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Re: part assigned sequence behavior puzzling

  • To: mathgroup at smc.vnet.net
  • Subject: [mg105275] Re: [mg105228] part assigned sequence behavior puzzling
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Wed, 25 Nov 2009 23:02:03 -0500 (EST)
  • Reply-to: hanlonr at cox.net

As stated in the documentation for sequence: assignment and replacement functions have attribute SequenceHold

Attributes[Set]

{HoldFirst,Protected,SequenceHold}

tmp = Range[15]
tmp[[7]] = Sequence @@ Range[2];
tmp[[7]] = Sequence @@ Range[2];
tmp[[7]] = Sequence @@ Range[2];
tmp

{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}

tmp = Range[15]
tmp[[7]] = Sequence @@ Range[2];
tmp = tmp;
tmp[[7]] = Sequence @@ Range[2];
tmp = tmp;
tmp[[7]] = Sequence @@ Range[2];
tmp

{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}


Bob Hanlon

---- mkr <mileskrains at gmail.com> 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?





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