I need help. First time writing C - arrays

Question

Can someone help me fixing the syntax errors in this code?
#include <stdio.h>
int result(int v, int size);
int main(void){
int arr[5], n;
for (n = -1; n < 4; n++){
arr[n] = n + 1;
printf("the product of entered values is %d", result(n, 5));
}
return 0;
}
int product(int a[]) {
int product, i;
for (i = 0; i <= sizeof(int); i++){
product *= a[i];
}
return product;
}

Answer 1

for (n = -1; n < 4; n++){
arr[n] = n + 1;
You start n at -1, then immediately use arr[n].
Negative indices are not allowed in C. Valid values are 0 to the size of the array-1.In otherwords, if you have an arr[5], then the valid indices are [0], [1], [2], [3], and [4].
int product(int a[]) {
int product, i;
You create a function named product and also a variable named product.
That makes it virtually impossible to refer to the proper object. Work on your name management, so that every item is unambiguous.

Answer 2

You are trying to call the result function, for which you have provided the forward declaration but not an actual implementation. You need to write an implementation for the result function. That is why the compiler complains about an undefined reference.

Answer 3

For starters the function result is declared
int result(int v, int size);
but not defined.
On the other hand, taking into account the message in this call of printf
printf("the product of entered values is %d", result(n, 5));
you mean a function that calculates a product of elements of an array like your function defined after main but that is not used
int product(int a[]) {
int product, i;
for (i = 0; i <= sizeof(int); i++){
product *= a[i];
}
return product;
}
So let's remove the declaration of the function result and declare before main a function with name product because the name result is not enough informative.
The function deals with an array. We need to pass to the function the number of elements in the array. As the array itself will not be changed within the function then it should be declared with the qualifier const
Also a product of integer numbers of the type int can be too big to be stored in an object of the type int.
So it is better to declare the return type of the function at least like long long int (or even like double or long double).
Thus the function declaration can look like
long long int product( const int a[], size_t n );
The function can be defined the following way
long long int product( const int a[], size_t n )
{
long long int result = n == 0 ? 0 : 1;
for ( size_t i = 0; i < n; i++ )
{
result *= a[i];
}
return result;
}
As for your function product then you forgot to initialize the variable product
int product, i;
The condition in the for loop
for (i = 0; i <= sizeof(int); i++){
does not make a sense because the expression sizeof( int ) does not yield the number of elements in an array.
Try to not use magic numbers like 5 in this declaration
int arr[5], n;
in your programs. Use named constants.
Indices of arrays always start from 0. So this loop
for (n = -1; n < 4; n++){
where the variable n is used as an index does not make sense.
Also this call of printf
printf("the product of entered values is %d", result(n, 5));
must be placed outside the for loop and in the the call of the function result that is used as an argument of the function printf
result(n, 5)
you even are not using the array.
Thus the program can look the following way
#include <stdio.h>
long long int product( const int a[], size_t n );
int main(void)
{
enum { N = 5 };
int a[N];
for ( size_t i = 0; i < N; i++ )
{
a[i] = i + 1;
}
printf( "The product of entered values is %lld", product( a, N ) );
return 0;
}
long long int product( const int a[], size_t n )
{
long long int result = n == 0 ? 0 : 1;
for ( size_t i = 0; i < n; i++ )
{
result *= a[i];
}
return result;
}
The program output is
The product of entered values is 120