[R] Using ifelse and grep

William Dunlap
Sat, 03 Apr 2010 16:55:01 -0000

> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On Behalf Of Sam Albers
> Sent: Saturday, April 03, 2010 9:18 AM
> To: r-help at r-project.org
> Subject: [R] Using ifelse and grep
>
> Good Morning,
>
> I am trying to create a new column of character strings based
> on the first
> two letters in a string in another column. I believe that I
> need to use some
> combination of ifelse and grep but I am not totally sure how
> to combine
> them. I am not totally sure why the command below isn't
> working. Obviously
> it isn't finding anything that matches my criteria but I am
> not sure why.
> Any ideas on how I might be able to modify this to get to
> work? Below is
> also a data example of what I would like to achieve with this command.
>
> > section <- ifelse(Sample==grep("^BU",
Sample),"up",
> ifelse(Sample==grep("^BM", Sample),
"mid","down"))
> > section
>  [1] "down" "down" "down" "down"
"down" "down" "down" "down"
> "down" "down"
> [11] "down" "down"

I'm not sure what Sample contains, but if you break this
nested set of functions calls down you can see what the problem is:
grep() returns the positions of strings that match the pattern.

E.g., if you have
    Sample <- c("BU1", "BU2", "BM1",
"BD1", "BU3")
then grep("^BU", Sample) returns c(1,2,5) and
Sample==c(1,2,5) doesn't make much sense.

If you instead grepl in a subscript, as in
   section <- character(length(Sample))
   section[grepl("^BU", Sample)] <- "up"
   section[grepl("^BM", Sample)] <- "mid"
   section[grepl("^BD", Sample)] <- "down"
then section is
c("up","up","mid","down","up").

There are lots of ways to make this mapping, especially
for such a simple set of patterns (they don't overlap
and they are all the initial 2 characters of a string).
E.g.,
   map <- c(BU="up", BM="mid", BD="down")
   section <- unname(map[substring(Sample, 1, 2)])
or
   from <- c("BU", "BM", "BD")
   to <- c("up", "mid", "down")
   section <- to[match(substring(Sample,1,2), from)]

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

>
> Thanks in advance.
>
> Sam
>
>  Sample Transmission section  BU1 0.39353 up  BU2 0.38778 up
> BU3 0.42645 up
> BM1 0.37510 mid  BM2 0.5103 mid  BM3 0.67224 mid  BD1 0.37482
> down  BD2
> 0.54716 down  BD3 0.50866 down  BU1 0.34869 up  BU2 0.32831
> up  BU3 0.59877
> up  BM1 0.52518 mid  BM2 0.94387 mid  BM3 0.94387 mid  BD1
> 0.46872 down  BD2
> 0.63115 down  BD3 0.45239 down
> n" "down" "down" "down" "down"
"down" "down"
>
>
>
>
> --
> *****************************************************
> Sam Albers
> Geography Program
> University of Northern British Columbia
> 3333 University Way
> Prince George, British Columbia
> Canada, V2N 4Z9
> phone: 250 960-6777
> *****************************************************
>
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