Calculates the number of positional arguments used in a call.

nposargs(x, a = FALSE)



a call object, usually obtained from


if a[1] is TRUE make a correction to distinguish x[] from x[i], see details.


nposargs is mainly for use in the body of function definitions, paricularly for functions or methods that wish to mimic the behaviour of "[".

nposargs gives the number of positional arguments used in a call. It also takes into account empty arguments like those used in expressions like x[1, ].

Optionally, it makes a particular correction that is peculiar for "[" - if there are no named arguments in the call and the count of the arguments is 2 and a[1]=TRUE, it decreases the count by one, i.e. returns 1. This is to distinguish between a x[] and x[i] which both would give 2 otherwise. I have forgotten the details but, roughly speaking, x[i] becomes "["(x,i) while x[] becomes "["(x,), i.e. R puts the comma after x in any case.


the number of positional arguments in the call


I wrote this function (a long time ago) for use in methods for "[".

a[1] above is typically obtained by a call missing(i) somewhere at the beginning of the function. In my application I put the results of several such calls in a vector, hence the check for a[1] rather than a, For "[", we may set a = c(missing(i), missing(j), missing(k)).


f <- function(x,y,z,...){ call <- nposargs(call) } f(a,b,c) # 3
#> [1] 3
f(a, , ) # 3
#> [1] 3
f(a, ) # 2
#> [1] 2
f(a) # 1
#> [1] 1
f(, ) # 2
#> [1] 2
f(, a, ) # 3
#> [1] 3
f() # 0
#> [1] 0