R help - Aug 2010 - diff in multiple columns by groups?

Hi,

Is there a way to apply the diff operator over a large data frame, by groups in
a single function?

I have a toy data frame that looks like this:

MEASUREMENT              BLOCK                  TRATMENT         VALUE1         
VALUE2
1                                             1                             T-01
15                           1
1                                             1                             T-02
32                           2
1                                             2                             T-01
21                           5
1                                             2                             T-02
32                           6
2                                             1                             T-01
19                           5
2                                             1                             T-02
37                           2
2                                             2                             T-01
29                           3
2                                             2                             T-02
36                           6
3                                             1                             T-01
21                           21
3                                             1                             T-02
45                           22
3                                             2                             T-01
38                           32
3                                             2                             T-02
40                           31
.
.
.

What I would like to calculate is a dataframe that takes the difference between
measurements over the same BLOCK-TRATMENT for VALUE1 and VALUE2

Any one know?

Cheers,

Cristián Montes

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