How does R treat positional arguments

Question

I'm a python guy and very new to R (so far, all I've done is copy-paste code and screen-shot the resulting, graph).

I would now like to actually learn the language so that I can draw useful plots (right now, I am trying to plot this).

In attempting my first plot, I came across this function call:

sets_options("universe", seq(from = 0, to = 25, by = 0.1))

Now, I would like to know if I can achieve the same result by calling

sets_options("universe", seq(0, 25, 0.1))

The help page for seq doesn't speak to this specifically (or I'm not reading it correctly), so I was hoping someone could shed some light on how R handles positional arguments

I tried calling the function that way in R and it worked (no syntax errors, etc), but I don't know how to test the output of that function, so I'm forced to ask here

The full details of how arguments are matched: http://cran.r-project.org/doc/manuals/r-release/R-lang.html#Argument-matching. Roughly, named arguments are matched first, then the remaining (passed) arguments are matched to the remaining (formal) arguments positionally. — Brian Diggs, Oct 26 '13 at 04:31
Some comments on when to use named vs. position at http://adv-r.had.co.nz/Functions.html#calling-functions — hadley, Oct 26 '13 at 12:42

score 0 · Answer 1 · answered Oct 26 '13 at 03:55

Calling sets_options() will display the current settings. From the following log, it seems that the positional arguments are treated as expected:

> sets_options("universe", seq(0,5,0.25))
> sets_options()
$quote
[1] TRUE

$hash
[1] TRUE

$openbounds
[1] "()"

$universe
 [1] 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00

> sets_options("universe", seq(from=0,to=5,by=0.25))
> sets_options()
$quote
[1] TRUE

$hash
[1] TRUE

$openbounds
[1] "()"

$universe
 [1] 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00

Correct. Positional arguments are always valid in R and the order that is present in python is not in R. You can also define your function args in whatever order you want. — Justin, Oct 26 '13 at 03:55

score 0 · Answer 2 · answered Oct 26 '13 at 19:08

0

The question is what seq is doing with positional versus named objects. The way to address this looking at the ?seq page which lays out the named arguments and their order:

seq(from = 1, to = 1, by = ((to - from)/(length.out - 1)),
length.out = NULL, along.with = NULL, ...)

So seq(0, 25, 0.1) will be interpreted the same way as seq(from = 0, to = 25, by = 0.1) since the order is the same as name in Usage listing.

answered Oct 26 '13 at 19:08

IRTFM

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I did read that page and try to arrive at the conclusion you propose. However, I noticed that `by` defaults to `((to - from)/(length.out - 1))`, and `length.out` defaults to `NULL`. So, I couldn't be as sure about what variables take on what values, since one of the default assignments uses a value I don't supply – inspectorG4dget Oct 26 '13 at 22:51
1

Your values replace the defaults. – IRTFM Oct 26 '13 at 23:36
But what if I don't supply a value for `((to - from)/(length.out - 1))`? – inspectorG4dget Oct 26 '13 at 23:48
Then `by` gets calculated using that expression based on what is given to `from`, `to` and `length.out` – IRTFM Oct 26 '13 at 23:56
But I still haven't specified `length.out`, which defaults to `NULL`. – inspectorG4dget Oct 27 '13 at 00:01
I guess you need to refer to the ultimate arbiter, the code for `seq.default`. – IRTFM Oct 27 '13 at 04:11
That is a fantastic idea. Thanks – inspectorG4dget Oct 27 '13 at 04:23

How does R treat positional arguments

2 Answers2