Problem with matching datatypes using a dynamic float* array and dynamic matrix

Question

Basically, I'm saving some coordinates in 2 arrays, arrayX and arrayY. Then, I want to change the value of a "blank" matrix in the coordinates that are saved in the previously mentioned arrays. Will post a simpler version of the code, just so you can understand:

float* arrayX = 0;
float* arrayY = 0;
int array_size = 0;
int width = 1440;
int height = 1080;

for (i=0;i<1234;i++){
    if(flag==TRUE){
        arrayX = realloc(arrayX, ++array_size * sizeof(*arrayX));
        arrayY = realloc(arrayY, array_size * sizeof(*arrayY));
        arrayX[array_size-1] = i
        arrayY[array_size-1] = 100;
    }
}

float *matrix1 = (float *)malloc(width * height * sizeof(float*));
for (i=0; i<width*height;i++){
    *(matrix+i) = 0;
}
for (i=0; i<array_size; i++){
    *(matrix1 + arrayY[i] * width + arrayX[i]) = 255;  //ERROR HERE
}

This code gives me this error:

error: invalid operands to binary + (have 'float *' and 'float')

And I'm not understanding why:

arrayY[l] is the float pointed by arrayY + l.

*(matrix1 + float * int + float) should also be a float.

I'm clearly messing up something here, and I've scrolled to a fair share of other posts (on how to allocate the matrix, and with this error) and don't understand it/don't find exactly what I'm looking for.

I'm sure it's a pretty silly question, and with a very easy solution.

Do your arrayY and arrayX need to be arrays of floats in your actual code? If not then they could just be int arrays. Otherwise, try casting the values to floats, i.e., *(matrix1 + (int)arrayY[l] * width + (int)arrayX[l]) = 255; — Matt Pitkin
– Matt Pitkin, Commented May 2, 2023 at 9:13
Also, maybe have a look at stackoverflow.com/questions/394767/pointer-arithmetic about pointer arithmetic. In pointer arithmetic, the things you add to pointers should be integers. — Matt Pitkin
– Matt Pitkin, Commented May 2, 2023 at 9:18
You're trying to add a float to a pointer. That's invalid. What type would the result even have? You need to add an integer type to a pointer. Or you need to add a float and another numeric type. — Tom Karzes
– Tom Karzes, Commented May 2, 2023 at 9:34
I guess the index l is also a transcription error (should be i)? — Ian Abbott
– Ian Abbott, Commented May 2, 2023 at 13:33

Vlad from Moscow · Accepted Answer · 2023-05-02 09:50:45Z

1

For starters the argument of the call of malloc in this declaration

float *matrix1 = (float *)malloc(width * height * sizeof(float*));
                                                  ^^^^^^^^^^^^^^

is incorrect. It seems you mean

float *matrix1 = (float *)malloc(width * height * sizeof(float));
                                                  ^^^^^^^^^^^^^

The pointer arithmetic is allowed between pointers and objects of integer types. However in this expression

*(matrix1 + arrayY[l] * width + arrayX[l])

you are using pointers and objects of the type float. So the compiler issues the error message.

From the C11 Standard (6.5.6 Additive operators)

2 For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to a complete object type and the other shall have integer type. (Incrementing is equivalent to adding 1.)

So either use integer arrays or at least use casting as for example

*(matrix1 + ( int )( arrayY[l] * width + arrayX[l] ))

edited May 2, 2023 at 9:50

answered May 2, 2023 at 9:44

Vlad from Moscow

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2 Comments

ATSlooking4things Over a year ago

Thanks. I doesn't give me building errors anymore. But, it's not doing what I want either. If I fill the new matrix with 255 (white) instead of 0 (black), and then save it using stbi_write_bmp("answer_blank.bmp", width, height, 1, matrix); I get an image with alternating black and white columns. It seems that my for (i=0; i<width*height;i++) is not running through each and every pixel/matrix element. Any idea why?

Vlad from Moscow Over a year ago

@ATSlooking4things You can ask a new question (closing this question y electing the best answer) providing a minimal complete updated program.

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