C/C++ :: Iterate 2D Matrix In Order To Exchange Columns And Rows Using Pointers

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

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;
   
[Code] ......