[R] Simple loop

[Woida71]

Hello everybody,
I am beginning with loops and functions and would be glad to have help in
the following question:
If i have a dataframe like this
Site  Prof  H        
1      1     24
1      1     16
1      1     67
1      2     23
1      2     56
1      2     45
2      1     67
2      1     46
And I would like to create a new column that subtracts the minimum of H from
H, but for S1 and P1
only the minimum of the data points falling into this category should be
taken.
So for example the three first numbers of the new column write: 24-16,
16-16, 67-16
the following numbers refering to Site1 and Prof2 write: 23-23, 56-23,
45-23.
I think with two loops one refering to the Site, the other to the Prof, it
should be possible to automatically
create the new column.
Thanks a lot for any help.

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