Question

I have a data frame with several columns; some numeric and some character. How to compute the sum of a specific column? I’ve googled for this and I see numerous functions (sum, cumsum, rowsum, rowSums, colSums, aggregate, apply) but I can’t make sense of it all.

For example suppose I have a data frame people with the following columns

people <- read.table(
  text = 
    "Name Height Weight
    Mary 65     110
    John 70     200
    Jane 64     115", 
  header = TRUE
)
…

How do I get the sum of all the weights?

Answer 1

You can just use sum(people$Weight).

sum sums up a vector, and people$Weight retrieves the weight column from your data frame.

Note – you can get built-in help by using ?sum, ?colSums, etc. (by the way, colSums will give you the sum for each column).