rowSums on one column

Question

Question

Is there a function, or a way to get rowSums to work on only one column?

Example Data

col1 <- c(1,2,3)
col2 <- c(1,2,3)
df <- data.frame(col1, col2)

I can use rowSums to sum each value in a row for two or more defined columns:

colsToAdd <- c("col1", "col2")
rowSums(df[,colsToAdd])
[1] 2 4 6 

However, this fails when only on column is passed

colsToAdd <- c("col1")
rowSums(df[,colsToAdd])
Error in rowSums(df[, colsToAdd]) : 
  'x' must be an array of at least two dimensions

which makes sense when looking at the rowSums() function:

> rowSums
function (x, na.rm = FALSE, dims = 1L) 
{
    if (is.data.frame(x)) 
        x <- as.matrix(x)
    if (!is.array(x) || length(dn <- dim(x)) < 2L) 
    ## This line 'stops' the function if only one column is passed

Additional information

The columns will be selected by a user in a Shiny app and stored in a variable. This variable is then passed to rowSums. The user can select one or more columns.

I could construct an ifelse statement to check the length of the variable, but was wondering if there is a better/more elegant/single line solution that I'm not seeing?

Answer 1

You could try

rowSums(df[,colsToAdd, drop=FALSE])

According to ?'[', by default,

x[i, j, ... , drop = TRUE]

Based on the documentation

drop: For matrices and arrays. If ‘TRUE’ the result is coerced to the lowest possible dimension