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

[Duncan Murdoch]

On 30/06/2009 7:13 PM, Mark Knecht wrote:
> 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]

It uses the logical vector pos to index two other vectors.  On the right 
hand side, that selects all the entries of y where pos is TRUE.  On the 
left hand side, that says to replace all the entries of p where pos is 
TRUE.  Since y and p are the same length (both being columns of a data 
frame), that's the same thing as replacing p by y in those cases where 
pos is TRUE.

Duncan Murdoch

> 
> 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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>>