Re: unevaluated, hold, holdform
- To: mathgroup at smc.vnet.net
- Subject: [mg50983] Re: unevaluated, hold, holdform
- From: rationalmean at hotmail.com ("D. Gomez")
- Date: Thu, 30 Sep 2004 04:52:52 -0400 (EDT)
- References: <vas7uiw8wg82@legacy>, <k0gbicmq07vc@legacy>
- Sender: owner-wri-mathgroup at wolfram.com
Many thanks. I understand mathematica does not have any "switch" to turn such automatic reduce-to-lowest off. Notice that if you make: a= unevaluated[[(4*x^2)/(2*x)] then, Numerator[a] also yields 2x I wonder if there are other packages which allow to work the real form of ratios, specially when considering that it is going to be the most important principle of modern mathematics in future times. Regards, D. Gomez On 26 Sep 04 13:31:43 -0400 (EDT), highegg wrote: >On 24 Sep 04 09:46:19 -0400 (EDT), D. Gomez wrote: >>Dear all, >> >>A friend of mine need to get both the numerator and denominator of >any >>expression but he does not want it to be reduced to its lowest >>form, i.e.: given the expression (4 x^2)/(2 x), he needs to extract >>its Numerator: >>Numerator[(4*x^2)/(2*x)]= 4 x^2, >>however Mathematica always simplify it to its lowest form yielding: >>(2*x) >>as its Numerator, that's not what we are looking for. >>We all know about the Hold, HoldForm, Unevaluated functions but >>don't know how to get the Numerator by using at the same time those >>hold functions. >>Many thanks for your help, indeed. >>D. Gomez > >hello Gomez, > >might not be what you will need, >but in general, arguments given to a function are evaluated first >(as an expression in brackets) >if you want to prevent an argument from evaluation, >Wrap it with the _Unevaluated_ function: >Numerator[Unevaluated[(4*x^2)/(2*x)]] > >but be careful, if you use this as a function: >f[x_]:=Numerator[Unevaluated[x]], > >f[(4*x^2)/(2*x)] won't work, the argument is pre-evaluated again! > >the best way to sort this out is to assign an argumant to the function >f: >AppendTo[Attributes[f],HoldAll] > >now f[(4*x^2)/(2*x)] will give us what we want! >(but note that f[x_]:=Numerator[x] still won't work!) ~ ~ ~ ~