This will return the number of columns that end with _TissueA in your dataframe (dat).
If you want to know the specific indices of those columns (i.e., where they are in your dataframe), just use the 'grep' part:
The return values will give the column indices (this output is just an example, it will be different for your dataframe):
 62 63 89 94 95
In addition, if you want to actually see a dataframe with the values in those columns (and only those columns), try the following: