From: Nick Pavey <nick@...>
Date: 2006-12-28T07:47:40+09:00
Subject: Re: Handling of arbitrary length arrays in arguments to printf builtin

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Hi Everyone,

Thanks very much for your fast responses - the "unary unarray"  
operator works great!

Thanks also for the kind welcome you've given me - it makes a big  
difference.

Best wishes,


Nick


On Dec 27, 2006, at 3:03 PM, Timothy Hunter wrote:

> Nick Pavey wrote:
>> Hi Folks,
>>
>> I'm porting an error reporting system from Perl to Ruby as a way  
>> of getting up the Ruby learning curve. Apologies in advance if my  
>> question has already been answered - I did a cursory search and  
>> got no hits.
>>
>> I'm running up against my lack of detailed Ruby knowledge, and I  
>> was wondering if someone could provide some advice - I'm sure that  
>> Ruby supports what I want to do.
>>
>> In the Perl world, I had a system where I could specify an error  
>> message and a set of parameters to that message, as follows:
>>
>>     do_error(message, [parameters]);
>>
>> for example:
>>
>>     do_error("param 1: '%s', param 2 : '%s', param 3 : '%s'",
>>              "detail1", "detail2", "detail3");
>>
>> would give the result:
>>
>>     param 1: 'detail1', param 2 : 'detail2', param 3 : 'detail3'
>>
>> The perl implementation looks like this:
>>
>> sub do_error {
>>   my ($message, @params) = @_;
>>
>>   printf($message, @params);
>> }
>>
>> As you can see, the idea is that the user is able to specify both  
>> their error message and where in their message string various  
>> parameters appear.
>>
>> In this case, Perl was very obliging because parameter lists are  
>> simply arrays. Passing an array into printf became equivalent to  
>> passing the appropriate number of parameters.
>>
>> So, in Ruby, I have the following code:
>>
>> printf "#{@message}\n", @params
>>
>> Where class instance variable @params is an array. I have verified  
>> that it really is an array and not getting mangled somewhere in my  
>> class by using '@params.inspect'.
>>
>> Unfortunately, it seems that Ruby is treating the array in what  
>> Perl folks would call a "scalar context". The printf is getting a  
>> mangled version of the array contents and this is triggering a  
>> runtime error from Ruby.
>>
>> For example:
>>
>>     @params = ["foo", "bar"]
>>     print @params.inspect
>>     >>>  ["foo", "bar"]
>>
>>     printf("%s %s", @params)
>>     >>> ErrorClasses.rb:32:in `printf': too few arguments  
>> (ArgumentError)
>>
>> I did some experiments, and it appears that printf is actually  
>> seeing:
>>     "foobar"
>> instead of the array of 2 elements. If I have the right number of  
>> '%s' conversion codes in the format string, I get this message  
>> because Ruby thinks I've only passed on parameter, not many.
>>
>> For the record, here's the Ruby version I'm using
>>
>>     BigWhite:nick> ruby -v
>>     ruby 1.8.4 (2005-12-24) [i686-darwin8.8.1]
>>
>>
>> What would Ruby's code look like to achieve what I want?
>>
>> Thanks in advance,
>>
>>
>> Nick
>>
>>
>>
>>
> Welcome to Ruby!
>
> Use the "*" (splat) operator to turn the array into a list of  
> arguments.
>
> printf "%s %s", *@params
>
> The unary asterisk is often called the "unary unarray" operator  
> because it has this effect on an array.



--

Nick Pavey
nick@skatingloons.com




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