[R] Choosing columns by number

[K. Elo]

Hi!

25.08.2015, 18:17, Sam Albers wrote:
> Hi all,
>
> This is a process question. How do folks efficiently identify column
> numbers in a dataframe without manually counting them. For example, if I
> want to choose columns from the iris dataframe I know of two options. I can
> do this:
>
>> str(iris)'data.frame':	150 obs. of  5 variables:
>   $ Sepal.Length: num  5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ...
>   $ Sepal.Width : num  3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ...
>   $ Petal.Length: num  1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ...
>   $ Petal.Width : num  0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ...
>   $ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1
> 1 1 1 1 1 1 ...
>
> or this:
>
>> names(iris)[1] "Sepal.Length" "Sepal.Width"  "Petal.Length" "Petal.Width"  "Species"
>
> Neither option explicitly identifies the column number so that I can
> do something like this:
>
> iris[,c(2,4)]
>
> I feel like there must be a better way to do this so I wanted to ask
> the collective wisdom here what people do to accomplish this.
> Obviously this is a trivial example, but the issue really becomes
> problematic when you have a large dataframe.

Maybe with 'which'?

 > which(colnames(iris)=="Sepal.Length")
[1] 1

Or did I somehow misunderstood what you are looking for?

HTH,
Kimmo