[R] Using functions to change values in a data.frame

[Duncan Murdoch]

On 30/06/2009 6:47 PM, Mark Knecht wrote:
> I'm having trouble with something that looks easy. (And I'm sure it
> will be easier within about 1 minute of receiving my first response.)
> Thanks in advance.
> 
> I have a collection of data frames that I need to add columns, do some
> calculations and then fill in the new columns. Since I have a large
> number of similar data frames I want to do this with functions to make
> the code more readable and ensure that every frame is done the same
> way. However my simple example doesn't work as I expected it to:
> 
> Here the R example code to cut and paste. The basic idea is to copy
> positive values of y into p and negative values into l.
> 
> <START COPY>
> 
> AddCols = function (MyFrame) {
> 	MyFrame$p<-0
> 	MyFrame$l<-0
> 	return(MyFrame)
> }
> 
> BinPosNeg = function (MyFrame) {
> 	ifelse(MyFrame$y>0,  MyFrame$p<-MyFrame$y,  MyFrame$l<MyFrame$y)
> 	return(MyFrame)
> }
> 
> F1 <- data.frame(x=1:10, y=-4:5)
> F1
> F1 <- AddCols(F1)
> F1
> F1 <- BinPosNeg(F1)
> F1
> 
> <END COPY>
> 
> My results below are weird. After the last function call F1 acts like
> BinPosNeg always evaluated MyFrame$y>0. All updates went into p - none
> into l.
> 
> What am I doing wrong?

Misunderstanding ifelse().  It evaluates both value args, and returns 
the elements from one of them, depending on the corresponding element of 
the condition.  So you don't want it.  I think you want

   pos <- MyFrame$y > 0
   MyFrame$p[pos] <- MyFrame$y[pos]
   MyFrame$l[!pos] <- MyFrame$y[!pos]

Duncan Murdoch

> 
> Thanks,
> Mark
> 
> 
> MY RESULTS:
> 
>> AddCols = function (MyFrame) {
> + MyFrame$p<-0
> + MyFrame$l<-0
> + return(MyFrame)
> + }
>> BinPosNeg = function (MyFrame) {
> + ifelse(MyFrame$y>0,  MyFrame$p<-MyFrame$y,  MyFrame$l<MyFrame$y)
> + return(MyFrame)
> + }
>> F1 <- data.frame(x=1:10, y=-4:5)
>> F1
>     x  y
> 1   1 -4
> 2   2 -3
> 3   3 -2
> 4   4 -1
> 5   5  0
> 6   6  1
> 7   7  2
> 8   8  3
> 9   9  4
> 10 10  5
>> F1 <- AddCols(F1)
>> F1
>     x  y p l
> 1   1 -4 0 0
> 2   2 -3 0 0
> 3   3 -2 0 0
> 4   4 -1 0 0
> 5   5  0 0 0
> 6   6  1 0 0
> 7   7  2 0 0
> 8   8  3 0 0
> 9   9  4 0 0
> 10 10  5 0 0
>> F1 <- BinPosNeg(F1)
>> F1
>     x  y  p l
> 1   1 -4 -4 0
> 2   2 -3 -3 0
> 3   3 -2 -2 0
> 4   4 -1 -1 0
> 5   5  0  0 0
> 6   6  1  1 0
> 7   7  2  2 0
> 8   8  3  3 0
> 9   9  4  4 0
> 10 10  5  5 0
> 
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