[R] Subtracting rows by id

[Dennis Murphy]

Hi:

Interesting problem. Here's one approach:

library(plyr)
# Read in your datasets as data frames rather than matrices
dataset1 <- data.frame(id1 = rep(1:3, each = 10),
                       value1 = sample(seq_len(100), 30, replace = TRUE))
dataset2 <- data.frame(id2 = 1:3, subtract.value = c(1, 3, 5))

# The idea is to use the rows of dataset2 as parameters for
# subsetting and removing the first n_i rows. The tail() function
# serves the purpose:
foo <- function(id2, subtract.value) tail(subset(dataset1, id1 ==
id2), -subtract.value)

# Use the mdply function in the plyr package:
> mdply(dataset2, foo)[, -(1:2)]
   id1 value1
1    1      2
2    1     55
3    1     18
4    1      4
5    1      3
6    1     76
7    1     74
8    1     21
9    1     97
10   2     19
11   2     49
12   2     20
13   2     73
14   2     79
15   2     95
16   2     52
17   3     60
18   3     58
19   3     68
20   3     59
21   3     13


HTH,
Dennis

On Wed, May 25, 2011 at 9:55 AM, Sara Maxwell <smaxwell at ucsc.edu> wrote:
> Dear R users,
>
> I have two datasets:
>
> id1 <- c(rep(1,10), rep(2,10), rep(3,10))
> value1 <- sample(1:100, 30, replace=TRUE)
> dataset1 <- cbind(id1,value1)
>
> id2 <- c(1,2,3)
> subtract.value <- c(1,3,5)
> dataset2 <- cbind(id2, subtract.value)
>
> I want to subtract the number of rows in the subtract.value that
> corresponds to the id value in dataset1.  So for the 1 in id1, I want
> to remove the first row, for 2 in id1 I want to remove the first 3
> rows, for 3 in id1 I want to remove the first 5 rows, finally creating
> a new dataframe with the remaining values.
>
> I am having trouble structuring a loop that can do this by the unique
> ids in the first dataset while matching the ids in the datasets.
>
> Any thoughts would be greatly appreciated.
>
> Thank you,
> Sara
>
>
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>
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