mistero(5, 6) =
2 + 3 + 4 + 5 + mistero(6, 6) =
2 + 3 + 4 + 5 + 6
= 20

Answer 2

Recursion works as below:
2 + mistero(3,6);
2 + 3 + mistero(4,6)
2 + 3 + 4 + mistero(5,6)
2 + 3 + 4 + 5 + mistero(6,6)
2 + 3 + 4 + 5 + 6 = 20

Answer 3

Just execute the code step by step and replace the function call by the return value.
When the function is first called :
return 2 + mistero(3,6);
mistero(3,6) returns 3 + mistero(4,6)
so the return statement eventually continues to get added till a==b
return 2+3+4+5+6

Answer 4

First your function definition has to be before your main, so that at compile time it is known in the main function otherwise you have to use a function prototype!
Also you forgot to define the return type and the types of the arguments. So your code should looks like this to work:
#include <stdio.h>
int mistero(int a, int b) {
//^ ^^^----^^^ variable type
//| Here return type
if( a == b ) {
return a;
}
return a + mistero(a+1, b);
}
int main() {
printf("%d \n",mistero(2, 6));
return 0;
}
Or with a function prototype:
#include <stdio.h>
int mistero(int a,int b);
int main() {
printf("%d \n",mistero(2, 6));
return 0;
}
int mistero(int a,int b) {
if( a == b ){
return a;
}
return a + mistero(a+1, b);
}
And this should show how it works:
return 2 + mistero(2 + 1, b);
return 2 + 3 + mistero(3 + 1, b);
return 2 + 3 + 4 + mistero(4 + 1, b);
return 2 + 3 + 4 + 5 + mistero(5 + 1, b);
return 2 + 3 + 4 + 5 + 6;
= 20

Answer 5

I am guessing the confusing aspect to this is the apparent lack of some sort of loop statement. I.e for or do while statement.
Think of it this way at the very end of the program.
return A + MISTERO(A+1, B) /* IS THE LOOP STATEMENT */
It causes the program to call the function MISTERO (A , B) from within the function MISTERO (A, B). So as long as this occurs you get a round about way of a loop. Until the loop is satisfied in the statement test if(A == B). Then the loop is escaped be a simple return A.
Thanks enjoy programming!!!