Qiita Teams that are logged in
You are not logged in to any team

Community
Service
Qiita JobsQiita ZineQiita Blog
1
Help us understand the problem. What is going on with this article?
@daikumatan

# Cで負のIndexが使える2次元配列を動的にかつ連続に確保する

More than 5 years have passed since last update.

2次元配列、myArray[j][i]のi, jは

imin <= i < imax
jmin <= j < imax

imin < 0
jmin < 0

imax >= 0
jmax >= 0

メモリアクセスが連続となる方をiとしています。i, jの順番に注意してください。

``````    #include <iostream>

int **allocateNegativeIndex2DimArray(
int jmin,       /* array[jmin][] jmin  < 0 */
int jmax,       /* array[jmax][] jmax >= 0 */
int imin,       /* array[][imin] imin  < 0 */
int imax        /* array[][imax] imax >= 0 */
)
{

const int ni = imax - imin;
const int nj = jmax - jmin;
const int numTotal = ni * nj;

int **tmp = (int **)malloc(nj * sizeof(int));
tmp[0] = (int *)malloc(numTotal * sizeof(int));

for (int j = 1; j < nj; j++) {
tmp[j] = tmp[j - 1] + ni;
}

const int icenter = abs(imin);
const int jcenter = abs(jmin);

int **val = &tmp[jcenter];
for (int j = jmin; j < jmax; j++) {
val[j] = &tmp[j + jcenter][icenter] + ni;
}

return val;
}

int main() {

int xmin = -2;
int xmax =  3;
int ymin = -1;
int ymax =  2;

int **myArray = allocateNegativeIndex2DimArray(ymin, ymax, xmin, xmax);

/*
* 実際に負のインデックスが入るか表示テストする
*/
for (int j = ymin; j < ymax; j++){
for (int i = xmin; i < xmax; i++) {
myArray[j][i] = i * j;
printf("myArray[%2d][%2d] = %3d\n", j, i, myArray[j][i]);
}
}
return 0;
}
``````
``````myArray[-1][-2] =   2
myArray[-1][-1] =   1
myArray[-1][ 0] =   0
myArray[-1][ 1] =  -1
myArray[-1][ 2] =  -2
myArray[ 0][-2] =   0
myArray[ 0][-1] =   0
myArray[ 0][ 0] =   0
myArray[ 0][ 1] =   0
myArray[ 0][ 2] =   0
myArray[ 1][-2] =  -2
myArray[ 1][-1] =  -1
myArray[ 1][ 0] =   0
myArray[ 1][ 1] =   1
myArray[ 1][ 2] =   2

``````
1
Help us understand the problem. What is going on with this article?
Why not register and get more from Qiita?
1. We will deliver articles that match you
By following users and tags, you can catch up information on technical fields that you are interested in as a whole
2. you can read useful information later efficiently
By "stocking" the articles you like, you can search right away
2002-2015: Fujixerox, Numerical simulation Engineer 2015-2016: NVIDIA Japan, BD Manager 2016-2020: Rescale Japan, Evangelist 2020-Present: XTREME-D, CTO