Mixing tapply and diff

[Tristan Lorino <tristan.lorino@lcpc.fr>]

Hi,

I have two columns: one concerning quantitative data (V1), the other
concerning the identification of the repeated measures made on the
same units (V2) ; here, there is 3 measures on the first unit, 4
measures on the 2d unit, and so one...

  V1 V2
 1  5  1
 2  3  1
 3  6  1
 4  4  2
 5  7  2
 6  8  2
 7  9  2
 8  6  3
 9  5  3
10  2  4
11 31  4

I would like to obtain (ideally...) a data frame or matrix with the
ident variable and the difference between measures made on the same
unit: something as returned by

> tapply(as.vector(SDF1$V1),SDF1$V2,diff)
$"1":
[1] -2  3

$"2":
[1] 3 1 1

$"3":
[1] -1

$"4":
[1] 29

but in a suitable way (data array), that means:

ident  diff
1      -2
1      3
2      3
2      1
2      1
.....

Thank you for your help,
Tristan

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