zoo create new column with dynamic column name

Question

I am trying to add a column to a zoo object. I found merge which works well

test = zoo(data.frame('x' = c(1,2,3)))
test = merge(test, 'x1' = 0)

However when I try to name the column dynamically, it no longer works

test = merge(test, paste0('x',1) = 0)
Error: unexpected '=' in "merge(test,paste0('x',1) ="

I have been working with data frames and the same syntax works

test = data.frame('x' = c(1,2,3))
test[paste0('x',1)] = 0

Can someone help explain what the problem is and how to get around this?

Answer 1

Try setNames :

setNames( merge(test, 0), c(names(test), paste0("x", 1)) )

or names<-.zoo like this:

test2 <- merge(test, 0)
names(test2) <- c(names(test), paste0("x", 1))

Answer 2

I found this solution very easy and elegant. It uses the eval() function to interpret a string as an R command. Thus, you are completely free to assemble the string exactly the way you want:

test = merge(test, paste0("x",1) = 0)
# does not work (see question)

test[,"x1"] <- 0
# does not work for uninitialized columns

test$x1 <- 0
# works to initialize a new column

# so lets trick R by assembling this command out of strings:
newcolumn <- "x1"
eval(parse(text=paste0("test$",newcolumn," <- 0")))
# welcome test$x1 :-)

Answer 3

Merge expects a string as variable name, it doesn't understand variable names that are return values of functions. Why not

test = zoo(data.frame('x' = c(1,2,3)))
var <- paste0('x',1)
test = merge(test, var = 0)