R help - Oct 2011 - How to create a new variable based on parts of another character variable.

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Use regular expressions

?grepl

On Sunday, October 23, 2011, Philipp Fischer <Philipp.Fischer@awi.de>
wrote:
> Hello,
> I am just starting with R and I am having a (most probably) stupid problem
by creating a new variable in a data.frame based on a part of another
character variable.
>
> I have a data frame like this one:
>
>
> A                       B               C
> AWI-test1       1               i
> AWI-test5       2               r
> AWI-tes75       56              z
> UFT-2           5               I
> UFT56           f               t
> UFT356          9j              t
> etc. etc.               89              t
>
>
> I now want to look in the variable A if the string AWI is present and then
create a variable D and putting "Arctic" inside. However, if the
string UFT
occurs in the variable A, then the variable D shall be "Boreal" etc.
etc.
>
> The resulting data.frame file should look like
> A                       B               C       D
> AWI-test1       1               i       Arctic
> AWI-test5       2               r       Arctic
> AWI-tes75       56              z       Arctic
> UFT-2           5               I       Boreal
> UFT56           f               t       Boreal
> UFT356          9j              t       Boreal
> etc. etc.               89              t
>
>
> I know how to do this when I want to look for the entire string of A means
when there is "AWI-test1" and then create the variable D with
"Arctic" but
not how to look only for a substring in A?
> Would be great if somebody might help.
> Thanks
> Philipp
>
>
>
> ***************************************************
>
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
-- 
Jim Holtman
Data Munger Guru

What is the problem that you are trying to solve?

	[[alternative HTML version deleted]]

On 10/24/2011 12:35 AM, Philipp Fischer wrote:
> Hello,
> I am just starting with R and I am having a (most probably) stupid problem
by creating a new variable in a data.frame based on a part of another character
variable.
>
> I have a data frame like this one:
>
>
> A			B 		C
> AWI-test1	1		i
> AWI-test5	2		r
> AWI-tes75	56		z
> UFT-2		5		I
> UFT56		f		t
> UFT356		9j		t
> etc. etc.		89		t
>
>
> I now want to look in the variable A if the string AWI is present and then
create a variable D and putting "Arctic" inside. However, if the
string UFT occurs in the variable A, then the variable D shall be
"Boreal" etc. etc.
>
> The resulting data.frame file should look like
> A			B 		C	D
> AWI-test1	1		i	Arctic	
> AWI-test5	2		r	Arctic
> AWI-tes75	56		z	Arctic
> UFT-2		5		I	Boreal
> UFT56		f		t	Boreal
> UFT356		9j		t	Boreal
> etc. etc.		89		t
>
>
Hi Philipp,
Since you mentioned that you were just starting with R, it might be a 
little optimistic to throw you into the regular expression cage and 
expect you to emerge unscathed. You can do this by constructing a 2 
column matrix or data frame of replacement values:

replacements<-matrix(c("AWI","UFT","Arctic","Boreal"),ncol=2)
replacements
      [,1]  [,2]
[1,] "AWI" "Arctic"
[2,] "UFT" "Boreal"

Then write a function using grep to replace the values:

swapLabels<-function(x,y) {
  for(swaprow in 1:dim(y)[1])
   if(length(grep(y[swaprow,1],x))) return(y[swaprow,2])
  return(NA)
}

Finally, apply the function to the first row of the data frame:

pf.df$D<-unlist(lapply(pf.df[,1],swapLabels,replacements))
pf.df$D
[1] "Arctic" "Arctic" "Arctic" "Boreal"
"Boreal" "Boreal"

Jim

Hi

If you want to get rid of regular expressions at all and your A values 
start AWI for Arctic and UFT for boreal you can

DF$D <- ifelse(substr(DF$A, 1,1) == "A", "Arctic",
"Boreal")

Regards
Petr


> 
> Hello,
> I am just starting with R and I am having a (most probably) stupid 
problem
> by creating a new variable in a data.frame based on a part of another 
> character variable.
> 
> I have a data frame like this one:
> 
> 
> A         B       C
> AWI-test1   1      i
> AWI-test5   2      r
> AWI-tes75   56      z
> UFT-2      5      I
> UFT56      f      t
> UFT356      9j      t
> etc. etc.      89      t
> 
> 
> I now want to look in the variable A if the string AWI is present and 
then
> create a variable D and putting "Arctic" inside. However, if the
string
> UFT occurs in the variable A, then the variable D shall be
"Boreal" etc.
etc.
> 
> The resulting data.frame file should look like 
> A         B       C   D
> AWI-test1   1      i   Arctic 
> AWI-test5   2      r   Arctic
> AWI-tes75   56      z   Arctic
> UFT-2      5      I   Boreal
> UFT56      f      t   Boreal
> UFT356      9j      t   Boreal
> etc. etc.      89      t
> 
> 
> I know how to do this when I want to look for the entire string of A 
means
> when there is "AWI-test1" and then create the variable D with
"Arctic"
but
> not how to look only for a substring in A?
> Would be great if somebody might help.
> Thanks
> Philipp
> 
> 
> 
> *************************************************** 
> 
> 
>    [[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.