Re: List Behavior
Re: List Behavior
- Subject: Re: List Behavior
- From: "Steven D. Majewski" <email@hidden>
- Date: Wed, 5 Mar 2008 14:52:12 -0500
On Mar 5, 2008, at 2:38 PM, John Mistler wrote:
I have discovered some peculiar behavior with lists. Can anyone
help me understand why?
This works as expected:
set a to {{1, 2, 3}, {"x", "y", "z"}}
set b to item 1 of a
set c to item 1 of a
set c to c & {4}
log b --> {1,2,3}
log c --> {1,2,3,4}
but why does this happen?:
set a to {{1, 2, 3}, {"x", "y", "z"}}
set b to item 1 of a
set c to item 1 of a
set end of c to 4
log b --> {1,2,3,4}
log c --> {1,2,3,4}
Both work as expected ( or as they should be expected to. ).
'set c' binds the name c to an object.
Evaluation of the new object happens before the binding,
so the "c & {4}" on the right-hand-side refers to the old binding.
"b" is still bound to that first object, so "b" and "c" name
two different objects.
'set end of c' is modifying the object named c by changing the end.
You haven't changed what "b" or "c" point to, but you've changed
the value of that thing by changing it's "end" .
See Lewis Carrol: <http://hubpages.com/hub/Lewis-Carroll> :
Alice was walking beside the White Knight in Looking Glass Land.
"You are sad." the Knight said in an anxious tone: "let me sing you a
song to comfort you."
"Is it very long?" Alice asked, for she had heard a good deal of
poetry that day.
"It's long." said the Knight, "but it's very, very beautiful.
Everybody that hears me sing it - either it brings tears to their
eyes, or else -"
"Or else what?" said Alice, for the Knight had made a sudden pause.
"Or else it doesn't, you know. The name of the song is called
'Haddocks' Eyes.'"
"Oh, that's the name of the song, is it?" Alice said, trying to feel
interested.
"No, you don't understand," the Knight said, looking a little vexed.
"That's what the name is called. The name really is 'The Aged, Aged
Man.'"
"Then I ought to have said 'That's what the song is called'?" Alice
corrected herself.
"No you oughtn't: that's another thing. The song is called 'Ways and
Means' but that's only what it's called, you know!"
"Well, what is the song then?" said Alice, who was by this time
completely bewildered.
"I was coming to that," the Knight said. "The song really is 'A-
sitting On a Gate': and the tune's my own invention."
-- sdm.
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