Copying part of 2D array into another 2D array in C

Question

I want to write C code that takes a block of a 2D array and copies it into another 2D array by using the memcpy function. In the particular code I have included, I have tried to copy the last 3 columns of the 2x4 array matrix into the 2x3 array mat_copy.

double matrix[2][4] = {{0, 1, 2, 3}, {4, 5, 6, 7}};
double mat_copy[2][3];
double sz = sizeof(mat_copy)/2;

for (int i = 0; i < 2; i++) {
    memcpy(&mat_copy[i][1], &matrix[i][1], sz);
}

I'm getting the output: [0, 1, 2; 3, 5, 6] instead of [1, 2, 3; 5, 6, 7]. How does the memcpy function choose which addresses to copy once you give the initial pointer in memory? I know my error has to do with addressing the 2D arrays, but I don't know how to change it.

Do you know how multidimensional arrays are laid out in the memory? — Eugene Sh.
– Eugene Sh., Commented Aug 2, 2017 at 18:55
You are not going to copy columns. You are going to copy rows. — Mike Nakis
– Mike Nakis, Commented Aug 2, 2017 at 18:55
The sizeof the last 3 columns is the same as the size of 3x any one element of mat_copy. size_t sz = 3 * sizeof mat_copy[0][0]; or in this case, the size of 1 row of size_t sz = sizeof mat_copy[0]; — chux
– chux, Commented Aug 2, 2017 at 19:10

dlasalle · Accepted Answer · 2017-08-02 19:01:28Z

2

You're starting from 1 in your destination instead of 0, so you're copying past the end of the row.

Change

memcpy(&mat_copy[i][1], &matrix[i][1], sz);

to

memcpy(&mat_copy[i][0], &matrix[i][1], sz);

answered Aug 2, 2017 at 19:01

dlasalle

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chqrlie · Accepted Answer · 2017-08-02 19:13:50Z

1

To copy the last 3 columns, you must pass the offset of columns 0 as destination. You can further simplify the code this way:

double matrix[2][4] = {{0, 1, 2, 3}, {4, 5, 6, 7}};
double mat_copy[2][3];

for (int i = 0; i < 2; i++) {
    memcpy(mat_copy[i], &matrix[i][1], sizeof(mat_copy[i]));
}

answered Aug 2, 2017 at 19:13

chqrlie

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