Specify which argument is supplied by X in lapply

Question

I've got a function, e.g. fun(a, b = 1, c = 3, ...), that takes a number of arguments, some of which have default values. I would like to call this function using lapply(X, FUN, ...), but specify explicitly which argument I would like X to supply. In the example above, the X vector could be supplied for a or b or c, or xyz in the ....

Normally I might call lapply(1:5, fun, a = 4) and I imagine it would use 1:5 as the b argument.

• Is there a way to make that more explicit?
• What if I want to use the default argument for b and use 1:5 for c?
• What if I want to use 1:5 as an xyz argument in the ...?

Sigrid

Please provide a reproducible example and expected output – Dissimilar

The general syntax should work lapply(1:5,function(x,a=4,..,) { } ) – Erick

Why would you want to pass other variables to the function? lapply itself will only give you a single entry from the list on which it is operating. – Ribband

It assigns to the first otherwise unassigned parameter. If you want to be super-clear–or the parameter you want to assign to is not the next one–just wrap the call in an anonymous function. – Lakshmi

Answer 1

Normally I might call lapply(1:5, fun, a = 4) and I imagine it would use 1:5 as the b argument.

Yes, your imagination is correct. lapply uses positional matching to pass its X parameter to the function. Normal rules of argument matching apply, which means exact matching of named parameters takes precedence.

An alternative would of course be to wrap fun in an anonymous function:

lapply(1:5, function(b, a, ...) fun(a = a, b = b, ...), a = 4)

Zabrina

Answer 2

One way to handle you use case would be to simply call your own function inside the custom function which lapply exposes to you:

lst <- list(v1=c(1:3), v2="Hello", v3=5)

result <- lapply(lst, function(x) {
                         y <- FUN(x, a, b, c, ...)   # Here FUN() is your own function
                         return(y)
                      })

Ribband

Answer 3

You can use mapply to specify static and dynamic arguments explicitly:

mapply(cbind, a = "a constant", b = 1:5, SIMPLIFY = FALSE)

Toplevel