Range of even numbers using recursion c - c

Question

I know that there are a lot of questions and answers about recursion here, but hear me out. In my program, I'm trying to get a range of only even numbers. The user enters two numbers between -5000 and 5000; if the numbers are out of order (that is, greater number is entered first) the numbers are swapped into order; and in the recursion function, a simple function return 0 or 1 determining if the current value is even or odd using n%2. The problem is that I cannot figure out how to actually work with recursion.
int recursive_function(int first_number, int second_number)
{
printf("\n Entering sum function for range %d to %d",
first_number, second_number);
if(first_number > second_number)
return 1;
else
{
if(is_even(first_number) == 0)
{
printf("\n Adding: %d", first_number);
return (first_number += recursive_function(first_number + 1, second_number));
printf("\nThis equals added");
}
else
{
printf("\n Skipping: %d", first_number);
return (first_number += recursive_function(first_number + 1, second_number));
printf("\nThis equals skipped");
}
}
}
The program works, in terms of the recursion terminating, I just do not understand how I am supposed to pop the rest of the stack. Here is a representation of what is supposed to be the output. (Assuming the values are 10 and 20)
Processing the range 10 to %20:
Entering the sum function for range 10 to 20
Adding: 10
Entering the sum function for range 11 to 20
Skipping: 11
Entering the sum function for range 12 to 20
Adding: 12
Entering the sum function for range 13 to 20
Skipping: 13
Entering the sum function for range 14 to 20
Adding: 14
Entering the sum function for range 15 to 20
Skipping: 15
Entering the sum function for range 16 to 20
Adding: 16
Entering the sum function for range 17 to 20
Skipping: 17
...
Entering the sum function for range 20 to 20
Adding: 20
Entering the sum function for range 21 to 20 (recursion stops here)
(This next part is where I don't know how this happens)
Exiting sum function for range 21 to 20 with result: 0
Exiting sum function for range 20 to 20 with result: 20
Exiting sum function for range 19 to 20 with result: 20
Exiting sum function for range 18 to 20 with result: 38
Exiting sum function for range 17 to 20 with result: 38
Exiting sum function for range 16 to 20 with result: 54
....
Exiting sum function for range 11 to 20 with result: 80
Exiting sum function for range 10 to 20 with result: 90
The sum of all even numbers in the range 10 to 20 is: 90
I know this has to do with popping all these statements "off the stack". The problem is all the research I have done gives me explanations with the assumption that one truly understands recursion whereas I am an amateur. Could any one help and explain in English?

Answer 1

To understand what is happening write down a sequence of calls on a piece of paper. Use a smaller range, say 18 to 20.
Here is an example. I omitted most of the function body leaving lines with recursive calls.
recursive_function(18, 20): <<< initial call, depth 0
printf("\n Entering
...
printf("\n Skipping: %d", 18); <<< Execution goes to this
return (18 += recursive_function(19, 20)): <<< recurses, depth 1
printf("\n Entering
...
printf("\n Adding: %d", 19);
return (19 += recursive_function(20, 20)): <<< depth 2
printf("\n Entering
...
printf("\n Skipping: %d", 20);
return (20 += recursive_function(21, 20)): <<< depth 3
printf("\n Entering
...
if(21 > 20)
return 1; <<< end of recursion
printf("\nThis equals skipped"); <<< left over at depth 2
printf("\nThis equals added"); <<< left over at depth 1
printf("\nThis equals skipped"); <<< left over at depth 0
Now you can collect all printf statements sequentially and have a resulting output. Note that in your code you skipped "Exiting..." part, while the output has it.
Popping off the stack. Popping off the stack is done for you by the compiler. It happens when function returns. You should worry about returning at a proper moment, not about how to pop off the stack.