dogmaxpeppe - 6 months ago 21

C Question

I had a square matrix typed dynamically, correctly allocated

`double **matrix;`

I want to delete a "x" row and "x" column from that matrix, in a way like that:

`SOURCE MATRIX:`

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

I want to remove, for example, the 2nd row/column. The output had to be like this:

`FINAL MATRIX:`

1 3 4

9 11 12

13 15 16

I've tried to write down and test many algorithms, without success.

How can I do?

Answer

Well, to do this, you'll have to realise that in memory your matrix is actually a *list of lists*.
This means that removing a column is slightly different from removing a row:

Assuming your syntax is `matrix[row][column];`

```
void removeColumn(int** matrix, int col){
MATRIX_WIDTH--;
//TODO check for empty matrix etc;
for(int i=0;i<MATRIX_HEIGHT; i++)
{
while(col<MATRIX_WIDTH)
{
//move data to the left
matrix[i][col]=matrix[i][col+1];
col++;
}
matrix[i] = realloc(matrix[i], sizeof(double)*MATRIX_WIDHT);
}
void removeRow(int** matrix, int row){
MATRIX_HEIGHT--;
//TODO check for empty matrix etc.
free(matrix[row]);
while(row<MATRIX_HEIGHT)
{
//move data up
matrix[row] = matrix[row+1];
row++;
}
}
```

so `removeColumn`

iterates over each row, and deletes the appropriate item, and `removeRow`

can just `free`

the row, and overwrite its pointer.

NOTE that you have to keep track of the size of the matrix size yourself. In the example i used `MATRIX_WIDTH`

and `MATRIX_HEIGHT`

but you'll have to implement something for this. (Maybe a struct with width height and pointer in it.)