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MathGroup Archive 2005

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Re: Turning on numerical computation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg63064] Re: Turning on numerical computation
  • From: David Bailey <dave at Remove_Thisdbailey.co.uk>
  • Date: Tue, 13 Dec 2005 03:40:45 -0500 (EST)
  • References: <dmuice$mlr$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

New Guy wrote:
> Mathematica outputs answers in symbolic form as the exact answer.
> 
> I can force numeric output by N or using a decimal.
> 
> Can I make numeric answers the default until I want to go back to
> symbolic.  In other words, can I turn numeric on and off as my default
> output?
> 
> Thank you.
> 
Hello,

As two people have already pointed out, you CAN use $Post to change the 
default behaviour of Mathematica. However, I wonder if this is really 
wise - particularly if, as your name suggests - you are fairly new to 
Mathematica.

Setting $Post=N will do many things - including converting integers to 
reals, and spoiling any algebraic calculations that you do (unless you 
like polynomials with fractional coefficients represented as real 
numbers!). You could make a more complicated assignment to $Post and 
avoid some of these problems, but even so, the point is, everyone learns 
Mathematica by using it, and you will learn the 'wrong' system.

In any particular situation there is usually a better alternative. Say 
you are doing a lot of calculations with Log with integer values - these 
don't usually evaluate because the result is not exact. However, you can 
always define your own Log function to get round that problem:

realLog[x_]:=N[Log[x]]

David Bailey
http://www.dbaileyconsultancy.co.uk


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