I’m working on an RPN calculator (don’t ask why…) and I’m having
trouble getting a number from gets - how am I supposed to know whether
it’s a number?

At the moment I’m doing [eval(oper) == oper.to_f], which is far from
ideal. Any ideas? You all get to win my calculator if you know of a
better way :D.

Alle giovedÃ¬ 22 novembre 2007, Peter B. ha scritto:

I’m working on an RPN calculator (don’t ask why…) and I’m having
trouble getting a number from gets - how am I supposed to know whether
it’s a number?

At the moment I’m doing [eval(oper) == oper.to_f], which is far from
ideal. Any ideas? You all get to win my calculator if you know of a
better way :D.

Do you mean you need to find out whether a string contains a number or
not? In
this case, you can either use Kernel#Float, which returns str.to_f if
the
string contains a number and raises an exception owtherwise, or try to
craft
a regexp which only matches strings containing numbers. For example, the
(untested) regexp

/[±]?\d+(.\d+)?(e[±]?\d+)?/

should match numbers with an optional + or - in front, followed by at
least
one digit, with an optional decimal part (if there’s a dot, there should
be
at least one digit following it; if you want to allow something like 11.
then
replace (.\d+) with (.\d*) ). The number can also be in exponential
form,
with a downcase e and an optional + or - in front of the exponent.

I’m working on an RPN calculator (don’t ask why…) and I’m having
trouble getting a number from gets - how am I supposed to know whether
it’s a number?

At the moment I’m doing [eval(oper) == oper.to_f], which is far from
ideal. Any ideas? You all get to win my calculator if you know of a
better way :D.

str = gets # chomp is not needed for Float
num = Float(str) rescue nil

Is that the only way to do it? There’s no built-in method for
telling if
a string is a number?

Strings are never numbers in ruby. Ever. That is the beauty of working
in a clean typesafe language.

What you’re doing, possibly without realizing it, is writing a parser
for RPN but it sounds like you’ve not addressed this project like a
parser. I suggest you go look at the screencast for treetop (a parser
generator). In this screencast they build up an infix parser, which is
much more than you need for RPN, but the tool is clean, powerful, and
worth learning.

class String
def is_number?
Float(self) == self.to_f
rescue false
end
end

That’s an error prone approach because it relies on comparing floats
(which is also superfluous here because the real work is done by Float
and rescue). Why don’t you just do

Alle giovedÃ¬ 22 novembre 2007…
In this case, you can either use Kernel#Float, which returns str.to_f
… or try to craft a regexp which only matches strings containing
numbers.
For example, the (untested) regexp

/[±]?\d+(.\d+)?(e[±]?\d+)?/

Two small additions:
a) the ‘dot’ should be literally a dot (else it matches any character)
b) the boundaries to the whole expresssion are needed (else “ab23.35de”
would be
validated).

So, the regexp should be something like:

/^ [±]? \d+ (?: [.]\d+)? (?: e[±]? \d+)? $ /x

(If capture of components is desired, parenthesis should enclose all
components, and the ?: removed).