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

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

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

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