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Still missing features in IDL 8 [message #72898] Tue, 12 October 2010 14:35 Go to previous message
penteado is currently offline  penteado
Messages: 866
Registered: February 2018
Senior Member
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I was showing IDL's lists and hashes to a friend, who immediately
noticed two important missing points:

1) A new method (or a new keyword, like, /pointer, to the existing
toarray()), that would create from an arbitrary list an array of
pointers, where each element in the array is a pointer to the
corresponding element of the list. That is, something equivalent to

function topointerarray,list
ret=ptrarr(n_elements(list))
foreach el,list,i do ret[i]=ptr_new(el)
return,ret
end

And a new keyword to list() and list::add (such as /dereference) that
would do the opposite: given a pointer array, would add to the list
the variables pointed to by each element of the pointer array (or !
null in case the element is a null pointer or a pointer to an
undefined variable).

Also, in some cases it would be desirable for list::toarray() and
hash::tostruct() to have a no_copy keyword, that would empty the list/
hash when making the array/structure, instead of making a copy. This
is already present in list() and list::add, but is also missing in
hash().

Why is (1) so needed? Besides better handling those conversions
(particularly when interfacing with old code that returns/expects
pointer arrays), for getting around the lack of:

2) This is not a simple additional feature as (1), but rather a change
to the language: Make a way for a list/hash element not be an
expression. One example of the problem is this error:

IDL> a=list()
IDL> a.add,bindgen(3)
IDL> a.add,-dindgen(2)
IDL> print,a
0 1 2
-0.0000000 -1.0000000
IDL> print,(a[1])[0]
-0.0000000
IDL> (a[1])[0]=1.0
% Expression must be named variable in this context: <DOUBLE
Array[2]>.
% Execution halted at: $MAIN$

This error happend because (a[1]) is an expression:

IDL> help,(a[1])
<Expression> DOUBLE = Array[2]

And I can see why it is, as I guess it is the result of a call to
list::overloadbracketsrightside(). But this makes it very inconvenient
to change pieces of list/hash elements, like in the line that caused
the error above. It would be necessary to retrieve the element,
assigning it to some variable, change that variable, then put it back
in the list/hash. Very awkward, and unnecessary if a pointer array is
used instead of the list:

IDL> a=ptrarr(2)
IDL> a[0]=ptr_new(bindgen(3))
IDL> a[1]=ptr_new(-dindgen(2))
IDL> print,(*a[1])[0]
-0.0000000
IDL> (*a[1])[0]=1.0
IDL> print,(*a[1])[0]
1.0000000

Which obviously works because (*a[1]) is a variable:

IDL> help,(*a[1])
<PtrHeapVar2> DOUBLE = Array[2]

I realize that this is a change to the language, and somewhat tricky
to implement. But it is probably easier than was the addition of
operator overloading.

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