C: why do I get invalid initializer? - c

Question

I have to do a function which returns the dot product of two vectors.
Here his the code:
float res;
float v1[3];
float v2[3];
v1[0] = 3;
v1[1] = 2;
v1[2] = 0;
v2[0] = 2;
v2[1] = 5;
v2[2] = 0;
float dotProd(float *vec1, float *vec2) {
return vec1[0]*vec2[0] + vec1[1]*vec2[1] + vec1[2]*vec2[2];
}
res = dotProd(v1, v2)
I thik the function is right, the problem is that I get invalid initializer error when I define the values of the vectors.
What am I missing?

Answer 1

What you are attempting to do is not initialization but assignment. The former is done at the time the variable is defined, while the later is done later as an executable statement. You're getting an error because executable statements are not allowed outside of a function.
You need to use an initializer at the time the arrays are defined:
float v1[3] = {3, 2, 0};
float v2[3] = {2, 5, 0};

Answer 2

float v1[3];
// ...
v1[0] = 3;
v1[1] = 2;
v1[2] = 0;
This: float v1[3]; is a declaration (and a definition). It defines an array object named v1. It can legally appear either at file scope (outside any function) or at block scope (inside a function definition).
This: v1[0] = 3; is a statement. Statements are permitted only inside a function definition.
If you want those statements to be executed just once, you can put them inside your main function.
Execution of your program begins with an implicit call to your main function. Very nearly everything that happens as your program runs results from that, as other functions are called from main, directly or indirectly. If you could have statements outside any function, there would be no way for them ever to be executed. File-scope declarations are permitted, but they're restricted to ones that don't execute any code. For example, a declaration like:
int n = 42;
is permitted either at file scope or at block scope; if it's at file scope the value 42 is loaded into n at program startup. But if the initializer isn't a constant expression, such as:
int r = rand();
then it can only appear at block scope; there's no way to call a function before main starts.
As for why you're getting that particular message, the compiler's parser isn't expecting to see a statement in that context, so it tries (and fails) to interpret v1[0] = 3; as a declaration. When a compiler sees something that's syntactically invalid, it tries to interpret it in a way that lets it give a meaningful error message, but it doesn't always succeed.

Answer 3

Your code is fine; you are probably just having a structure problem.
Try something like:
#include<stdio.h>
float dotProd(float * vec1, float * vec2) {
return vec1[0] * vec2[0] + vec1[1] * vec2[1] + vec1[2] * vec2[2];
}
int main() {
float res;
float v1[3];
float v2[3];
v1[0] = 3;
v1[1] = 2;
v1[2] = 0;
v2[0] = 2;
v2[1] = 5;
v2[2] = 0;
res = dotProd(v1, v2);
printf("%lf", res);
return 0;
}