Hi all, I am trying to write a loop to return the matched index for a column of values. Here is an simple example of this, A B 0 5 1 2 2 2 3 4 4 1 5 4 6 2 In this case, A is an index column for B. Now I have this new column C with just two values of 2 and 4. I want to match column C with column B, and return the matched indexes. So what I desire is to return: [1] 1 2 6 [2] 3 5 Since value 2 corresponds to indexes 1,2,6, and 4 corresponds to indexes 3,5. Is there any way to write a loop to have this done? Thank you for the help. Xueke
simple loop questions
3 messages · xueke at pdx.edu, David Winsemius
On Apr 23, 2011, at 6:31 AM, <xueke at pdx.edu> wrote:
Hi all, I am trying to write a loop to return the matched index for a column of values. Here is an simple example of this, A B 0 5 1 2 2 2 3 4 4 1 5 4 6 2 In this case, A is an index column for B. Now I have this new column C with just two values of 2 and 4. I want to match column C with column B, and return the matched indexes. So what I desire is to return: [1] 1 2 6 [2] 3 5 Since value 2 corresponds to indexes 1,2,6, and 4 corresponds to indexes 3,5.
Assuming this to be in a data.frame named dat: > dat$A[which(dat$B==2) ] [1] 1 2 6 > dat$A[which(dat$B==4) ] [1] 3 5 Results of varying lengths generally need to be returned in list form: > sapply(c(2,4), function(x) dat$A[which(dat$B==x) ] ) [[1]] [1] 1 2 6 [[2]] [1] 3 5
Is there any way to write a loop to have this done? Thank you for the help. Xueke
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David Winsemius, MD West Hartford, CT
1 day later
On Apr 23, 2011, at 9:34 AM, David Winsemius wrote:
On Apr 23, 2011, at 6:31 AM, <xueke at pdx.edu> wrote:
Hi all, I am trying to write a loop to return the matched index for a column of values. Here is an simple example of this, A B 0 5 1 2 2 2 3 4 4 1 5 4 6 2 In this case, A is an index column for B. Now I have this new column C with just two values of 2 and 4. I want to match column C with column B, and return the matched indexes. So what I desire is to return: [1] 1 2 6 [2] 3 5 Since value 2 corresponds to indexes 1,2,6, and 4 corresponds to indexes 3,5.
Assuming this to be in a data.frame named dat:
dat$A[which(dat$B==2) ]
[1] 1 2 6
I've been reminded that in many instances one gets comparable results with dat$A[ dat$B==2 ] (logical indexing). The reason I choose to use which() is that it returns a numeric vector and leaves out any NA's that result from dat$B==2. the "[" function returns all logical NA rows. It may be important to return such NA's to alert the analyst to their presence. (This does result in extensive/ useless/unexpected screen output when large datasets with even a small faction of NA's are being manipulated.)
David > > dat$A[which(dat$B==4) ] > [1] 3 5 > > Results of varying lengths generally need to be returned in list form: > > > sapply(c(2,4), function(x) dat$A[which(dat$B==x) ] ) > [[1]] > [1] 1 2 6 > > [[2]] > [1] 3 5 > >> David Winsemius, MD West Hartford, CT