[R] create data set from selection of rows

[e-letter]

On 15/03/2011, Francisco Gochez <fjgochez at googlemail.com> wrote:
> Hi,
>
> What you are after is:
>
> datasubset <- dataset[ dataset[,3] == "text3", ]

Thank you. For the set

text1,23,text2,45
text1,23,text3,78
text1,23,text3,56
text1,23,text2,45

Is it possible to write a function that selects rows containing
'text3' and applies the function 'sum' to values '78' and '56'? The
control statements described in the document 'an introduction to r'
(Venables and Smith, 2010) suggest that the if statement would return
'true', to prevent a sum function being applied to 'true' results.