Invalid type for array subscript error

Question

I'm getting an error with a piece of my code... In the following, "vertices" is defined as a vector of gVector3's, where gVector3 is a float array of length three (x,y,z coordinates of a point). The [] operator has been overloaded for the gVector3 class, so vertices[i][k] returns a float.

I have an error in this line: (*result)[i+k] = vertices[i][k]. The full code as well as the error message is below. Any insights would be appreciated!

float* Polygon::getVertices(){
    float* result = new float[vertices.size()*3];
    for (int i = 0; i < vertices.size(); i++){
        for (int k = 0; k < 3; k++){
            (*result)[i+k] = vertices[i][k];   // invalid types for array subscript
        }
    }
    return result;
 }

Barmar · Accepted Answer · 2014-02-11 05:12:40Z

1

Since result is declared as float*, (*result) is just a float. It's not an array, so you can't subscript it. The assignment should be:

result[3*i + k] = vertices[i][k];

or:

*(result + 3*i + k) = vertices[i][k];

Notice that you also need to multiply i by 3, otherwise you're going to repeatedly overwrite the same elements of result. E.g. you'll write into result[1] when i = 0, k = 1 and when i = 1, k = 0. This is called row major indexing.

edited Feb 11, 2014 at 5:12

answered Feb 11, 2014 at 5:06

Barmar

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bb94 · Accepted Answer · 2014-02-11 05:12:36Z

0

Try substituting result[3*i+k] for (*result)[i+k]. result is a pointer to a float, not a pointer to a float array.

edited Feb 11, 2014 at 5:12

answered Feb 11, 2014 at 5:05

bb94

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1 Comment

bb94 Over a year ago

Exactly, change that to result[i+k].

Programmer · Accepted Answer · 2014-02-11 05:16:30Z

0

float* result = new float[vertices.size()*3]; is creating a pointer of float type. But in the nested for loop you tend to treat it as a pointer to array type hence the error. Either make the pointer an array type, or use *(result + 3*i + k) = vertices[i][k];

answered Feb 11, 2014 at 5:16

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