My program prints 0 instead of the expected number - c

Question

So I made a function that returns a value which is the connection (sum) of 2 numbers that the user gives. But everytime I enter these 2 numbers, the result is somehow always 0.
float connection(int num1, int num2, float result)
{
result= num1 + num2;
return (result);
}
int main()
{
int num1= 0;
int num2= 0;
float result= 0.0;
printf("\nChoose the first number you'd like to connect: ");
scanf("%d", &num1);
getchar();
printf("choose the second number you'd like to connect: ");
scanf("%d", &num2);
connection(num1, num2, result);
printf("The result is: %f", result);
}

Answer 1

The problem here is (I believe) that you expect the argument result you pass to the function to change value. That's not how C works, instead all arguments are passed by value and the variable result inside the called function is a local variable which has no connection to the result variable in the calling function. Changing one variable will not affect the other.
Instead use the value the function returns:
result = connection(num1, num2, result);
Of course, this means passing result as an argument is totally meaningless, and you don't even need it in the function:
int connection(int num1, int num2)
{
return num1 + num2;
}
Also note that I changed the return-type, as adding two integers will never result in a floating-point number.
On a side-note to modify the argument means you have to pass it by reference, which is not supported by C. It can be emulated though, by using pointers:
void connection(int num1, int num2, int *result)
{
*result = num1 + num2;
}
Then call it as
int result;
connection(num1, num2, &result);

Answer 2

You are mixing up two possibilities: returning a value and changing a variable by passing it to a method as a reference.
This is how to return a value (in this case, you don't need the result parameter):
float connection(int num1, int num2)
{
float result= num1 + num2;
return result;
}
...
result = connection(num1, num2);
This is how to pass a variable as a reference (in this case you don't need a return value):
void connection(int num1, int num2, float *result)
{
*result= num1 + num2;
}
...
connection(num1, num2, &result);

Answer 3

I think forgot to assign your result back to your variable:
connection(num1, num2, result); // returned value is missed
result = connection(num1, num2, result); // returned value saved
If, instead, you want your function to modify the variable, C is passed by value so you need some pointers:
void connection(int num1, int num2, float *result)
{
*result = num1 + num2;
}
connection(num1, num2, &result);

Answer 4

Simply make your function call pass by reference
#include<stdio.h>
float connection(int *num1, int *num2, float *result)
{
*result= *num1 + *num2;
return (*result);
}
int main()
{
int num1= 0;
int num2= 0;
float result= 0.0;
printf("\nChoose the first number you'd like to connect: ");
scanf("%d", &num1);
getchar();
printf("choose the second number you'd like to connect: ");
scanf("%d", &num2);
connection(&num1, &num2, &result); //Function Called by reference
printf("The result is: %f", result);
}
For further details on function calls Read It