[R] creating and then executing command strings

[Philipp Schmidt]

Greg:

Thanks for this concise explanation! I will have a look at the
fortunes you mention. Best - P

On Fri, May 15, 2009 at 7:36 PM, Greg Snow <Greg.Snow at imail.org> wrote:
> The arrow "<-" is used to assign a value to a variable, the equals sign "=" is used to specify the value for a function argument.  Recent versions of R allow "=" to be used for "<-" at the top level and certain circumstances which some people find more convenient, but can also lead to confusion (purists always keep them separate).
>
> The code:
>
>> parse( text <- paste( ...
>
> Will take the results of paste, save them in a variable named text, then pass a copy to the first argument of parse, which is file, not text, so parse will just get confused (looking for a file named what your code is).
>
> The code:
>
>> parse( text = paste( ...
>
> Will take the results of paste and pass them to the parse function as the text argument.
>
> But having said that, you should refer to fortune(106) (type that after loading the fortunes package) and possibly fortune(181).
>
> There are probably better ways to do what you want, Romain's second example is one way.
> --
> Gregory (Greg) L. Snow Ph.D.
> Statistical Data Center
> Intermountain Healthcare
> greg.snow at imail.org
> 801.408.8111
>
>
>> -----Original Message-----
>> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
>> project.org] On Behalf Of Philipp Schmidt
>> Sent: Friday, May 15, 2009 8:35 AM
>> To: Romain Francois
>> Cc: r-help at r-project.org
>> Subject: Re: [R] creating and then executing command strings
>>
>> On Fri, May 15, 2009 at 3:38 PM, Romain Francois
>> <romain.francois at dbmail.com> wrote:
>> > Hi,
>> >
>> > You can either parse and eval the string you are making, as in:
>> >
>> > eval( parse( text = paste("avg_",colname, " <- 0;", sep='') ) )
>> >
>> >
>> > Or you can do something like this:
>> >
>> > df[[ paste( "avg_", colname, sep = "" ) ]] <- 0
>> >
>>
>> Thanks you so much! I used the first version and it worked.
>>
>> What puzzles me, is that I am not able to use <- instead of = (my R
>> book says the two can be exchanged) or break the command into
>> different parts and execute them one after another.
>>
>> I get various error messages when I try:
>>
>> eval( parse( text <- paste("avg_",colname, " <- 0;", sep='') ) )
>>
>> or
>>
>> text = paste("avg_",colname, " <- 0;", sep='')
>> parse(text)
>> eval(parse(text))
>>
>> Anyway, thanks a lot - you greatly improved the likelihood of me not
>> working on the weekend!
>>
>> Best - P
>>
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>



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