From: Sebastian Hungerecker <sepp2k@...>
Date: 2008-02-15T07:18:58+09:00
Subject: Re: Problem usning 'OR' operator in 'IF' condition?

dare ruby wrote:
> but it works fine when the condition changes like,
>
>      if @character == 'R'  || @character == 'r'
>         raise " Expected 'R' after 'E' in version Declaration"
>       end

Here you are saying: if character is 'R' or if it is 'r'. This condition 
obviously can be true (if it is one of 'r' or 'R') or false (if it's 
something else).
Previously you were saying: if character is something other than 'R' or it is 
something other than 'r'. This will, as Paul pointed out, always be true. 
Just think about: If character is 'R' than the first part won't be true ('R' 
is not different from 'R'), but the second part will (because 'R' is 
different from 'r'). If it is 'r', it's the other way around. And since the 
whole expression is true when (at least) one of the parts is true (that's 
what "or" means), it's always true. What you want is: !(@c == 'R' || @c=='r') 
which is equivalent @c != 'R' && @c != 'r'. Note how here you have && instead 
of ||. That's the law of DeMorgan that Paul mentioned:
!(a && b) <-> !a || !b
!(a || b) <-> !a && !b


HTH,
Sebastian
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