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From: Amit Zaroo on 2 Feb 2010 03:41
Hi,
I wanted to know the difference if any between using the following
expressions. I am getting different results. The manual says any valid
numeric operands are supported. It seems only positive operands are
supported.

$ tclsh
% set exp -7
-7
% set result [expr 2**$exp]
0
% set result [expr 2**(-1*$exp)]
128
% set result [expr pow(2,$exp)]
0.0078125
% set result [expr pow(2,-$exp)]
128.0
From: Arjen Markus on 2 Feb 2010 03:51
On 2 feb, 09:41, Amit Zaroo <amit.za...(a)gmail.com> wrote:
> Hi,
>  I wanted to know the difference if any between using the following
> expressions. I am getting different results. The manual says any valid
> numeric operands are supported. It seems only positive operands are
> supported.
>
> $ tclsh
> % set exp -7
> -7
> % set result [expr 2**$exp]
> 0
> % set result [expr 2**(-1*$exp)]
> 128
> % set result [expr pow(2,$exp)]
> 0.0078125
> % set result [expr pow(2,-$exp)]
> 128.0

The essential difference between ** and pow() is that pow() treats its
arguments as double precision reals, so pow(2,-7) is equivalent to
2.0**(-7.0)
and the ** operation treats its arguments according to the values,
i.e.
integers are distinguished from reals, both in the exponent and the
base number.

Hence the differences!

Regards,

Arjen
From: Amit Zaroo on 2 Feb 2010 09:58
Thanks for the explanation. It is now clear!
Regards,
Amit Zaroo

Arjen Markus wrote:
> On 2 feb, 09:41, Amit Zaroo <amit.za...(a)gmail.com> wrote:
> > Hi,
> >  I wanted to know the difference if any between using the following
> > expressions. I am getting different results. The manual says any valid
> > numeric operands are supported. It seems only positive operands are
> > supported.
> >
> > $ tclsh
> > % set exp -7
> > -7
> > % set result [expr 2**$exp]
> > 0
> > % set result [expr 2**(-1*$exp)]
> > 128
> > % set result [expr pow(2,$exp)]
> > 0.0078125
> > % set result [expr pow(2,-$exp)]
> > 128.0
>
> The essential difference between ** and pow() is that pow() treats its
> arguments as double precision reals, so pow(2,-7) is equivalent to
> 2.0**(-7.0)
> and the ** operation treats its arguments according to the values,
> i.e.
> integers are distinguished from reals, both in the exponent and the
> base number.
>
> Hence the differences!
>
> Regards,
>
> Arjen
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