Iterate a 2D matrix in order to exchange columns and rows using pointers

[geo20]

Hello! I am trying to iterate a matrix in order to exchange rows and columns element by element. Although the function that exchanges the rows has come out well, i don't seem to figure out how to do the same on columns.The columns switch for a matrix like
1 2 3
1 2 3
1 2 3
if i try to switch column 3 and 2,is:
1 3 2
1 3 2
1 2 3

Any suggestion would be much appreciated. Here are both of the functions:

void interchange_rows(int *p,int n,int r1,int r2){
    int temp;
    for(int i=0;i<n;i++){
            temp=*(p+r1*n+i);
            *(p+r1*n+i)=*(p+r2*n+i);
            *(p+r2*n+i)=temp;
        }
}
 
void interchange_columns(int *p,int n, int c1,int c2){
    int temp;
    for(int i=0;i<n;i++){
        temp=*(p+(i*n)-c1);
        *(p+(i*n)-c1)=*(p+(i*n)-c2);
        *(p+(i*n)-c2)=temp;
    }
}
 

[weaknessforcats]

This works:

void interchange_columns(int *p,int n, int c1,int c2){
    int temp;
    for(int i=0;i<n;++i){
         temp = *(p+ c1 + i * 3);
                 *( p + c1 + i * 3) =   * ( p + c2 + i*3);
                 * ( p + c2 + i*3) = temp;
    }
}

However, I made the loop cycle for the number of columns and I had to hard-code the width of the row since it is not an argument. Probably, you need to supply that value as an argument.

The logic is based on incrementing p by a column number, like 0 1 2, incremented by the row width.