[R] Create macro_var in R

[MacQueen, Don]

Repeat many times:   R does not work the same way as SAS, do not expect it
to

If I needed to construct an SQL statement in which the name of a field is
provided by the value of another variable, I would consider this:

key <- 'pop'
sql.stmt <- paste("select grade,
 count(*) as cnt,
 min(",key,") as min,
 max(",key,") as max,
 avg(",key,") as mean,
 median(",key,") as median,
 stdev(",key,") as stdev
from tab
group by grade"

)

Then
  print(sql.stmt, quote=FALSE)
to make sure I got what I wanted.

Use paste0() instead of paste() if you don't want the extra space
characters, even though they don't matter to SQL.

And, no, the double [[...]] does not remove double quotes. That's not how
it works.
  tab[['pop']]
  tab[ , 'pop']
  tab$pop
are three different ways to reference the pop column in the tab data
frame. Of the three, the value 'pop' can be stored in a variable with the
first two, but not the third one.

Remember, R is not SAS, so you have to use a different technique to
construct the desired SQL statement; there is nothing in R that works
exactly like SAS macro variables. (R has other advantages).

-Don

-- 
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062





On 2/3/16, 11:16 AM, "R-help on behalf of Amoy Yang via R-help"
<r-help-bounces at r-project.org on behalf of 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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>
> 
>
>  
>	[[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.