[R] multi-Operations on a matrix

[alcesgabbo]

Hello everybody.

I have this object

          procedure property sensor_data              sensor_date
1    S_10     nord    626821.0 2002-09-30T00:00:00+0200
2     S_10     nord    626821.0 2002-12-05T00:00:00+0100
3    S_10     nord    626821.1 2008-07-31T00:00:00+0200
4  S_1000     nord    626496.8 2002-09-30T00:00:00+0200
5  S_1000     nord    626496.8 2002-12-05T00:00:00+0100
6  S_1000     nord    626496.9 2009-04-23T00:00:00+0200
7  S_1000     nord    626497.0 2009-11-10T00:00:00+0100
8     S_1     nord    626485.3 2002-09-30T00:00:00+0200
9    S_1     nord    626485.3 2002-12-05T00:00:00+0100
10    S_1     nord    626485.3 2003-02-07T00:00:00+0100

the third colomn (sensor_data) is a distance in meters.
each distance has a date (sensor_date)
for each row I would like to find the difference from the first distance
(with the min date) and the distance.

I know it's difficult to understand.. I give you an example:

for the first procedure S_10 I should find the first distance based on the
date:

distance:    626821.0   with the date: 2002-09-30T00:00:00+0200

then I should do the difference between the current distance and the first
distance:

626821.0 - 626821.0 = 0.0
 626821.0 -  626821.0 = 0.0
626821.1 - 626821.0 = 0.1

Then I should do this also for the other procedures.

I would like a resut like this:

     procedure property sensor_data              sensor_date     difference
1    S_10     nord    626821.0 2002-09-30T00:00:00+0200    0.0
2     S_10     nord    626821.0 2002-12-05T00:00:00+0100   0.0
3    S_10     nord    626821.1 2008-07-31T00:00:00+0200   0.1
4  S_1000     nord    626496.8 2002-09-30T00:00:00+0200  0.0
5  S_1000     nord    626496.8 2002-12-05T00:00:00+0100   0.0
6  S_1000     nord    626496.9 2009-04-23T00:00:00+0200    0.1
7  S_1000     nord    626497.0 2009-11-10T00:00:00+0100   0.2
8     S_1     nord    626485.3 2002-09-30T00:00:00+0200     0.0
9    S_1     nord    626485.3 2002-12-05T00:00:00+0100      0.0
10    S_1     nord    626485.3 2003-02-07T00:00:00+0100     0.0

How can I do this procedure?????


Thanks

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