[R] Help with R.oo

[Henrik Bengtsson]

On Sat, Jul 18, 2009 at 3:21 AM, Henrik Bengtsson<hb at stat.berkeley.edu> wrote:
> On Fri, Jul 17, 2009 at 11:41 PM, Jim Nemesh<nemesh at broad.mit.edu> wrote:
>> Hi!  I'm trying to learn about object oriented R, as it seems like it
>> would be very useful.
>>
>> I'm going over an example from the documentation, and I'm very confused:
>>
>> http://www1.maths.lth.se/help/R/R.oo/
>>
>> [assume you've called library (R.oo)]
>>
>> setConstructorS3("SavingsAccount", function(balance=0) {
>>   if (!is.numeric(balance) && length(balance) != 1)
>>     throw("Argument 'balance' must be a single numeric value: ",
>> mode(balance));
>>   if (balance < 0)
>>     throw("Trying to create an account with a negative balance.");
>>
>>   extend(Object(), "SavingsAccount",
>>     .balance = balance
>>   );
>> })
>>
>> Why would I get the following result?
>>
>>  > SavingsAccount(5)
>> [1] "SavingsAccount: 0x18706032"
>>  > SavingsAccount("fruit")
>> [1] "SavingsAccount: 0x605387128"
>>
>> The second time, the input should fail, balance is not numeric.
>
> Nope, because:
>
>> balance <- "fruit"
>> (!is.numeric(balance) && length(balance) != 1)
> [1] FALSE

...and shame on me - that was my own example code, which should be:

  (!is.numeric(balance) || length(balance) != 1)

to work as expected.

/H

>> (balance < 0)
> [1] FALSE
>
>
>>
>> ------------------------------
>>
>> A second attempt, with my own code:
>>
>> setConstructorS3("StreamingFileReader", function(fileName,
>> hasHeader=T, sep="\t") {
>>        if (missing (fileName)) throw ("Must supply a file name!");
>>        if (missing(hasHeader)) hasHeader <-T;
>>        if (missing(sep)) sep="\t";
>>
>>        extend(Object(), "StreamingFileReader",
>>        .fileName=fileName,
>>        .hasHeader=hasHeader,
>>        .sep=sep);
>> })
>>
>>  > z= StreamingFileReader("temp")
>> Error in list(`StreamingFileReader("temp")` = <environment>,
>> `extend(Object(), "StreamingFileReader", .fileName =
>> fileName, .hasHeader = hasHeader, .sep = sep)` = <environment>,  :
>>
>> [2009-07-17 17:37:15] Exception: Must supply a file name!
>>   at throw(Exception(...))
>>   at throw.default("Must supply a file name!")
>>   at throw("Must supply a file name!")
>>   at constructor()
>>   at getStaticInstance.Class(class)
>>   at getStaticInstance(class)
>>   at getStaticInstance.Object(this)
>>   at getStaticInstance(this)
>>   at extend.Object(Object(), "StreamingFileReader", .fileName =
>> fileName, .hasHeader = hasHeader, .sep =
>>   at extend(Object(), "StreamingFileReader", .fileName =
>> fileName, .hasHeader = hasHeader, .sep = sep)
>>   at StreamingFileReader("temp")
>>  > z= StreamingFileReader("temp")
>>  > z$.fileName
>> [1] "temp"
>>
>> Why is it that the first time I try to construct the object it fails,
>> and the second time it seems to work just fine?  Is there something I
>> should be doing between declaring the constructor and using the object
>> (besides adding S3 methods?)
>
> From help(setConstructorS3): "Note: The constructor must be able to be
> called with no arguments, i.e. use default values for all arguments or
> make sure you use missing() or similar! ...".
>
> In your case you must design your constructor function so that you can
> call it without arguments, e.g. StreamingFileReader().  The reason for
> this is that a *static* instance of the class is always created
> automatically by calling the constructor with empty arguments.  This
> static instance is create the first time the constructor is called in
> your code, but not the following times.  This is why you only get the
> error the first time, but not in subsequent calls.  This "feature" is
> by design of R.oo/setConstructorS3() - too long explanation/take my
> word on it.
>
> There should be no more surprises after this one (few people ever discover it).
>
> /Henrik
>
>>
>> Thanks for any help, I find this entirely confusing.
>>
>> -Jim Nemesh
>>
>>
>>
>>
>>        [[alternative HTML version deleted]]
>>
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>> and provide commented, minimal, self-contained, reproducible code.
>>
>