From: Brian Candler <b.candler@...>
Date: 2009-08-25T22:07:47+09:00
Subject: Re: Ruby eval

Brandon Larocque wrote:
> ( $jobarea == "hi" or $jobarea == "es" ) ? (loop = 9) : ( ($jobarea ==
> "ca") ? (loop = 8) : (loop = 7) )
> expcost = [] ; exppay = [] ; pay = [] ; name = []
> for i in (1..loop) do
>   eval("expcost.push $#{$jobarea}_job#{i}.expcost")
>   eval("exppay.push $#{$jobarea}_job#{i}.exppay")
>   eval("pay.push $#{$jobarea}_job#{i}.pay")
>   eval("name.push $#{$jobarea}_job#{i}.name")
> end
> 
> Is there any way that I might be able to do this without eval? Or is it
> alright the way I have done it?

There are two big problems with string eval: it's very slow (especially 
if used in a loop, as you have done), as it recompiles the string every 
time it runs; and it may leave your program open to security exploits if 
any of the data you're eval'ing comes from an external source. So here 
are some alternatives.

Look at 'send' for dynamic method dispatch:

  x = "hello"
  m = "upcase"   # or m = :upcase
  x.send(m)

Look at 'const_get' for dynamic class/constant access:

  klass = "String"
  obj = Object.const_get(klass).new

Look at 'instance_variable_set' / 'instance_variable_get' for dynamic 
access to instance variables:

  var = :@foo
  instance_variable_set(var, 123)
  instance_variable_get(var)

I didn't find an equivalent to these methods for global variable access, 
but that's a pretty horrible way to work anyway. Others have shown that 
a Hash is the right data structure to use in your code.

Look at instance_eval if you just want to run fixed code on a dynamic 
receiver.

  s = "hello"
  s.instance_eval { upcase }
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