Adding multiple columns, transforming with multiple variables

Question

How do I add the values from many variables?

If I just had two variables (columns) I could simply go:

summation.variable <- variable1 + variable2

or if it was all in a dataframe:

transform(dataframe, summation.col = column1 + column2)

How do I do it if I have about 10 variables and I do not want to type each one as in col1+col2+col3+col4. To make matters worse my columns have quite long names and at times the exact columns that I use can change. I have a character vector with all the relevant column names in it but cannot think how to use it.

The following is useless since it adds every value in every column in every row and gives a single value for the whole lot.

sum(metrics)

Answer 1

You want to use rowSums (see the indexing with a character vector.)

tmp <- data.frame(a=1:2,b=3:4,d=5:6)
rowSums(tmp[,c("a","d")])

or, more generally, apply:

apply(tmp[,c("a","d")], 1, sum)

Answer 2

There are many ways to do this kind of operation (ie. apply a function across a row or column), but as Eduardo points out, apply is the most basic:

tmp <- data.frame(a=1:2,b=3:4,d=5:6)
apply(tmp, 1, prod)

This is a very flexible function. For instance, you can do both operations at once with this call:

apply(tmp, MARGIN=1, function(x) c(sum(x), prod(x)))

Performing the same analysis across columns is also simple (the MARGIN parameter describes whether you use rows or columns):

apply(tmp, MARGIN=2, function(x) c(sum(x), prod(x)))