Need some help initializing an array

Hi, Im trying to initialize the counter array to all zeros, but I keep getting this error when trying to compile:

xray.cpp:30: error: variable-sized object ‘counter’ may not be initialized

Code:

---

#include <iostream>
#include <stdio.h>
#include "CImg.h"


using namespace cimg_library;

//main program
int main()
{
CImg<double> img("xray.jpg");

//retrieve image dimensions
const unsigned int width = img.dimx();
const unsigned int height = img.dimy();

//converts the image to black and white
img.RGBtoYCbCr().channel(0).resize(-100,-100,1,3).RGBtoLUT(CImg<>(2,1,1,3).fill(0.0f,255.0f),false);

//displays image
CImgDisplay display(img,"Black and White");
while (!display.is_closed)
display.wait();

//display dimensions
printf("The height of the image is %d\n",height);
printf("The width of the image is %d\n",width);

//initialize counter array of size height
int counter[height]={0};

//go through image one row at a time from left to right
for (int i=0;i<width;i++)
{
    for (int j=0;j<height;j++)
    {
        //if pixel equals white
        //add total number of white pixels to counter 
           
    }
}

return 0;
}

---

The error message seems pretty clear: you cannot initialize a variable-length object. (I.e., initialization is a compile-time activity, and the compiler has no idea what length is going to be in the future.) You'll have to use a loop.

Quote:

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Originally Posted by tabstop

The error message seems pretty clear: you cannot initialize a variable-length object. (I.e., initialization is a compile-time activity, and the compiler has no idea what length is going to be in the future.) You'll have to use a loop.

---

thats tabstop, that makes sense. I have one last question Im stumped on. Im trying to calculate the number of white pixels per row, but so far I can only get it calculate the total number of white pixels in the image. How do I make it calculate the total number of white pixels per row? Here is the code I have so far:

Code:

---

#include <iostream>
#include <stdio.h>
#include "CImg.h"
#include <vector>

using namespace cimg_library;


//main program
int main()
{
CImg<float> img("xray.jpg");

const unsigned int width = img.dimx();
const unsigned int height = img.dimy();

//converts the image to black and white
img.RGBtoYCbCr().channel(0).resize(-100,-100,1,3).RGBtoLUT(CImg<>(2,1,1,3).fill(0.0f,255.0f),false);

//displays image
CImgDisplay display(img,"Black and White");
while (!display.is_closed)
display.wait();


printf("The height of the image is %d\n",height);
printf("The width of the image is %d\n",width);

//counter array
int counter[height];
int number = 0;

//go through image left to right, top to bottom
for (int i=0;i<width;i++)
{
    for (int j=0;j<height;j++)
    {
        //if pixel equals white, add to number
        if (img(i,j,0,0) == 255 && img(i,j,0,1) == 255 && img(i,j,0,2) == 255)
        {
                number=number + 1;
        }

    }
}
printf("The number of white pixels is %d\n", number);

int x;

//for each row, print the total number of white pixels for that row
for (x =0; x < height; x++)
{
        printf("%d %d\n", x, number);
}

return 0;
}

---

So, you're looping over the entire image. Don't do that, if you only want to look at a single row.

You'll have to keep track of how many white pixels you get per row, just like you want.

Code:

---

// this is kinda pseudo-code

int height = 100; // get the actual height, i just use 100 for example
int width = 100; // ditto

int numWhites[height][1];

// init all to zero
for(int h = 0; h < height; h++){
    numWhites[h][0] = 0;
}

// find all the values == 255
for(int y = 0; y < height; y++){
    for(int x = 0; x < width; x++){
        if(image[x][y] == 255){
            numWhites[y][0]++;
        }
    }
}

// now count how many per row
for(int y = 0; y < height; y++){
    printf("number of whites for line %i: %i\n", y, numWhites[y][0]);
}

---

I typed that in the reply window so if there's any syntax errors have mercy.

BTW, variable length arrays like the one you're using aren't legal in standard C++, they are just provided as an extension on some the gcc compiler (because they are in the latest C standard).

Normally, in standard C++ you'd use a vector for a dynamically sized array. The vector is automatically initialized with the loop.