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Consider the following snippet:

---------------------------------------------

#!/usr/bin/ruby

class Thing

def initialize(name)
@name = name
end

def to_s
@name
end

def to_i
@name.length
end
end

a = Bla.new("foo")

printf "%s\n", a
printf "%d\n", a
printf "%c\n", a

---------------------------------------------

This code breaks at the last line with the message :

../go7:22:in `printf': can't convert Thing into Integer (TypeError)

It seems that the printf conversion to %c tries to call a specific
method of my class to convert the object to the appropriate format. I am
a bit confused about the message "can't convert to integer", since the
to_i method is defined, and sucessfully called that the printf "%d"
line.

So, two questions arise

- what method am I supposed to provied for printf so it can handle the
'%s' format string

- How can I figure this out myself. It seems that printf is trying to
call something, but I have no way of telling exactly what it is
trying to do. Is there some kind of tracing/debugging that can be
switched on to see what's printf is trying to do ?

Thank you very much,

Gerald schrieb:

> It seems that the printf conversion to %c tries to call a specific
> method of my class to convert the object to the appropriate format. I am
> a bit confused about the message "can't convert to integer", since the
> to_i method is defined, and sucessfully called that the printf "%d"
> line.
>
> So, two questions arise
>
> - what method am I supposed to provied for printf so it can handle the
> '%s' format string
>
> - How can I figure this out myself. It seems that printf is trying to
> call something, but I have no way of telling exactly what it is
> trying to do. Is there some kind of tracing/debugging that can be
> switched on to see what's printf is trying to do ?

I guessed that it would try to implicitly convert the object to an
Integer. Ruby usually uses to_int() for that. to_i() is used for
explicit conversions.

I verified that guess by doing this:

irb(main):002:0> "%c" % Class.new { def to_int() ?A end }.new
=> "A"

Hope this helps.

--
http://flgr.0x42.net/

Florian Gross <> wrote:
> Gerald schrieb:
>
>> It seems that the printf conversion to %c tries to call a specific
>> method of my class to convert the object to the appropriate format. I am
>> a bit confused about the message "can't convert to integer", since the
>> to_i method is defined, and sucessfully called that the printf "%d"
>> line.
>>
>> So, two questions arise
>>
>> - what method am I supposed to provied for printf so it can handle the
>> '%s' format string
>>
>> - How can I figure this out myself. It seems that printf is trying to
>> call something, but I have no way of telling exactly what it is
>> trying to do. Is there some kind of tracing/debugging that can be
>> switched on to see what's printf is trying to do ?
>
> I guessed that it would try to implicitly convert the object to an
> Integer. Ruby usually uses to_int() for that. to_i() is used for
> explicit conversions.
>
> I verified that guess by doing this:
>
> irb(main):002:0> "%c" % Class.new { def to_int() ?A end }.new
> => "A"
>
> Hope this helps.

Yes, that was the solution, thank you very much. I must say it is a bit
confusing that the string and float methods are called to_s and to_f,
that to_i works for %d, but not for %c, and to_int works for both %d and
%c. Can you give me a referer to some official documentation describing
%these conversion functions ?

Thanks a lot,

On Jul 29, 2006, at 6:15 AM, Gerald wrote:

> Florian Gross <> wrote:
>> Gerald schrieb:
>>
>>> It seems that the printf conversion to %c tries to call a specific
>>> method of my class to convert the object to the appropriate
>>> format. I am
>>> a bit confused about the message "can't convert to integer",
>>> since the
>>> to_i method is defined, and sucessfully called that the printf "%d"
>>> line.
>>>
>>> So, two questions arise
>>>
>>> - what method am I supposed to provied for printf so it can
>>> handle the
>>> '%s' format string
>>>
>>> - How can I figure this out myself. It seems that printf is
>>> trying to
>>> call something, but I have no way of telling exactly what it is
>>> trying to do. Is there some kind of tracing/debugging that can be
>>> switched on to see what's printf is trying to do ?
>>
>> I guessed that it would try to implicitly convert the object to an
>> Integer. Ruby usually uses to_int() for that. to_i() is used for
>> explicit conversions.
>>
>> I verified that guess by doing this:
>>
>> irb(main):002:0> "%c" % Class.new { def to_int() ?A end }.new
>> => "A"
>>
>> Hope this helps.
>
> Yes, that was the solution, thank you very much. I must say it is a
> bit
> confusing that the string and float methods are called to_s and to_f,
> that to_i works for %d, but not for %c, and to_int works for both %
> d and
> %c. Can you give me a referer to some official documentation
> describing
> %these conversion functions ?
>
> Thanks a lot,
>
>
>

The rule for things like to_int, to_ary and to_str are basically if
your object has a canonical representation as an integer it should
support to to_int. If it can be represented as an integer in more
than one way, e.g. ("10" is this a binary 2? a decimal 10? an octal
8?) you pick a representation and use to_i. The same rules follow for
to_s vs. to_str and to_a vs. to_ary. What this also means is that
unless your Thing is an _integer_ its likely "bad form" to implement
to_int. But like most things in ruby this more a string suggestion
than anything enforced by the language.