# Function to count the number of positional arguments used in a call

`nposargs.Rd`

Calculates the number of positional arguments used in a call.

## Arguments

- x
a call object, usually obtained from

`sys.call()`

.- a
if

`a[1]`

is`TRUE`

make a correction to distinguish`x[]`

from`x[i]`

, see details.

## 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.

## Note

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))`

.

## Examples

```
f <- function(x,y,z,...){
call <- sys.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
```