R help - May 2013 - How can I extract part of the data with a selection criterion?

Hi,

   As an example, how can I get the data such that field a of ab,
ab["a"],
equals 3? I expect the answer to be the union of 2 and 4, as

   Thanks,
> a<-c(1,3,4,3,5,6,5)
> b<-c(2,4,6,7,3,1,2)
> ab<-data.frame(a,b)
> ab
  a b
1 1 2
2 3 4
3 4 6
4 3 7
5 5 3
6 6 1
7 5 2
> ab[a==3]
Error in `[.data.frame`(ab, a == 3) : undefined columns
selected
> ab[ab["a"]==3]
[1] 3 3 4 7

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Hi,

Try ab[a==3,]

You're selecting only rows for which a==3 (before the comma), and all
columns (empty condition for column index, after the comma)

Patrick

2013/5/9 jpm miao <miaojpm at gmail.com>:
> Hi,
>
>    As an example, how can I get the data such that field a of ab,
ab["a"],
> equals 3? I expect the answer to be the union of 2 and 4, as
>
>    Thanks,
>
>> a<-c(1,3,4,3,5,6,5)
>> b<-c(2,4,6,7,3,1,2)
>> ab<-data.frame(a,b)
>> ab
>   a b
> 1 1 2
> 2 3 4
> 3 4 6
> 4 3 7
> 5 5 3
> 6 6 1
> 7 5 2
>
>> ab[a==3]
> Error in `[.data.frame`(ab, a == 3) : undefined columns selected
>> ab[ab["a"]==3]
> [1] 3 3 4 7
>
>         [[alternative HTML version deleted]]
>
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On May 9, 2013, at 7:06 PM, jpm miao wrote:
> Hi,
> 
>   As an example, how can I get the data such that field a of ab,
ab["a"],
> equals 3? I expect the answer to be the union of 2 and 4, as
> 
>   Thanks,
> 
>> a<-c(1,3,4,3,5,6,5)
>> b<-c(2,4,6,7,3,1,2)
>> ab<-data.frame(a,b)
>> ab
>  a b
> 1 1 2
> 2 3 4
> 3 4 6
> 4 3 7
> 5 5 3
> 6 6 1
> 7 5 2
> 
>> ab[a==3]
> Error in `[.data.frame`(ab, a == 3) : undefined columns selected
>> ab[ab["a"]==3]
> [1] 3 3 4 7
> 
Try;

ab[ ab[["a"]]==3, ]

And then read ?Extract to help drive home the point the ab["a"] is a
list with one element and ab[["a"]] is a vector with 7 elements.

--
David Winsemius
Alameda, CA, USA