Error in output of dynamically initialized 2D array - c++

Question

I'm working on a school project written in c++. I am trying to generate an array of rgb colours based on user input, for a program that generates fractals(irrelevant).
I found the need to use a pointer to a 2D array. The pointer has a global scope because it needs to be accessed across multiple files, in multiple functions.
There is no problem with the declaration of the pointer. It is as such:
in "global.h"
int **ptr;
int palette[10][3];
//all the statements like #ifndef GLOBAL_H_INCLUDED, etc are present
in main.cpp
extern int **ptr;
extern int palette[10][3];
void gen_array();
int main()
{
//initialize values of palette
//palette is basically list of colors that the user selects for the
//desired blend of colors to appear on the screen
gen_array();
}
in void gen_array(), in main.cpp
void gen_array()
{
int i=0, size=0, size_arr=0;
//size is a variable helpful in calculating "size_arr", which is the size of the dynamic array to be initialized
while(palette[i][0]!=256)
{ //calculates "size" }
for(int i=0; i<size-1; i++)
{
//to find "size_arr"
}
ptr= new int*[size_arr]();
for(int i = 0; i < size_arr; ++i)
{
ptr[i] = new int[3];
}
//the following piece of code enter's values column-wise into the array
int s=0;
for(int i=0; i<size-1; i++)
{
for(int k=0; k<3; k++)
{
int x=-1, a=0, b=0;
a= palette[i][k] - palette[i+1][k];
b= palette[i][k];
if(a<0)
{
a= -a;
x=1;
}
for(int j=0; j<=a; j++, s++)
{
ptr[i][k] = b;
//cout<<*(*(ptr+i)+k)<<' ';
b= b+ x;
}
b= b-x;
for(int j=a+1; j<=diff[i]; j++, s++)
{
ptr[i][k] = b;
cout<<ptr[i][k]<<' ';
}
cout<<endl;
}
}
//this output's the array that the pointer points to, on the screen
for(int i=0; i<size_arr; i++)
{
for(int j=0; j<3; j++)
{
cout<<ptr[i][j]<<' ';
}
cout<<endl;
}
}
The problem is that, the data is getting generated and inputted in the array correctly [in the second-last for loop], but when outputting it [the last for loop] I'm getting junk values.
The data inputted [the correct values] as obtained from the cout statements [that are now commented] is this [printed column-wise] whereas the O/P given out is this [printed row-wise].
If it is required to be know, "size" (in this case) =2, size_arr=256 and palette[0]={0, 0, 255} and palette[1]={0,255,0}.
Can anybody point out what the problem is, and whether it has something to do with the pointer initialization?

Answer 1

Even without reading all the program code like
for(int j=0; j<=a; j++, s++)
{
ptr[i][k] = b;
//cout<<*(*(ptr+i)+k)<<' ';
b= b+ x;
}
looks clearly wrong. At each iteration you are writing to ptr[i][k] and the loop doesn't change ptr, i or k. This means that it will keep writing to the very same location during the loop.

Answer 2

Look at your int i in the loop you're using to fill int** ptr. If size = 2 then you're never going to advance ptr far enough to fill the array. Consider filling the array like this:
for(int i = 0; i < size_arr; ++i){//for every pixel in the main array
for(int k = 0; k < 3; ++k){//for every subpixel
int my_value = ...;//calculate the value for this entry
*(*(ptr+i)+k) = my_value;//assign
Better yet, use a struct named pixel containing Uint8 r,g,b and allocate as pixel* ptr = new pixel[size] and iterate over pixels and assign ptr[i].r ptr[i].g ptr[i].b