Re: Bug with Sequence

*To*: mathgroup at smc.vnet.net*Subject*: [mg104633] Re: Bug with Sequence*From*: Szabolcs Horvát <szhorvat at gmail.com>*Date*: Thu, 5 Nov 2009 03:51:19 -0500 (EST)*References*: <hcr7ac$8di$1@smc.vnet.net>

On 2009.11.04. 7:34, dh wrote: > Hello, > > has anybody an explanation for the behavior of "Sequence"? I think it is > > an ugly bug. > > Consider the following that shoud succesively shorten the list t: > > > > t = {1, 2, 3}; j = 0; > > While[ ++j< 4, t[[1]] = Sequence[]; Print[t]] > > > > this returns: {2,3} three times.Dropping of the first element only seems > > to work once. > > If you say Information[t] you get: > > t={Sequence[],2,3} > Hi Daniel, What seems to be happening when t[[1]] = x is evaluated is that first x is evaluated, then the first element of t is changed to (the result of the evaluation of) x. But t itself does not go through an evaluation process. Thus an assignment like t[[1]] is only capable of changing the first element of t but nothing else. If we force t to be evaluated in its entirety after every assignment, then the program will work as expected: While[ ++j < 4, t[[1]] = Sequence[]; t = t; Print[t]] The fact that t[[1]] = x type assignments are not able to affect anything else than the first element is not necessarily a bad thing. Consider the following (admittedly unusual) example: u = Unevaluated[{RandomInteger[], RandomInteger[]}] If u[[1]] = 2 would trigger an evaluation of the entire contents of u, the second element would change too. However, I wouldn't expect that to happen in this particular example. I hope this helps, Szabolcs

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