[R] Need help figuring out sapply (and similar functions) with multiple parameter user defined function

[Walter Anderson]

On 12/06/2013 10:43 AM, William Dunlap wrote:
>> I have been researching and it appears that I should be using the sapply
>> function to apply the evaluate.question function above to each row in
>> the data frame like this
> Read the documentation more closely: sapply(dataFrame, func)
> applies func() to each column, not row, of dataFrame.
I misunderstood.  I thought it was apply the func to each row...  My mistake
>> preferences <- sapply(df, evaluate.questions, function(x,y,z)
>> evaluate.questions(df['Q1'],df['Q2'],df['Q3']))
> Furthermore,
>      sapply(X = dataFrame, FUN = func, extraArgument)
> calls
>      func(dataFrame[, i], extraArgument)
> for i in seq_len(ncol(dataFrame).
>
> One problem is that FUN=evaluate.questions takes 3 arguments and
> you give it only 2.  Another problem is that the third argument you
> pass to sapply is a function (of 3 arguments) and FUN is not expecting
> any of its arguments to be functions.
I will need to think about this, I am not sure I understand.  I really 
don't seem to understand how any of the apply functions seem to work.
> It may be easier for you to not use sapply here, but to use for-loops and
> come up with something that works.  (Write tests that will indicate whether
> it works or not in a variety of situations.)  Then transform it to use things
> like ifelse() and sapply() to make it more readable and run faster.
I already have tested my functions by using a for loop, and they work.  
Here is the for loop I use.

for (indx in 1:length(df$ID)) {
     df$Preference <- 
evaluate.questions(df$Q1[indx],df$Q2[indx],df$Q3[indx])
}

I understand that such for loops aren't 'best practice' in R and am 
trying to learn its approach.  Thank you for the suggestions!
>> Unfortunately this doesn't work and the problem appears that the sapply
>> function is not feeding the parameters to the evaluate.questions
>> function as I expect.  Can someone provide some guidance on what I am
>> doing wrong?
> Bill Dunlap
> Spotfire, TIBCO Software
> wdunlap tibco.com
>
>