That worked perfectly.  Thank you Dennis - I very much appreciate the  
help!

Sara Maxwell, PhD, Postdoctoral Fellow
Marine Conservation Institute
University of California Santa Cruz
Long Marine Laboratory
100 Shaffer Road
Santa Cruz CA 95060 USA
+1 206 355 3249
Sara.Maxwell@Marine-Conservation.org
www.Marine-Conservation.org



On May 25, 2011, at 12:58 PM, Dennis Murphy wrote:

> 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@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
>>
>>
>>        [[alternative HTML version deleted]]
>>
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>


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