How to convert a 2D array into a 1D array-CodePudding

I am trying to convert a 2D array into a 1D array using for loops. Am I able to accomplish this using the described method? Currently my code just assigns the final element of the 2D array to all the elements of the 1D array.

#include <iostream>
using namespace std;

int main() {
    int myArray[4][2]={{1,2},{3,4},{5,6},{7,8}};
int ray[8];

for(int i=0; i<8; i  ){
    for(int j=0; j<4;j  ){
        for(int k=0;k<2;k  ){
            ray[i]=myArray[j][k];
        }
    }
}

for(int z = 0; z<8; z  ){
    cout << ray[z] << endl;
}
}

CodePudding user response：

The 3 for loops will copy every value in myArray to ray over and over overwriting the value every time until you assign myArray[3][1] to every element of ray.

You have to compute the value of i based on j and k instead or vice versa instead of 3 nested loop.

But you can also use the fact how 2D arrays are layed out in memory. Specifically a 2D array is a single contigos block of memory. So you can just copy the whole block at once:

std::copy_n(&my_Array[0][0], 4 * 2, &ray[0]);

CodePudding user response：

Here's the example of taking advantage of the memory layout.

Basically a 2D array is just a bunch of 1-d arrays. So all we basically need to do is say going from the first 1-d array, just place the next one after it, until they are all stacked in a line.

**Memory 2-d array**
[1-d A]
[1-d B]
[1-d C]

**We just need to reshuffle**
[1-d A][1-d B][1-d C]

[1-d A] <- at the end of this guy put [1-d B]
results: [1-d A][1-d B] with length (A width)   (B width)

[1-d A][1-d B] <- at the end of this guy put [1-d C]
results: [1-d A][1-d B][1-d C] with length (A width)   (B width)   (C width)

Example with code

int main()
{
    //Create some variables so naming is easier to follow
    const int rows = 4;
    const int rowWidth = 2;

    //Make our 2d array
    int twoDArray[rows][rowWidth] = { {1,2},{3,4},{5,6},{7,8} };
    //Make our one d array
    int oneDDArray[rows * rowWidth] = {};

    //For each row we just need to copy that block of memory in
    for(int currentRow = 0; currentRow < rows; currentRow  )
    {
        //Calculate how far we've gone th equivalent 1d distance # rows * row width
        const int oneDIndex = currentRow * rowWidth;

        //The next row to copy is just the next row in our for loop at the first position
        //Then we copy a widths worth of memory, note we need to know how big an int memory block is hence "* sizeof(int)"
        memcpy(&oneDDArray[oneDIndex], &twoDArray[currentRow][0], rowWidth * sizeof(int));
    }

    for (const int i : oneDDArray)
    {
        std::cout << i << ", ";
    }

}