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What is recursion really and explain the output of this program?
What is really happening in this code?

I have a recursive function which I don’t understand. I don’t even know if the code is valid or not:

int count(int x) // suppose x is 3
{
  if(x>0)
  {
    count(--x);
    printf(x);
    count(--x);  // control reaches here or not?
  }
}

This is just pseudocode. Take the initial variable as 3. Please explain with context the concept of stacks. This code is confusing me for days, and I can’t really find the answer for this code.

Answer 1

Ok, just because I needed to wake my brain up a bit 🙂

count(3) will enter the function and call count(2) (2:nd level).
count(2) will enter the function and call count(1) (3:rd level).
count(1) will enter the function and call count(0) (4:th level).
count(0) will enter the function, but since x>0 is false, it won’t do anything and just return to 4:th level where x is still 0.

4:th level will output 0, and call count(-1) (5:th level)

count(-1) will enter the function, but since x>0 is false, it won’t do anything and just return to 4:th level where x is still -1.
4:th level returns to 3:rd level where x is still 1.

3:rd level will output 1 and call count(0) (4:th level)

count(0) will enter the function, but since x>0 is false, it won’t do anything and just return to 3:rd level where xis still 0.
3:rd level returns to 2:nd level where x is still 2.

2:nd level will output 2 and call count(1) (3:rd level)

count(1) will enter the function and call count(0) (4:th level).
count(0) will enter the function, but since x>0 is false, it won’t do anything and just return to 3:rd level where x is still 0.

3:rd level will output 0 and call count(-1) (4:th level)

count(-1) will enter the function, but since x>0 is false, it won’t do anything and just return to 3:rd level where xis still -1.

3:rd level returns to 2:nd level where x is still 1.
2:nd level returns to 1:st level and we’re done.

Output is 0 1 2 0.

I suggest that if you really want to understand this, try it yourself with count(4).