Recursively return if statement calls - c++

Question

I'm trying to design a program that takes an integer array as input, and then returns all combinations of values that add up to a predetermined sum. For the sake of clarity, my recursive function will return true when the total adds up to 10.
However, I also want it to return the values from the array that comprise of this total, so my definition is as follows;
If suminarray returns true, print each number from the array.
My hope was, once my base clause is reached, the recursion would unwind, and my if statements would all be evaluated, and each value would be printed from my if statement. However, all that is printed is last value from the array which made up the target total, not all the values that preceded it.
I've likely misunderstood the recursive behaviour of C++. I know how to work with recursive return calls, but logically, if the if statement can't be evaluated until the recursive function returns true or false, shouldn't they unwind, also?
#include <iostream>
bool suminarray(int *numbers, const int &size, int startPos, int total);
using namespace std;
int main()
{
int numbers[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
int startPos = 0;
int total = 0;
suminarray(numbers, 10, 0, total);
return 0;
}
bool suminarray(int *numbers, const int &size, int startPos, int total)
{
if(total == 10)
{
cout << "result. " << endl;
return true;
}
else if(total > 10)
{
return false;
}
else
{
for(int i = startPos; i < size; i++)
{
cout << " loop " << i << endl;
cout << " total" << total << endl;
if(suminarray(numbers, size, i+1, total+numbers[i]) == true)
{
cout << "Uses " << numbers[i] << endl;
}
}
}
}
Edit: correction to source code.

Answer 1

The immediate problem (which your compiler should be warning you about), is that you have no return statement in the final else block, which causes the function to fall off the end without returning either true or false, leading to undefined behavior. If you fix that in the most obvious way:
else
{
for(int i = startPos; i < size; i++)
{
cout << " loop " << i << endl;
cout << " total" << total << endl;
if(suminarray(numbers, size, i, total+numbers[i]) == true)
{
cout << "Uses " << numbers[i] << endl;
return true;
}
}
return false;
}
your program then works, but it only prints the FIRST set of values that add up to 10 that it finds.
That immediately shows you the problem with your approach -- each function call can only return ONCE -- you can't (easily) have it both return success AND continue to try more alternatives.

Answer 2

As you've already guessed, the way you're imagining the recursive function unwinding is wrong.
In your example, you want to get to a total of 10, with numbers starting at 1, and ending at 9. So, first your recursive function will make the total = 1. Then it will add another 1 to that until it gets all the way down to the tenth 1. Then it will print result, then it will unwind 1 step, and print 1.
Good so far right? Well, here's where it goes off track. At this point it doesn't unwind all the way. It still stays at 9, but this time it adds a 2 to that, fails goes back until it unwinds to a sum of 8. Now it tries 2 and works this time! Printing result, then 2.
This, as i've understood, isn't what you want. What you should be doing is make something else to hold your array of answers, and not just print them.

Answer 3

I know how to work with recursive return calls, but logically, if the if statement can't be evaluated until the recursive function returns true or false, shouldn't they unwind, also?
This is correct. The reason you are only seeing one print statement at the end is because only one call is returning 'true'. Therefore the 'if' statement is only true once and you only see one print statement for "Uses ".
I believe your mistake is that you didn't add a return statement after your for loop. This means the return value for your function is actually undefined. Start using the -Wall flag when you compile to make sure you don't make this mistake. Here is a version where I added a "return true;" after the 'for' loop:
#include <iostream>
bool suminarray(int *numbers, const int &size, int startPos, int total);
using namespace std;
int main()
{
int numbers[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
int startPos = 0;
int total = 0;
suminarray(numbers, 10, startPos, total);
return 0;
}
bool suminarray(int *numbers, const int &size, int startPos, int total)
{
if(total == 10)
{
cout << "result. " << endl;
return true;
}
else if(total > 10)
{
return false;
}
else
{
for(int i = startPos; i < size; i++)
{
cout << " loop " << i << endl;
cout << " total" << total << endl;
if(suminarray(numbers, size, i, total+numbers[i]) == true)
{
cout << "Uses " << numbers[i] << endl;
}
}
}
return true;
}