How to write a separate function that computes scalar product? - c++

Question

I am aware there exists a C++ function template (std::inner_product), but I want to try writing my own. Here's some code I found but it runs in the main function:
#include <iostream>
using namespace std;
int main(){
float vectorA[3], vectorB[3], scalar=0.0;
int i;
// Get input vectors from user.
cout << "Enter elements of first vector: " << endl;
for(i=0;i<3;i++)
{
cin >> vectorA[i];
}
cout << "Enter elements of second vector: " << endl;
for(i=0;i<3;i++)
{
cin >> vectorB[i];
}
// Calculate scalar product.
for(i=0;i<3;i++)
{
scalar = scalar + (vectorA[i] * vectorB[i]);
}
// Output result.
cout << "The scalar product is " << scalar << endl;
return 0;
}
Next, I want to write this into a separate reusable function that I can call from my main loop. This is the best I could come up with.
float scalarProduct(float a1, float a2, float a3, float b1, float b2, float b3) {
float vectorA[3], vectorB[3], scalar;
vectorA[0]=a1;
vectorA[1]=a2;
vectorA[2]=a3;
vectorB[0]=b1;
vectorB[1]=b2;
vectorB[2]=b3;
for(int i=0;i<3;i++) // Calculate scalar product.
{
scalar = scalar + (vectorA[i] * vectorB[i]);
}
return scalar;
}
int main() {
cout << scalarProduct(1,2,3,4,5,6);
}
So my questions are:
How can I pass an array into this function? There must be a better way than having six parameters but I can't figure out how.
When I run the program in Xcode, I get the warning 'Variable scalar may be uninitialised when used here' at the line
scalar = scalar + (vectorA[i] * vectorB[i]);
The program still runs and computes the correct answer but how can I make this warning go away?

Answer 1

Question
How can I pass an array into this function? There must be a better way than having six parameters but I can't figure out how.
Change the function to accept two arrays as argument. For the sake of safety, also pass the number of elements in the array.
float scalarProduct(float a[], float b[], size_t num);
Change the function to accept two std:vectors as argument.
float scalarProduct(std::vector<float> const& a, std::vector<float> const& b);
Change the function to accept two std:arrays as argument.
float scalarProduct(std::array<float, 3> const& a, std::array<float, 3> const& b);
In all of these cases, you can access the elements of the collection using the array syntax.
float scalarProduct(std::array<float, 3> const& a, std::array<float, 3> const& b)
{
// Initialize scalar to 0
float scalar = 0.0f;
for(int i=0;i<3;i++) // Calculate scalar product.
{
scalar = scalar + (a[i] * b[i]);
}
return scalar;
}
The implementations will be a little different if you use the other signatures but not too different.
Question
When I run the program in Xcode, I get the warning 'Variable scalar may be uninitialised when used here' at the line
I have already added the line to initialize scalar. Without that, the initial value of scalar is not predictable. Also, accessing the value of an uninitialized variable is cause of undefined behavior.

Answer 2

How can I pass an array into this function? There must be a better way than having six parameters but I can't figure out how.quote
To pass an array into your function, simply do:
float scalarProduct(float arr[6])
In your main(), it will look like:
float array[6] = {1.0, 2.0, 3.0, 4.0, 5.0, 6.0};
cout << scalarProduct(array);
From there you can use your array like:
vectorA[0]=arr[0];
vectorA[1]=arr[1];
vectorA[2]=arr[2];
vectorB[0]=arr[3];
vectorB[1]=arr[4];
vectorB[2]=arr[5];
When I run the program in Xcode, I get the warning 'Variable scalar may be uninitialised when used here' at the line
Maybe try and initialize scalar with an initial value:
float scalar = 0.0;

Answer 3

Just have your function take in a reference to a vector (or array as stated by Hayden). A vector is better as you don't have to hard-code the size of the vector.
Also, it is useful to have a template function for this case.
#include <iostream>
#include <vector>
using namespace std;
template <typename T>
T scalarProduct(const vector<T>& a, const vector<T>& b)
{
// Check that the size of a and b match here.
// If not, do what you deem is necessary.
T product = 0; // Not initializing variable is cause of your warning.
for (int i = 0; i < a.size(); i++)
product += a[i] * b[i];
return product;
}
int main(void)
{
// Compile for c++ 11
// vector<float> a = {1.0, 2.0, 3.0};
// vector<float> b = {1, 1, 1};
// Else
vector<float> a;
vector<float> b;
a.push_back(1);
a.push_back(2);
a.push_back(3);
b.push_back(1);
b.push_back(1);
b.push_back(1);
cout << scalarProduct<float>(a, b) << endl;
return 0;
}