From: Lyle Johnson <lyle@...>
Date: 2004-01-14T02:56:36+09:00
Subject: Re: Warnings on assignments in conditionals?

David Heinemeier Hansson wrote:

> I consider the following statements to be perfectly legit (although 
> perhaps slightly ambigious) and Ruby does obey, but throws this 
> mandating warning. Am I just being paranoid or is Ruby forcing a style 
> upon me :)?

A single equals sign does assignment, so a statement like:

     if a = 1 then true else false end

assigns the value 1 to 'a', and then evaluates the "truthfulness" of 
'a'. In Ruby, only 'false' and and 'nil' evaluate to false, so this 
statement will *always* evaluate to true (i.e. because 'a' is 1, and 1 
is true).

If you're wanting to check whether the current value of 'a' is 1, you 
need to use two equals signs instead, i.e.

     if a == 1 then true else false end

This statement doesn't change the value of 'a' (as the previous 
statement did). It just compares its current value against another value 
(1) and returns true if they are equal.

Apologies in advance if you already understand this distinction, but it 
is one that has tripped many a programmer, even those who should know 
better ;) If you indeed mean to assign the value to 'a' (as in the first 
example), you can probably avoid the warning from Ruby by breaking this 
into two lines, e.g.

     a = 1
     if a then true else false end

Hope this helps,

Lyle