From murdoch.duncan at gmail.com  Fri Sep 30 15:56:00 2011
From: murdoch.duncan at gmail.com (Duncan Murdoch)
Date: Fri, 30 Sep 2011 09:56:00 -0400
Subject: [Rd] documenting a behavior of adding named item to a list
In-Reply-To: <CAE9ddGCmEwx1t7aATrGYrBeKm-eh3azZbnB5wtnVvghkuZkboQ@mail.gmail.com>
References: <CAE9ddGCmEwx1t7aATrGYrBeKm-eh3azZbnB5wtnVvghkuZkboQ@mail.gmail.com>
Message-ID: <4E85CA70.30102@gmail.com>

On 29/09/2011 11:42 AM, Suraj Gupta wrote:
> Folks,
>
> Per a post to StackOverflow, I'm looking for an opinion on whether something
> should be documented:
> When adding a named item to a list, its guaranteed that the item will be
> added to the end of the list.

I'd say not.  There's an easy way to guarantee the order:  instead of using

mylist$newname <- 1

use

mylist <- c(mylist, newname=1)

The former construction doesn't necessarily put it last, only when 
newname is a new name.  The latter guarantees that it goes last.

Currently, you can do something like this:

x <- list(1)   # one element list
x[[10]] <- 10  # grows x to a 10 element list, filling in NULLs in 
positions 2 to 9
x$newname <- 11  # puts the newname in the 11th position

It's not obvious to me that the newname shouldn't go into the 2nd 
position (if I ignore the comments).  I wouldn't want to rule that out 
as a future change.

Duncan Murdoch

> SO post:
> http://stackoverflow.com/questions/7599349/adding-named-item-to-list-guaranteed-to-append-to-end-of-list/7599610#7599610
>
> This is my first email to r-devel, so I hope this is an appropriate
> question.
>
> -Suraj
>
> 	[[alternative HTML version deleted]]
>
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[Rd] documenting a behavior of adding named item to a list
Duncan Murdoch