| More LIST weirdness? [message #73980] |
Tue, 14 December 2010 11:55 |
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Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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Hello,
Based on the other thread about list access errors, does the following make sense to anyone:
IDL> q = list()
IDL> help, q
Q LIST <ID=1 NELEMENTS=0>
IDL> help, q[0]
<Expression> UNDEFINED = !NULL
IDL> help, q[1]
<Expression> UNDEFINED = !NULL
IDL> help, q[2]
<Expression> UNDEFINED = !NULL
IDL> x=q[5]
IDL> help, x
X UNDEFINED = !NULL
??
I was expecting the "index out of range" type of error. Or e.g.
IDL> y=q[*]
% Illegal subscript range.
% Error occurred at: LIST::_OVERLOADBRACKETSRIGHTSIDE
% $MAIN$
% Execution halted at: $MAIN$
Now I'm confused. Removing an item causes the "LIST::REMOVE: Index is out of range" error. Why wouldn't simply accessing
an invalid index do a similar thing? The behaviour seems inconsistent. Or am I?
I guess the lesson is to always check the list length before removing from it, or referencing in it. That way there is
no special case and if/when ITTVIS changes the behaviour, no harm done. Right?
cheers,
paulv
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