[R] Choosing columns by number

[Sam Albers]

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.

Thanks in advance!

Sam

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