Vector dot product calculation wrong sum - c

Question

I want to make a program that can calculate the vectors with the dot product but my sum is always 0 what did I do wrong in this code? I'm new to pointers and functions if that is important to you. I coded everything only in the main function and it worked but I want the calculation in another function because its clearer instead of writing everything in the main function. I compiled it with -Wall and -Werror and I got 0 warnings.
Would be nice if you could help me here :)
Here is the code:
#include<stdio.h>
#include<stdlib.h>
double calculation(int o, double *a, double *b, int sum) {
int i;
for(i=0;i<o;++i) {
printf("Components of first vector\n");
printf("Component %d: ", i+1);
scanf("%lf", &a[i]);
}
for(i=0;i<o;++i) {
printf("Components of second vector\n");
printf("Component %d: ", i+1);
scanf("%lf", &b[i]);
}
for(i=0;i<o;++i) {
sum += a[i] * b[i];
}
return(0);
}
int main() {
int o;
int sum=0;
printf("How many dimensions should the vectors have?\n");
scanf("%d", &o);
double *a = malloc(o * sizeof(double));
double *b = malloc(o * sizeof(double));
if(a==NULL) {
printf("Memoryallocation was not successfull!!!");
return(1);
}
if(b==NULL) {
printf("Memoryallocation was not successfull!!!");
return(1);
}
calculation(o,a,b,sum);
printf("The dot product is: %d\n", sum);
free(a);
free(b);
a=NULL;
b=NULL;
return(0);
}

Answer 1

In C, variables are passed by value. That means that when you change the value of sum in calculation you only change a local copy.
The idiomatic way when you only need one value is to return it:
double calculation(int o, double *a, double *b) {
double sum = 0.;
...
return sum;
}
and in main:
...
double sum = calculation(o, a, b);
...
Alternatively, if you need to return multiple values, you can pass pointers to the output variables

Answer 2

In double calculation(int o, double *a, double *b, int sum) the last argument is passed by value1. That is, when you call calculation(..., sum);, the variable sum is copied and then passed to the function.
Inside this function, you're modifying sum, but this only modifies the copy of the original variable that was passed. To solve this, pass a pointer or just return the sum from your function.
Actually, all arguments are passed by value. The second and the third ones just happen to be pointers.

Answer 3

This function
double calculation(int o, double *a, double *b, int sum) {
should change
double calculation(int o, double *a, double *b, double & sum) {

Answer 4

The problem with your code is that the sum is a local variable in the calculation function. So when you return back to main your variable is destructed and the sum in main is still 0.
Also if you are getting floating point values for dimensions then you must use double for sum too.
One possible solution could be like this:
#include<stdio.h>
#include<stdlib.h>
double calculation(int o, double *a, double *b) {
int i;
double sum =0.0;
for(i=0;i<o;++i) {
printf("Components of first vector\n");
printf("Component %d: ", i+1);
scanf("%lf", &a[i]);
}
for(i=0;i<o;++i) {
printf("Components of second vector\n");
printf("Component %d: ", i+1);
scanf("%lf", &b[i]);
}
for(i=0;i<o;++i) {
sum += a[i] * b[i];
}
return sum;
}
int main() {
int o;
double sum =0;
printf("How many dimensions should the vectors have?\n");
scanf("%d", &o);
double *a = malloc(o * sizeof(double));
double *b = malloc(o * sizeof(double));
if(a==NULL) {
printf("Memoryallocation was not successfull!!!");
return(1);
}
if(b==NULL) {
printf("Memoryallocation was not successfull!!!");
return(1);
}
sum = calculation(o,a,b);
printf("The dot product is: %lf\n", sum);
free(a);
free(b);
a=NULL;
b=NULL;
return(0);
}
Or you can pass the address of sum to calculation function as:
#include<stdio.h>
#include<stdlib.h>
void calculation(int o, double *a, double *b,double *sum) {
int i;
for(i=0;i<o;++i) {
printf("Components of first vector\n");
printf("Component %d: ", i+1);
scanf("%lf", &a[i]);
}
for(i=0;i<o;++i) {
printf("Components of second vector\n");
printf("Component %d: ", i+1);
scanf("%lf", &b[i]);
}
for(i=0;i<o;++i) {
*sum += a[i] * b[i];
}
}
int main() {
int o;
double sum=0;
printf("How many dimensions should the vectors have?\n");
scanf("%d", &o);
double *a = malloc(o * sizeof(double));
double *b = malloc(o * sizeof(double));
if(a==NULL) {
printf("Memoryallocation was not successfull!!!");
return(1);
}
if(b==NULL) {
printf("Memoryallocation was not successfull!!!");
return(1);
}
calculation(o,a,b,&sum);
printf("The dot product is: %lf\n", sum);
free(a);
free(b);
a=NULL;
b=NULL;
return(0);
}