What method should a class provide for printf's %c format st

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,

Florian G. [email protected] wrote:

  • what method am I supposed to provied for printf so it can handle the

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,

Gerald schrieb:

  • 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.

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

switched on to see what’s printf is trying to do ?
Hope this helps.
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.

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