Adding to Individual Array Elements - C++ - c++

Question

I need to add either +0.25 or -0.25 to all elements within an array. Here is what I have so far. Any help would be appreciated.
int main() {
double i;
// Arrays
double x[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
double x2[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
for(i=1; i<=10; i++) {
const double pM[2] = {-1, 1};
int randoid = rand() % 2;
for(i=1; i<=10; i++){
x2[i] = x[i] + pM[randoid]*0.25; //Error Line
}
}
cout << x;
cout << x2;
}
I get this error at the marked line: "invalid types 'double[10][double] for array subscript"

Answer 1

The problem is that i is a double. Then you write x2[i].
It's not a very good error message; however with the [] operator, one of the operands must be a pointer and the other must be an integer. There is no implicit conversion of floating-point to integer when using this operator.
To fix this change double i; to int i;
Another issue is that your code accesses out of bounds of the arrays. double x2[10] means that there are 10 elements whose indices are 0 through 9. But your loop tries to write to x2[10]. This causes undefined behaviour, which could explain your strange output.
There is also a potential logic error. Maybe you meant to use a different variable for the inner loop than the outer loop. As it stands, the inner loop will take i to 11 (or 10 if you fix the code) and then the outer loop will be complete and not execute any more iterations.
Based on your description though, perhaps you only meant to have one loop in the first place. If so, remove the outer loop and just leave the contents there.
Also you do not need two separate arrays, you could just perform the addition in-place.
Regarding the output, cout << x and cout << x2 will output the number of the memory address at which the array is located. To output the contents of the array instead you will need to write another loop, or use a standard library algorithm that iterates over containers.

Answer 2

I see 3 issues -
Change the type of i to int.
x and x2 are arrays of size 10. You need to loop from i =
0 to i = 9. But you are looping from i = 1 to i = 10. x[10]
is out of bounds since arrays are 0 indexed.
cout << x - This is a wrong way to print an array. You need
to loop through the array and print - e.g. -
for(i = 0; i < 10; i++)
cout << x[i] << " ";

Answer 3

Try this, it works, I converted to C
int main( )
{
int i = 0;
// Arrays
double x[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
double x2[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
for ( i = 1; i < 10; i++ )
{
const double pM[2] = { -1, 1 };
int randoid = rand( ) % 2;
for ( i = 1; i <= 10; i++ )
{
x2[i] = x[i] + pM[randoid] * 0.25; //Error Line
printf( "\nx[%d]==%2.2f", i, x[i] );
printf( "\nx2[%d]==%2.2f", i, x2[i] );
}
}
}