[sv-bc] RE: associativity of ** operator

Right-associativity would be more conventional for **.

Left-associativity degenerates to the following identity
 
            (x**y)**z = x**(y*z)

For example, consider (2**3)**5

    2 ** 3 = 

        2 * 2 * 2

   (2 ** 3) ** 5 =

        2 * 2 * 2 
     * 2 * 2 * 2   
     * 2 * 2 * 2 
     * 2 * 2 * 2 
     * 2 * 2 * 2 

Notice there are 3*5 factors of 2.

-- Brad

________________________________________
From: owner-sv-bc@eda.org [owner-sv-bc@eda.org] On Behalf Of Bresticker, Shalom [shalom.bresticker@intel.com]
Sent: Monday, October 26, 2009 9:59 PM
To: Marq Kole; verilog-ams@server.eda.org
Cc: sv-bc@server.eda.org
Subject: [sv-bc] RE: associativity of ** operator

Marq,

Writing 2**3**2 is not the same as writing on paper 2 followed by a superscripted 3 followed by a double-superscripted 2.

Those superscripts define an implicit associativity that does not appear when written linearly.

In these programming languages, it is a matter of definition.

In any case, changing the associativity now would cause a problem of backwards compatibility, which is the main reason it won't be changed in SystemVerilog (There won't be any more 1364 as it has been merged into SystemVerilog.).

I also don't see why you think that with left-to-right associativity it becomes equivalent to * except for the left-most operator. If that were the case, then 2**3**2 would give you (2**3)*2 = 16, not 64.

Regards,
Shalom

> In LRM 2.3.1 I believe there is a mistake in operator
> associativity. According to section 4.2.2 all operators
> associate left to right except the contional operator. This
> is the wrong choice in the case of the exponentiation
> operator '**' because this operator should also associate
> right to left. With left-to-right associativity this operator
> becomes equivalent to the multiplication operator except for
> the leftmost operand.
>
> Here is a small example to show the difference:
>
> 2**3**2 = ?
>
> left-to-right associativity:
>
> 2**3**2 = (2**3)**2 = 8**2 = 64 = 2**(3*2)
>
> right-to-left associativity:
>
> 2**3**2 = 2**(3**2) = 2**9 = 512
>
> This should probably be changed in IEEE 1364 (Verilog) and
> IEEE 1800 (SystemVerilog) as well.
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