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

[Mark Knecht]

On Tue, Jun 30, 2009 at 3:57 PM, Duncan Murdoch<murdoch at stats.uwo.ca> wrote:
> 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
>

OK, clearly I have some learning to do here. Only my 3rd day with R.

Your response and Phil's are the same and they both work which is
great. The following evaluates correctly TRUE or FALSE looking at the
data.frame. Good so far.

pos <- MyFrame$y > 0

But how does the following actually work? What's it doing?

MyFrame$p[pos] <- MyFrame$y[pos]

Anyway, I have a solution which is great. Thanks. And now I'll try to
get comfortable with how these last two equations actually work.

Cheers,
Mark.


>>
>> 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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>
>