[R] Unexpected behavior when giving a value to a new variable basedon the value of another variable

[David Winsemius]

On Aug 29, 2014, at 8:54 PM, David McPearson wrote:

> On Fri, 29 Aug 2014 06:33:01 -0700 Jeff Newmiller <jdnewmil at dcn.davis.ca.us 
> >
> wrote
>
>> One clue is the help file for "$"...
>>
>> ?" $"
>>
>> In particular there see the discussion of character indices and the  
>> "exact"
>> argument.
>>
>
> <...snip...>
>>
>> On August 29, 2014 1:53:47 AM PDT, Angel Rodriguez
>> <angel.rodriguez at matiainstituto.net> wrote: >
>>> Dear subscribers,
>>>
>>> I've found that if there is a variable in the dataframe with a name
> <...sip...>
>>>> N <- structure(list(V1 = c(67, 62, 74, 61, 60, 55, 60, 59, 58),  
>>>> V2 =
>>> c(NA, 1, 1, 1, 1,1,1,1,NA)),
>>> +                     .Names = c("age","samplem"), row.names = c(NA,
>>> -9L), class = "data.frame")
>>>> N$sample[N$age >= 65] <- 1
>>>> N
>>> age samplem sample
>>> 1  67      NA      1
>>> 2  62       1      1
>>> 3  74       1      1
>>> 4  61       1      1
>>> 5  60       1      1
>>> 6  55       1      1
>>> 7  60       1      1
>>> 8  59       1      1
>>> 9  58      NA     NA
> <...snip...>
>
> Having seen all the responses about partial matching I almost  
> understand. I've
> also replicated the behaviour on R 2.11.1 so it's been around  
> awhile. This
> tells me it ain't a bug - so if any of the cognoscenti have the time  
> and
> inclination can someone give me a brief (and hopefully simple)  
> explanation of
> what is going on under the hood?
>
> It looks (to me) like N$sample[N$age >= 65] <- 1 copies N$samplem to  
> N$sample
> and then does the assignment. If partial matching is the problem  
> (which it
> clearly is) my expectation is that  the  output should look like
>
>   age samplem
> 1   67       1
> 2   62       1
> 3   74       1
> 4   61       1
> 5   60       1
> 6   55       1
> 7   60       1
> 8   59       1
> 9   58      NA
> That is - no new column.
> (and I just hate it when the world doesn't live up to my  
> expectations!)

Not sure what you are seeing. I am seeing what you expected:

 > test <- data.frame(age=1:10, sample=1)
 > test$sample[test$age<5] <- 2
 > test
    age sample
1    1      2
2    2      2
3    3      2
4    4      2
5    5      1
6    6      1
7    7      1
8    8      1
9    9      1
10  10      1

-- 
David
>
> Bewildered and confused,
> DMcP
>
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David Winsemius, MD
Alameda, CA, USA