| Re: Arrays of Structures [message #52452 is a reply to message #20472] |
Thu, 08 February 2007 11:11   |
Brian Larsen
Messages: 270
Registered: June 2006
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Senior Member |
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Paulv,
I think this is really just a wholly bad idea to do that at all. The
reason is what about this structure?
IDL> dat=replicate({a:findgen(50)}, 50)
IDL> help, dat
DAT STRUCT = -> <Anonymous> Array[50]
IDL> help, dat, /str
** Structure <84ccb14>, 1 tags, length=200, data length=200, refs=1:
A FLOAT Array[50]
Here the difference between dat[23].a and dat.a[23] is really
obvious.
IDL> print, dat[23].a
0.00000 1.00000 2.00000 3.00000 4.00000
5.00000 6.00000 7.00000 8.00000
9.00000 10.0000 11.0000 12.0000 13.0000
14.0000 15.0000 16.0000 17.0000
18.0000 19.0000 20.0000 21.0000 22.0000
23.0000 24.0000 25.0000 26.0000
27.0000 28.0000 29.0000 30.0000 31.0000
32.0000 33.0000 34.0000 35.0000
36.0000 37.0000 38.0000 39.0000 40.0000
41.0000 42.0000 43.0000 44.0000
45.0000 46.0000 47.0000 48.0000 49.0000
IDL> print, dat.a[23]
23.0000 23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000
23.0000 23.0000 23.0000 23.0000 23.0000
Be sure to just use the structure correctly and put the [ ] on the
thing that is an array whether that is the structure or the thing in
the structure (all the dereferencing stuff is true but tends to just
confuse the issue in my opinion)
Brian
------------------------------------------------------------ ---------
Brian A. Larsen
Dept. of Physics
Space Science and Engineering Lab (SSEL)
Montana State University - Bozeman
Bozeman, MT 59717
On Feb 8, 12:02 pm, Paul van Delst <Paul.vanDe...@noaa.gov> wrote:
> Mick Brooks wrote:
>> On Feb 8, 5:41 pm, "Brian Larsen" <balar...@gmail.com> wrote:
>
>>> This is what you did and it is true this is an array with 50 elements
>>> IDL> HELP, structs.a
>>> <Expression> INT = Array[50]
>
>>> but if you look at structs by itself not structs.a, it is the array,
>>> NOT structs.a
>
>>> IDL> HELP, structs
>>> STRUCTS STRUCT = -> <Anonymous> Array[50]
>
>>> So what is inside the structure?
>
>>> Here a is inside the struct and it is an int
>
>>> IDL> HELP, structs, /str
>>> ** Structure <84bd40c>, 1 tags, length=2, data length=2, refs=1:
>>> A INT 1
>
>> (I think) I'm following you to here...
>
>>> Meaning that structs.a[23] doesn't make sense because structs.a is an
>>> int.
>
>> ... but this is where you lose me. Doesn't the the first line that you
>> showed say that structs.a is an array of 50 ints?
>> Maybe I just can't read the output of HELP properly, but struts.a
>> certainly sometimes behaves as an array. Try PRINTing it, for
>> instance.
>> Or let's compare the output of HELP on a real array of 50 ints:
>
>> IDL> HELP, structs.a
>> <Expression> INT = Array[50]
>> IDL> HELP, indgen(50)
>> <Expression> INT = Array[50]
>
>>> While structs[23].a does make sense because structs is an array.
>
>> Yes, I'm happy with why this is right, but still not clear why
>> structs.a[23] is wrong.
>
> You know, I'm confused now too. Check out the precedence in the IDL help:
>
> Table 12-9: Operator Precedence
>
> Priority Operator
> First (highest) ( ) (parentheses, to group expressions)
> [ ] (brackets, to concatenate arrays)
> Second . (structure field dereference)
> [ ] (brackets, to subscript an array)
> ( ) (parentheses, used in a function call)
> etc...
>
> So, you see that the the structure field dereference operator, ".", and the array
> subscript operator, "[]" , have the same precedence. Operators with equal precedence are
> evaluated from left to right.
>
> So,
>
> structs.a[23]
>
> means FIRST dereference the structure field (structs.a), THEN index the array ([23]). The
> way I see it,
>
> structs.a[23]
> and
> (structs.a)[23]
>
> should be equivalent.
>
> Maybe the weirdness has something to do with where the result of the dereference "goes" ?
> Hence the "<No name>" in the error message?
>
> Hmm.
>
> paulv
>
> --
> Paul van Delst Ride lots.
> CIMSS @ NOAA/NCEP/EMC Eddy Merckx
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