R, subtract value from previous row, group by -

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in r, lets have data.

data id      date        value 2380    10/30/12    21.01 2380    10/31/12    22.04 2380    11/1/12     22.65 2380    11/2/12     23.11 20100   10/30/12    35.21 20100   10/31/12    37.07 20100   11/1/12     38.17 20100   11/2/12     38.97 20103   10/30/12    57.98 20103   10/31/12    60.83

and want subtract previous value of value grouped id (in order date), create this:

id      date        value   diff 2380    10/30/12    21.01   0 2380    10/31/12    22.04   1.03 2380    11/1/12     22.65   0.61 2380    11/2/12     23.11   0.46 20100   10/30/12    35.21   0 20100   10/31/12    37.07   1.86 20100   11/1/12     38.17   1.1 20100   11/2/12     38.97   0.8 20103   10/30/12    57.98   0 20103   10/31/12    60.83   2.85

you can ave function:

data$diff <- ave(data$value, data$id, fun=function(x) c(0, diff(x))) data #       id                date value diff # 1   2380 2012-10-30 00:15:51 21.01 0.00 # 2   2380 2012-10-31 00:31:03 22.04 1.03 # 3   2380 2012-11-01 00:16:02 22.65 0.61 # 4   2380 2012-11-02 00:15:32 23.11 0.46 # 5  20100 2012-10-30 00:15:38 35.21 0.00 # 6  20100 2012-10-31 00:15:48 37.07 1.86 # 7  20100 2012-11-01 00:15:49 38.17 1.10 # 8  20100 2012-11-02 00:15:19 38.97 0.80 # 9  20103 2012-10-30 10:27:34 57.98 0.00 # 10 20103 2012-10-31 12:24:42 60.83 2.85

the first argument data operated on, second argument group, , last argument function applied data each group.


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