Re: How to evaluate parts of an expression, but not other parts?

*To*: mathgroup at smc.vnet.net*Subject*: [mg122726] Re: How to evaluate parts of an expression, but not other parts?*From*: Alexey Popkov <lehin.p at gmail.com>*Date*: Tue, 8 Nov 2011 07:14:18 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <j930l9$a3d$1@smc.vnet.net>

Hi Julian, Please see strongly related thread on Stack Overflow: http://stackoverflow.com/questions/6633236/replace-inside-held-expression Two techniques are presented and discussed there: 1) The Trott-Strzebonski in-place evaluation technique http://library.wolfram.com/conferences/devconf99/villegas/UnevaluatedExpressions/Links/index_lnk_30.html 2) An undocumented, but very convenient, way to make replacements in held expressions using RuleCondition: In[3]:= Hold[{1, 2, 3, 4, 5}] /. n_Integer :> RuleCondition[n^2, OddQ[n]] Out[3]= Hold[{1, 2, 9, 4, 25}] HTH, Alexey On 5 Nov, 13:46, Julian Francis <julian.w.francis at gmail.com> wrote: > Dear all, > > I'd like to use the TreePlot function to visualise the expression of a > dynamic programming problem I am working on. > > If I have something like: ( (a+b) + (c+d ) > > Mathematica helpfully simplifies this to: a + b + c + d > > But I'd prefer it to be in the original form. > > I can't write Hold[ ( (a+b) + (c+d) )] because I do want a,b,c & d to > be evaluated. > > I want to write something like: > Hold[ ( (Evaluate[a]+Evaluate[b]) + (Evaluate[c]+Evaluate[d]) ) ] > > But this just leaves the Evaluate expressions unevaluated. > > Any help greatly appreciated. > > Thanks, > Julian.