asterisk users - Jan 2013 - Single = sign and double == sign.What is the difference and when to use the two properly?

quick question that leaves alittle confusion here. Im confused on the
difference or when to use the other if i have 1 = sign or 2 == signs .. so
If i had

exten => _XXXX,1,answer()
    same=> n,Set($[${a}==1]?true:false]  <--double equal sign
    same => n(true),Goto(main,s,1)
    same=> n(false), Hangup()
would this be saying the same thing as above then?

exten => _XXXX,answer()
    same=> n,Set($[${a}=1]?true:false] <-- single equal sign

in essence wouldn't i get the same result ? im confused on the double and
single equal sign and when to use the difference of the two. Would i get
the same result in both these expressions?
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On Friday 11 January 2013, penguin wrote:
> quick question that leaves alittle confusion here. Im confused on the
> difference or when to use the other if i have 1 = sign or 2 == signs .. so
> If i had
> 
> exten => _XXXX,1,answer()
>     same=> n,Set($[${a}==1]?true:false]  <--double equal sign
>     same => n(true),Goto(main,s,1)
>     same=> n(false), Hangup()
> would this be saying the same thing as above then?
> 
> exten => _XXXX,answer()
>     same=> n,Set($[${a}=1]?true:false] <-- single equal sign
> 
> in essence wouldn't i get the same result ? im confused on the double
and
> single equal sign and when to use the difference of the two. Would i get
> the same result in both these expressions?
Generally, one = sign means you're telling. Two == signs means you're
asking.

It's amusing  (for sadists)  to see ex-BASIC programmers trip up over this
and
write something like this:

if (denominator = 0) {
    printf ("Can't divide by zero!\n");
} else {
    answer = numerator / denominator
};

This will never print "Can't divide by zero!" because you are
actually
assigning a value to a variable right there in the conditional, and returning 
the assigned value.  Since this is zero, the if() will fail and drop through 
to the else clause -- and then, just to confuse you, the program will crash 
with "Floating point exception" anyway.

-- 
AJS

Answers come *after* questions.

In Asterisk extensions.conf and extensions.ael inside $[] "=" and
"==" are the same comparison operator.  I can't quote where I saw
this, but it has been documented somewhere.  The == was added to make things
more "programmer friendly".



-----Original Message-----
From: asterisk-users-bounces at lists.digium.com [mailto:asterisk-users-bounces
at lists.digium.com] On Behalf Of penguin
Sent: Friday, January 11, 2013 10:50 AM
To: asterisk-users at lists.digium.com
Subject: Re: [asterisk-users] Single = sign and double == sign.What is the
difference and when to use the two properly?

quick question that leaves alittle confusion here. Im confused on the difference
or when to use the other if i have 1 = sign or 2 == signs .. so If i had

exten => _XXXX,1,answer()
    same=> n,Set($[${a}==1]?true:false]  <--double equal sign
    same => n(true),Goto(main,s,1)
    same=> n(false), Hangup()
would this be saying the same thing as above then?

exten => _XXXX,answer()
    same=> n,Set($[${a}=1]?true:false] <-- single equal sign

in essence wouldn't i get the same result ? im confused on the double and
single equal sign and when to use the difference of the two. Would i get the
same result in both these expressions?