| When Ptr_New doesn't work [message #28954] |
Tue, 22 January 2002 03:42  |
badenes
Messages: 5
Registered: January 2002
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Junior Member |
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I have the following problem:
rCoefficients=PtrArr(nElems)
FOR i=0, nElems-1 DO BEGIN
rCoefficients[i]=Ptr_New(PtrArr(elems[i]))
ENDFOR
since elems is a vector of integers, each element i of
rCoefficients is a pointer to an array of elems[i] pointers.
But, for some reason,
((*rCoefficients[j])[k]) = Ptr_New(FltArr(2), /NO_COPY)
doesn't work. k and j are within the allowed range, of course, and
((*rCoefficients[j])[k]) is a null pointer, as expected from the
initialization above. Ptr_New is supposed to allocate memory for the
specified pointer to store an array of 2 floats, but I get the message
Expression must be named variable in this context: <POINTER
(<NullPointer>)>.
I must be doing something wrong. Can you help?
Thanks,
Carles
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| Re: When Ptr_New doesn't work [message #29016 is a reply to message #28954] |
Wed, 23 January 2002 11:13  |
John-David T. Smith
Messages: 384
Registered: January 2000
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Senior Member |
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"Pavel A. Romashkin" wrote:
>
> I think the key here is, anything in parenthesis gets *evaluated* and as
> such is an expression. And it is not really surprising that one is not
> allowed to assign a value to an expression. This is the strength of IDL
> - it creates temporary variables as expressions, and allows to use them
> just like named variables. Unlike, say, IGOR Pro that requres you to
> define a named variable to even plot it.
>
Right. Another example of parentheses forming an expression is:
IDL> a=(b=1)
which is the only way to do chained assignments.
But as for the necessity of this, compare Perl, where things like:
($first, $second, @rest)=make_list();
are allowed. But heh, it's IDL.
JD
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| Re: When Ptr_New doesn't work [message #29017 is a reply to message #28954] |
Wed, 23 January 2002 10:07 |
Pavel A. Romashkin
Messages: 531
Registered: November 2000
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Senior Member |
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I think the key here is, anything in parenthesis gets *evaluated* and as
such is an expression. And it is not really surprising that one is not
allowed to assign a value to an expression. This is the strength of IDL
- it creates temporary variables as expressions, and allows to use them
just like named variables. Unlike, say, IGOR Pro that requres you to
define a named variable to even plot it.
Pavel
Richard Younger wrote:
>
> This seems very strange to me. I think of parenthesis as not modifying
> their arguments at all, since they should (in my own little idl, er,
> ideal world) change only precedence and not value.
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