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comp.lang.ruby

Working with kernel.eval

Ian Whitney

2/13/2006 8:07:00 PM

I'm trying to calculate the results of formulas that are stored in
text files.

For example, a file may contain the line: 1+2/3

I want to take that line & get the result of the formula, 1.66 (roughly)

The way to do this seems to be Kernel.eval. But I can't get it to
work correctly. Here's my irb session:

irb(main):001:0> a = "1+2/3"
=> "1+2/3"
irb(main):002:0> b = eval(a)
=> 1
irb(main):004:0> b.class
=> Fixnum

I would think that eval(a) should return a Float, not a Fixnum.

I thought maybe that eval wasn't following the standard order of
operations, which could explain the return value of 1. So then I
tried this:

irb(main):005:0> a = "2/3"
=> "2/3"
irb(main):006:0> b = eval(a)
=> 0
irb(main):007:0> b.class
=> Fixnum


So, it's not an order of operations thing.

Maybe I shouldn't be using eval. But I can't find another option. Any
thoughts?

Thanks,

Ian
6 Answers

Kent Sibilev

2/13/2006 8:11:00 PM

0

$ irb
irb(main):001:0> 2/3
=> 0
irb(main):002:0> 2.0/3
=> 0.666666666666667
irb(main):003:0>

Kent.

On 2/13/06, Ian Whitney <iwhitney@ssa-i.org> wrote:
> I'm trying to calculate the results of formulas that are stored in
> text files.
>
> For example, a file may contain the line: 1+2/3
>
> I want to take that line & get the result of the formula, 1.66 (roughly)
>
> The way to do this seems to be Kernel.eval. But I can't get it to
> work correctly. Here's my irb session:
>
> irb(main):001:0> a = "1+2/3"
> => "1+2/3"
> irb(main):002:0> b = eval(a)
> => 1
> irb(main):004:0> b.class
> => Fixnum
>
> I would think that eval(a) should return a Float, not a Fixnum.
>
> I thought maybe that eval wasn't following the standard order of
> operations, which could explain the return value of 1. So then I
> tried this:
>
> irb(main):005:0> a = "2/3"
> => "2/3"
> irb(main):006:0> b = eval(a)
> => 0
> irb(main):007:0> b.class
> => Fixnum
>
>
> So, it's not an order of operations thing.
>
> Maybe I shouldn't be using eval. But I can't find another option. Any
> thoughts?
>
> Thanks,
>
> Ian
>
>

Joel VanderWerf

2/13/2006 8:15:00 PM

0

Ian Whitney wrote:
> I'm trying to calculate the results of formulas that are stored in text
> files.
>
> For example, a file may contain the line: 1+2/3
>
> I want to take that line & get the result of the formula, 1.66 (roughly)
>
> The way to do this seems to be Kernel.eval. But I can't get it to work
> correctly. Here's my irb session:
>
> irb(main):001:0> a = "1+2/3"
> => "1+2/3"
> irb(main):002:0> b = eval(a)
> => 1
> irb(main):004:0> b.class
> => Fixnum
>
> I would think that eval(a) should return a Float, not a Fixnum.

It's nothing to do with eval, but rather that if you start with fixnums,
you do fixnum arithmetic. Try replacing 2 with 2.0. That invokes float
division, and every result depending on that input will be float. In
general, you can use x.to_f if you want to force a value to be treated
as float.

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

David Vallner

2/13/2006 8:19:00 PM

0

require "mathn" for a batch of tweaks that will make Ruby maths expression
more maths-like than C-like.

David Vallner

Dna Pondelok 13 Február 2006 21:07 Ian Whitney napísal:
> I'm trying to calculate the results of formulas that are stored in
> text files.
>
> For example, a file may contain the line: 1+2/3
>
> I want to take that line & get the result of the formula, 1.66 (roughly)
>
> The way to do this seems to be Kernel.eval. But I can't get it to
> work correctly. Here's my irb session:
>
> irb(main):001:0> a = "1+2/3"
> => "1+2/3"
> irb(main):002:0> b = eval(a)
> => 1
> irb(main):004:0> b.class
> => Fixnum
>
> I would think that eval(a) should return a Float, not a Fixnum.
>
> I thought maybe that eval wasn't following the standard order of
> operations, which could explain the return value of 1. So then I
> tried this:
>
> irb(main):005:0> a = "2/3"
> => "2/3"
> irb(main):006:0> b = eval(a)
> => 0
> irb(main):007:0> b.class
> => Fixnum
>
>
> So, it's not an order of operations thing.
>
> Maybe I shouldn't be using eval. But I can't find another option. Any
> thoughts?
>
> Thanks,
>
> Ian

James Gray

2/13/2006 8:30:00 PM

0

On Feb 13, 2006, at 2:24 PM, Ian Whitney wrote:

> Any thoughts on how I can take the string as it stands, "1+2/3" and
> get the right answer?

Well, depending on how many operators you need to support, it's
fairly easy to parse them into an Abstract Syntax Tree and run the
calculation.

James Edward Gray II

Joel VanderWerf

2/13/2006 9:10:00 PM

0

Ian Whitney wrote:

> Any thoughts on how I can take the string as it stands, "1+2/3" and get
> the right answer? I could use regex to replace every integer with a
> floating point number, I guess. But that seems kludgey.

David's suggestion to require 'mathn' is a good way to go, but be aware
that 2/3 will evaluate to a rational--precise, but in general slower to
compute with.

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Ian Whitney

2/14/2006 2:59:00 AM

0

That does appear to be the best solution. I plugged that in & my unit
tests look good.

Thanks,

Ian

On Feb 13, 2006, at 2:19 PM, David Vallner wrote:

> require "mathn" for a batch of tweaks that will make Ruby maths
> expression
> more maths-like than C-like.
>
> David Vallner
>
> Dna Pondelok 13 Február 2006 21:07 Ian Whitney napísal:
>> I'm trying to calculate the results of formulas that are stored in
>> text files.
>>
>> For example, a file may contain the line: 1+2/3
>>
>> I want to take that line & get the result of the formula, 1.66
>> (roughly)
>>
>> The way to do this seems to be Kernel.eval. But I can't get it to
>> work correctly. Here's my irb session:
>>
>> irb(main):001:0> a = "1+2/3"
>> => "1+2/3"
>> irb(main):002:0> b = eval(a)
>> => 1
>> irb(main):004:0> b.class
>> => Fixnum
>>
>> I would think that eval(a) should return a Float, not a Fixnum.
>>
>> I thought maybe that eval wasn't following the standard order of
>> operations, which could explain the return value of 1. So then I
>> tried this:
>>
>> irb(main):005:0> a = "2/3"
>> => "2/3"
>> irb(main):006:0> b = eval(a)
>> => 0
>> irb(main):007:0> b.class
>> => Fixnum
>>
>>
>> So, it's not an order of operations thing.
>>
>> Maybe I shouldn't be using eval. But I can't find another option. Any
>> thoughts?
>>
>> Thanks,
>>
>> Ian
>