# Working with kernel.eval

**URL:** <https://www.ruby-forum.com/t/working-with-kernel-eval/53699>\
**Category:** Ruby\
**Created:** [February 13, 2006, 9:07pm UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699 "2006-02-13T21:07:10Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![ian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@ian](https://www.ruby-forum.com/u/ian)\
**Post date:** [February 13, 2006, 9:07pm UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699/1 "2006-02-13T21:07:10Z")

</div>

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

---

<div class="post-metadata">

**Author:** ![ian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@ian](https://www.ruby-forum.com/u/ian)\
**Post date:** [February 13, 2006, 9:11pm UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699/2 "2006-02-13T21:11:39Z")

</div>

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

Kent.

---

<div class="post-metadata">

**Author:** ![ian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@ian](https://www.ruby-forum.com/u/ian)\
**Post date:** [February 13, 2006, 9:19pm UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699/3 "2006-02-13T21:19:11Z")

</div>

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

David V.

DÅ?a Pondelok 13 Február 2006 21:07 Ian W. napísal:

---

<div class="post-metadata">

**Author:** ![ian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@ian](https://www.ruby-forum.com/u/ian)\
**Post date:** [February 13, 2006, 9:15pm UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699/4 "2006-02-13T21:15:23Z")

</div>

Ian W. wrote:

> 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.

---

<div class="post-metadata">

**Author:** ![ian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@ian](https://www.ruby-forum.com/u/ian)\
**Post date:** [February 13, 2006, 9:29pm UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699/5 "2006-02-13T21:29:44Z")

</div>

On Feb 13, 2006, at 2:24 PM, Ian W. 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 G. II

---

<div class="post-metadata">

**Author:** ![ian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@ian](https://www.ruby-forum.com/u/ian)\
**Post date:** [February 13, 2006, 9:24pm UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699/6 "2006-02-13T21:24:46Z")

</div>

This works and I appreciate the response. But it doesn’t solve the  
problem that the formulas in the text files already exist & they  
weren’t written with Ruby’s arithmetic in mind. My life would be  
easier if I didn’t have to go update all of those text files.

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.

---

<div class="post-metadata">

**Author:** ![ian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@ian](https://www.ruby-forum.com/u/ian)\
**Post date:** [February 13, 2006, 10:09pm UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699/7 "2006-02-13T22:09:45Z")

</div>

Ian W. 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.

---

<div class="post-metadata">

**Author:** ![ian](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@ian](https://www.ruby-forum.com/u/ian)\
**Post date:** [February 14, 2006, 3:59am UTC](https://www.ruby-forum.com/t/working-with-kernel-eval/53699/8 "2006-02-14T03:59:00Z")

</div>

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

Thanks,

Ian