Re: Please Help Eliminate My Ignorance
- To: mathgroup at smc.vnet.net
- Subject: [mg118571] Re: Please Help Eliminate My Ignorance
- From: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>
- Date: Tue, 3 May 2011 05:46:30 -0400 (EDT)
- References: <ipm2cl$56f$1@smc.vnet.net>
In 1. because of SetDelayed (:=) the Part ([[ ]]) statement is not executed directly. It lays dormant until test is called. In 2. Append and the Part contained in it are executed immediately. Here the problem with p being not a List that can be indexed directly leads to an error message. If you call test in 1. with a argument that is not a list you get the same error message. Cheers -- Sjoerd Mathematica questions also answered at Stackoverflow: http://stackoverflow.com/questions/tagged/mathematica May 2, 12:52 pm, Michael Mandelberg <mmandelb... at comcast.net> wrote: > Why does Mathematica like the first, but not the second? > > 1. > Clear[x, testp, p]; > test[p_] := Append[p, p[[4]]]; > > 2. > Clear[x, testp, p]; > test[p_] := x = 1; Append[p, p[[4]]]; > > Part::partd: Part specification p[[4]] is longer than depth of object. > > > > Append::normal: Nonatomic expression expected at position 1 in > Append[p,p[[4]]]. >>