[R] else problem

[David Winsemius]

On Jun 23, 2011, at 12:05 PM, Kara Przeczek wrote:

> Thank you for all your help! I did not know to use "" when searching  
> for help, as ?mean, etc, had always worked for me in the past.
> It makes perfect sense why 'else' was causing me the trouble the way  
> I was using it. I think it was working in my other code, despite the  
> same format, because it was part of a function and thus would have  
> been executed completely within the function?

Right. When done at the console or sourced, the problem will arise,  
but not when inside a function. Bert's strategy of enclosing the whole  
call in "{}" can be used to demonstrate at the console:

 > {fntest <- function() if (FALSE)  print("T")
+                        else  print("F") }
 > fntest()
[1] "F"

Or you can enclose just the body:

 > fntest <- function() { if (FALSE)   print("T")
+                  else { print("F") } }
 > fntest()
[1] "F"
-- 
David


> Cheers,
> Kara
> ________________________________________
> From: Bert Gunter [gunter.berton at gene.com]
> Sent: June 23, 2011 8:27 AM
> To: David Winsemius
> Cc: Kara Przeczek; r-help at r-project.org
> Subject: Re: [R] else problem
>
> Perhaps some additional clarification...  (???)
>
>>> if (length(dat2f$year)%%2==0) {
>>> md <-dat2f[, list(med_year = max(year[which(abs(tot_km3y -
>>> median(tot_km3y)) == min(abs(tot_km3y - median(tot_km3y)))) ]),  
>>> med_TotQ =
>>> median(tot_km3y))]  }
>>> else {
>>
>> If this line is executed at a console session it will fail because  
>> the
>> interpreter does not keep a copy of the last condition. If you  
>> moved the
>> closing curley-brace to just befor the 'else', you should get the  
>> behavior
>> you expect (if your other code is correct):
>
> Well, um.. not sure if this is what you meant, but what is happening
> at the console is that when you type <return>, the interpreter checks
> for a syntactically complete statement. If it finds what has been
> given to it **thus far** is, it tries to execute it (if not, it gives
> a continuation character and waits for more input) and, as you said,
> then starts anew to interpret the next line(s) entered, "forgetting"
> all previous. The problem above is that the" if()" statement up to the
> close bracket, "}" is syntactically complete, and so the "else{" that
> follows makes no sense as the beginnig of a new line to  be
> interpreted.
>
> The simplest and universal solution to this is to simply enclose the
> whole conditional in" { }":
>
> {if(length ...
> ...
> else {...}
> }
>
> This forces the interpreter to wait for the last "}" before it will
> interpret and execute.
>
> Hoping this clarifies rather than obfuscates.
>
> -- Bert
>
>
>
>>
>> Try instead:
>>
>> if (length(dat2f$year)%%2==0) {
>> md <-dat2f[, list(med_year = max(year[which(abs(tot_km3y -
>> median(tot_km3y)) == min(abs(tot_km3y - median(tot_km3y)))) ]),  
>> med_TotQ =
>> median(tot_km3y))]
>> } else {
>> md <-dat2f[, list(med_year = year[which(tot_km3y ==  
>> median(tot_km3y))],
>> med_TotQ = median(tot_km3y))]
>> }
>>
>>
>>> md <-dat2f[, list(med_year = year[which(tot_km3y ==  
>>> median(tot_km3y))],
>>> med_TotQ = median(tot_km3y))]
>>> }
>>>
>>> Each individual piece works perfectly on its own, but together I  
>>> get the
>>> following error:
>>>
>>>> if (length(dat2f$year)%%2==0) {
>>>
>>> +   md <-dat2f[, list(med_year = max(year[which(abs(tot_km3y -
>>> median(tot_km3y)) == min(abs(tot_km3y - median(tot_km3y)))) ]),  
>>> med_TotQ =
>>> median(tot_km3y))] }
>>>>
>>>> else {
>>>
>>> Error: unexpected 'else' in "else"
>>>>
>>>> md <-dat2f[, list(med_year = year[which(tot_km3y ==  
>>>> median(tot_km3y))],
>>>> med_TotQ = median(tot_km3y))]
>>>> }
>>>
>>> Error: unexpected '}' in "  }"
>>>>
>>>
>>> When I tried to look up "else" I got this error:
>>>
>>>> ?else
>>>
>>> Error: unexpected 'else' in "?else"
>>
>> Try instead:
>>
>> ?"else"
>>
>> --
>> David
>>>
>>> I have used exactly the same set up with if...else in other code  
>>> and it
>>> worked fine then. I tried to run it again, and I got the same  
>>> error as
>>> above. What is the problem? I hope it isn't something simple and  
>>> silly!
>>>
>>> I realize that I can use the first line:
>>>
>>> md <- dat2f[, list(med_year = max(year[which(abs(tot_km3y -
>>> median(tot_km3y)) == min(abs(tot_km3y - median(tot_km3y)))) ]),  
>>> med_TotQ =
>>> median(tot_km3y))]
>>>
>>> for all data sets and it will give me the median for both odd and
>>> even-length data sets, but it is now about the principle; why  
>>> won't the
>>> if...else work?
>>>
>>> Thank you very much for your time!
>>>
>>> Kara
>>
>> David Winsemius, MD
>> West Hartford, CT
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> 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.
>>
>
>
>
> --
> "Men by nature long to get on to the ultimate truths, and will often
> be impatient with elementary studies or fight shy of them. If it were
> possible to reach the ultimate truths without the elementary studies
> usually prefixed to them, these would not be preparatory studies but
> superfluous diversions."
>
> -- Maimonides (1135-1204)
>
> Bert Gunter
> Genentech Nonclinical Biostatistics

David Winsemius, MD
West Hartford, CT