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Re: part assigned sequence behavior puzzling

  • To: mathgroup at smc.vnet.net
  • Subject: [mg105274] Re: part assigned sequence behavior puzzling
  • From: dh <dh at metrohm.com>
  • Date: Wed, 25 Nov 2009 23:01:51 -0500 (EST)
  • References: <heimgd$58a$1@smc.vnet.net>


mkr wrote:

> I am puzzled by the following behavior:

> 

> tmp = Range[15]

> tmp[[7]] = Sequence @@ Range[2];

> tmp[[7]] = Sequence @@ Range[2];

> tmp[[7]] = Sequence @@ Range[2];

> tmp

> 

> yields

> 

> {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}

> {1, 2, 3, 4, 5, 6, 1, 2, 8, 9, 10, 11, 12, 13, 14, 15}

> 

> I would have expected the repeated assignment to have a repeated

> effect, thus obtaining

> 

> {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}

> {1, 2, 3, 4, 5, 6, 1, 2, 2, 2, 8, 9, 10, 11, 12, 13, 14, 15}

> 

> Where/why am I wrong?

> 

Hi,

I asked the same question some time ago.

Set(=) has the attribute SequenceHold and does NOT evaluate Sequence. 

What is assigne to tmp[[7]] is simply Sequence[1,2]. You can convince 

yourselfe by: ??tmp

Only when you e.g.  display tmp, Sequence will be evaluated.

Daniel




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