Function that counts arbitrary function 1 to 10 in C - c

Question

How do I write a function that have a input function (is objective to any function), array of input numbers and length of input array?
Function:
double accumulator(double (*function)(double, double), double array[], int length)
Main:
int main(){
double array[10];
for (int i=0; i<10; i++)
array[i] = i+1;
printf("Sum is: %g\n", accumulator(sum,array,10));
printf("Product is: %g\n", accumulator(product,array,10));
return 0;
}
For example sum should be 55 (1 + 2 + .... + 10) and product 362880 (1 * 2 * ... * 10).
I guess the function should by recursive but I still cant get the right results :/
I have got this non-recursive solution but it of course works only for sum...
double accumulator(double (*function)(double, double), double array[], int length)
{
int temp = 0;
for (int i = 0;i<length;i++)
{
temp = (*function)(temp, array[i]);
}
return temp;
}
on the top of course:
double sum(double x, double y){
return x+y;
}
double product(double x, double y){
return x*y;
}

Answer 1

What is wrong with:
double multiplicator(double (*function)(double, double), double array[], int length)
{
int temp = 1;
for (int i = 0;i<length;i++)
{
temp = (*function)(temp, array[i]);
}
return temp;
}
Either a different function or you need to supply the neutral element for the operation (0 for sum, 1 for product).

Answer 2

It doesn't work for multiplication because multiplying anything by 0 gives, well 0
you need to use first element as an initial value
double accumulator(double (*function)(double, double), double array[], int length)
{
int temp = array[0];
for (int i = 1; i < length;i++) // start from #1
{
temp = (*function)(temp, array[i]);
}
return temp;
}

Answer 3

Your solution is almost there if you set temp = array[0] and start your loop at i = 1 instead of i = 0.

Answer 4

Two thoughts:
You should use double temp rather than int temp.
You need to have a different starting value for addition versus multiplication. A sum should start at temp = 0, but a product should start at temp = 1. Otherwise the product will always be 0.
You could add add another initial value parameter:
double accumulator(double (*function)(double, double), double array[], int length, double initial)
Or you use the first array element as the starting value (but then you'll need to check for the special case where the array is empty):
double temp = array[0];
For what it's worth, your "accumulator" function is alternatively known as "reduce" in other functional programming contexts. That may help if you want to Google the term.