[R] Create macro_var in R

Fri Feb 5 19:30:21 CET 2016

You REALLY NEED to read the "Introduction to R" document discussion of indexing. 

tab is a variable. It is apparently a data.frame. tab[[mvar]] is an expression that retrieves part of the data in the tab data.frame. The data it returns is a vector, not a data.frame. 

The "[[" operator extracts an element of a list.  A data.frame is a list of vectors (all of the same length). A vector of mode "numeric" is just numbers, not a list. 

You created a new variable new.pop that holds a numeric vector. You printed it and confirmed that that is what it is. 

You then tried to refer to a variable that you have NOT created, new.tab. However,  if you had tried the expression new.pop[[mvar]] you would have been trying to treat a numeric vector as a list, which it is not... and if it was it would have to have an element named pop inside it already to extract something, which it doesn't. 

A key step in getting out of your state of confusion is to learn how objects can contain other objects, and when to work with containing objects and when to work work contained objects. 

Some possible solutions:

new.tab <- data.frame( pop=new.pop )

or

new.tab <- data.frame( pop = tab[[mvar]] )

or

new.tab <- tab[ , "pop", drop=FALSE )

With which you can then add new columns

new.tab$pop2 <- new.pop ^2
new.pop[[ "pop3" ]] <- new.pop^3

-- 
Sent from my phone. Please excuse my brevity.

On February 5, 2016 8:53:28 AM PST, Amoy Yang via R-help <r-help at r-project.org> wrote:
>
>
>One more question (see below). I cannot use macro-var, mvar, for
>creating new name, as shown below. Any advice is highly appreciated!
>
>> mvar<-"pop"
>> new.pop<-tab[[mvar]]; new.pop
> [1]  698 1214 1003 1167 2549  824  944 1937  935  570    0
>> new.tab[[mvar]]<-d$pop; 
>Error in new.tab[[mvar]] <- d$pop : object 'new.tab' not found 
>
>On Thursday, February 4, 2016 11:02 AM, Amoy Yang <amoy_y at yahoo.com>
>wrote:
> 
>
>This works although it looks rare by using min(",key,"). Don't know why
>but just have to remember it. This is a tough part in R.
>Thanks for helps!
>Amoy 
>
>On Wednesday, February 3, 2016 5:25 PM, Gabor Grothendieck
><ggrothendieck at gmail.com> wrote:
> 
>
> See
>
>  Example 5.  Insert Variables
>
>on the sqldf home page.
>
>  https://github.com/ggrothendieck/sqldf
>
>
>On Wed, Feb 3, 2016 at 2:16 PM, Amoy Yang via R-help
><r-help at r-project.org> wrote:
>> First, MVAR<-c("population) should be the same as "population'".
>Correct?
>> You use tab[[MVAR]] to refer to "population" where double [[...]]
>removes double quotes "...", which seemingly work for r-code although
>it is tedious in comparison direct application in SAS %let
>MVAR=population. But it does not work for sqldef in R as shown below.
>>
>>> key<-"pop"
>>> library(sqldf)
>>> sqldf("select grade, count(*) as cnt, min(tab[[key]]) as min,
>> + max(pop) as max, avg(pop) as mean, median(pop) as median,
>> + stdev(pop) as stdev from tab group by grade")
>> Error in sqliteSendQuery(con, statement, bind.data) :
>>  error in statement: near "[[key]": syntax error
>>
>>
>>
>>
>>    On Wednesday, February 3, 2016 12:40 PM, "ruipbarradas at sapo.pt"
><ruipbarradas at sapo.pt> wrote:
>>
>>
>>  Hello,
>>
>> You can't use tab$MVAR but you can use tab[[MVAR]] if you do MVAR <-
>"population" (no need for c()).
>>
>> Hope this helps,
>>
>> Rui Barradas
>>  Citando Amoy Yang via R-help <r-help at r-project.org>:
>> population is the field-name in data-file (say, tab).
>MVAR<-population takes data (in the column of population) rather than
>field-name as done in SAS:  %let MVAR=population;
>> In the following r-program, for instance, I cannot use ...
>tab$MVAR...or simply MVAR itself since MVAR is defined as "population"
>with double quotes if using MVAR<-c("population")
>>
>>    On Wednesday, February 3, 2016 11:54 AM, Duncan Murdoch
><murdoch.duncan at gmail.com> wrote:
>>
>>
>> On 03/02/2016 12:41 PM, Amoy Yang via R-help wrote:
>>  There is a %LET statement in SAS: %let MVAR=population; Thus, MVAR
>can be used through entire program.
>> In R, I tried MAVR<-c("population"). The problem is that MAVR comes
>with double quote "...." that I don't need. But MVAR<-c(population) did
>NOT work out. Any way that double quote can be removed as done in SAS
>when creating macro_var?
>> Thanks in advance for helps!
>> R doesn't have a macro language, and you usually don't need one.
>>
>> If you are only reading the value of population, then
>>
>> MAVR <- population
>>
>> is fine.  This is sometimes the same as c(population), but in general
>> it's different:  c() will remove some attributes, such as
>> the dimensions on arrays.
>>
>> If you need to modify it in your program, it's likely more
>complicated.
>> The normal way to go would be to put your code in a function, and
>have
>> it return the modified version.  For example,
>>
>> population <- doModifications(population)
>>
>> where doModifications is a function with a definition like
>>
>> doModifications <- function(MAVR) {
>>    # do all your calculations on MAVR
>>    # then return it at the end using
>>    MAVR
>> }
>>
>> Duncan Murdoch
>>
>>
>>
>>        [[alternative HTML version deleted]]
>>
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>http://www.r-project.org/posting-guide.htmlandprovide commented,
>minimal, self-contained, reproducible code.
>>
>>
>>
>>
>>        [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>
>
>-- 
>Statistics & Software Consulting
>GKX Group, GKX Associates Inc.
>tel: 1-877-GKX-GROUP
>email: ggrothendieck at gmail.com
>
>   
>
>   
>
>  
>	[[alternative HTML version deleted]]
>
>______________________________________________
>R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.

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