[R] Seeking help with a loop

Tony Plate
Wed, 03 Aug 2005 16:00:52 -0000

> x <- data.frame(q33a=3:4,q33b=5:6,q35a=1:2,q35b=2:1)
 > y <- list()
 > for (i in grep("q33", colnames(x), value=TRUE))
+    y[[sub("q33","",i)]] <-
ifelse(x[[sub("q33","q35",i)]]==1, x[[i]], NA)
 > as.data.frame(y)
    a  b
1  3 NA
2 NA  6
 > # if you really want to create new variables rather
 > # than have them in a data frame:
 > # (use paste() or sub() to modify the names if you
 > #  want something like "newfielda")
 > for (i in names(y)) assign(i, y[[i]])
 > a
[1]  3 NA
 > b
[1] NA  6
 >

hope this helps,

Tony Plate

Greg Blevins wrote:
> Hello R Helpers,
>
> After spending considerable time attempting to write a loop (and searching
the help archives) I have decided to post my problem.
>
> In a dataframe I have columns labeled:
>
> q33a q33b q33c...q33r    q35a q35b q35c...q35r
>
> What I want to do is create new variables based on the following logic:
> newfielda <- ifelse(q35a==1, q33a, NA)
> newfieldb <- ifelse(q35b==1, q33b, NA)
> ...
> newfieldr
>
> What I did was create two new dataframes, one containing q33a-r the other
q35a-r and tried to loop over both, but I could not get any of the loop syntax I
tried to give me the result I was seeking.
>
> Any help would be much appreciated.
>
> Greg Blevins
> Partner
> The Market Solutions Group, Inc.
> Minneapolis, MN
>
> Windows XP, R 2.1.1
>
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