| Re: structures still confusing [message #49069] |
Tue, 13 June 2006 09:33 |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Tue, 13 Jun 2006 09:13:49 +0100, Allan Whiteford wrote:
> Martin,
>
> print,(s.m)[0]
>
> is probably less confusing.
But alas, way less efficient, since to access just that one element, it
creates an (arbitrarily large) temporary vector (s.m). I don't
specifically cover multi-dimensional struct slices, but some info on
precedence etc. can be found in:
http://www.dfanning.com/misc_tips/precedence.html
The basic hint here is that structure dereference and array dereference
are at the same level of precedence, and don't step on eachother's toes
(both left-right associative), so unless you have pointers mixed in, you
shouldn't need *any* parentheses to get to an arbitrarily deeply nested
array of structure of structure of array of ... and if you do have
pointers mixed in, you just need a *single* pair of parentheses around
each and every pointer expression (except a top level pointer). Though
they result in bizarre looking expressions, the rules are quite simple.
JD
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| Re: structures still confusing [message #49070 is a reply to message #49069] |
Tue, 13 June 2006 08:45  |
rother
Messages: 9
Registered: April 2000
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Junior Member |
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Hi,
On Jun13 10:54, Ben Tupper wrote:
> Martin Rother wrote:
>> (X). print, s.m[2]
>> 3 3 3
> Which is different than asking for the ith elements
> of the m array in ALL the structures in the vector s.
> IDL> print, s.m[0]
> 1 1 1
[...]
> I agree it can be confusing (wait till you have
> pointers in there!) but is very handy sometimes, too.
mmmhhh. I already used nested structures with
and without pointers, but I always used a vast amount
of brackets... so, stupid well-behaving, I indeed never
was aware of this method of 'slicing'! I'm getting
more and more comfortable with that idea... :-)
thanks for all that hints,
m.
--
Martin Rother (rother@gfz-potsdam.de) +331 / 288-1272 Section 2.3
GeoForschungsZentrum Potsdam, Germany
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| Re: structures still confusing [message #49073 is a reply to message #49070] |
Tue, 13 June 2006 07:54 |
btt
Messages: 345
Registered: December 2000
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Senior Member |
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Martin Rother wrote:
> Hi Gurus,
>
> finally something, where I don't know,
> if it's odd idl syntax or not...
>
> something about structures.
>
> FUNCTION test_struct
>
> s = [{ m : [1L, 2L, 3L], n :1L }]
>
> FOR i = 1L, 2L DO BEGIN
> ;
> s = [s, { m : [1L, 2L, 3L], n : i }]
> ;
> ENDFOR
>
> return, s
>
> END
>
> this creates an array of structures:
>
> (X). s = test_struct()
> (X). help,/struct,s
> ** Structure <fa330>, 2 tags, length=16, data length=16, refs=1:
> M LONG Array[3]
> N LONG 1
> (X). print, n_elements(s)
> 3
> (X).
>
> so far ok, I guess. but
>
> (X). help,s.m
> <Expression> LONG = Array[3, 3]
> (X). print, s.m
> 1 2 3
> 1 2 3
> 1 2 3
> (X). print, s.m[0]
> 1 1 1
> (X). print, s.m[1]
> 2 2 2
> (X). print, s.m[2]
> 3 3 3
>
> is a *bit* confusing. isn't it?
>
>
> best regards,
> martin.
Hi,
Perhaps you are looking to get the entire m array of the ith element in
vector s?
IDL> s = [{ m : [1L, 2L, 3L], n :1L }]
IDL> FOR i = 1L, 2L DO s = [s, { m : [1L, 2L, 3L], n : i }]
IDL> print, s[0].m
1 2 3
Which is different than asking for the ith elements of the m array in
ALL the structures in the vector s.
IDL> print, s.m[0]
1 1 1
Or how about the ith element of the m array in the jth element of the
vector s?
IDL> print, s[0].m[0]
1
I agree it can be confusing (wait till you have pointers in there!) but
is very handy sometimes, too.
Ben
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| Re: structures still confusing [message #49077 is a reply to message #49073] |
Tue, 13 June 2006 01:13 |
Allan Whiteford
Messages: 117
Registered: June 2006
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Senior Member |
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Martin,
print,(s.m)[0]
is probably less confusing. Similarly you can do:
print,(s.m)[0,1]
but you can't do:
print,s.m[0,1]
or, you can also do:
print,(s.m[0])[1]
Thanks,
Allan
Martin Rother wrote:
> Hi Gurus,
>
> finally something, where I don't know,
> if it's odd idl syntax or not...
>
> something about structures.
>
> FUNCTION test_struct
>
> s = [{ m : [1L, 2L, 3L], n :1L }]
>
> FOR i = 1L, 2L DO BEGIN
> ;
> s = [s, { m : [1L, 2L, 3L], n : i }]
> ;
> ENDFOR
>
> return, s
>
> END
>
> this creates an array of structures:
>
> (X). s = test_struct()
> (X). help,/struct,s
> ** Structure <fa330>, 2 tags, length=16, data length=16, refs=1:
> M LONG Array[3]
> N LONG 1
> (X). print, n_elements(s)
> 3
> (X).
>
> so far ok, I guess. but
>
> (X). help,s.m
> <Expression> LONG = Array[3, 3]
> (X). print, s.m
> 1 2 3
> 1 2 3
> 1 2 3
> (X). print, s.m[0]
> 1 1 1
> (X). print, s.m[1]
> 2 2 2
> (X). print, s.m[2]
> 3 3 3
>
> is a *bit* confusing. isn't it?
>
>
> best regards,
> martin.
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| Re: structures still confusing [message #49079 is a reply to message #49077] |
Mon, 12 June 2006 18:42 |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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Martin Rother wrote:
> ...
> so far ok, I guess. but
>
> (X). help,s.m
> <Expression> LONG = Array[3, 3]
> (X). print, s.m
> 1 2 3
> 1 2 3
> 1 2 3
> (X). print, s.m[0]
> 1 1 1
> (X). print, s.m[1]
> 2 2 2
> (X). print, s.m[2]
> 3 3 3
>
> is a *bit* confusing. isn't it?
Not as confusing as a procedure named "!". You're never going to live
that one down!
--
Mark Hadfield "Kei puwaha te tai nei, Hoea tahi tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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