| Re: taming the shrew, a.k.a. structure [message #25990] |
Thu, 02 August 2001 10:30  |
HILBERMAN
Messages: 7
Registered: July 2001
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Junior Member |
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David Fanning wrote:
> Here is what I tried:
>
> IDL> month_struct = {month_struct, name: ptr_new( ), day: ptr_new(), $
> temp_c: Ptr_New()}
> IDL> station = {station, number:0L, month:{month_struct}}
> IDL> po_basin = replicate (station, 2)
> IDL> po_basin.month.name = ptr_new( strarr(2190) )
> IDL> *((po_basin)[0].month.name) = 'dad'
> IDL> *((po_basin)[1].month.name) = 'mom'
>
> What is the problem!?
>
Good question!
I took snipets of advice from all the responses and played around with them a
bit, but I'm still not successful at getting the pointers to work.
Unfortunately, I'm not quite sure what I'm doing wrong. Since I only started
programming in march, and that was in C (IDL just a few weeks ago), my comfort
with pointers is...well...closer to discomfort. I appreciate you all taking the
time to help out. If you want to get the full picture, I just did a quick and
dirty job of posting the .pro file (along with the data file it requires) on my
website at
http://instaar.colorado.edu/~hilberma/
click on the idl link.
Two questions come to mind:
David, you gave the nice example of *((po_basin)[0].month.name) = 'dad' as a way
to define the zeroth element of the po_basin array; is this the same sytax you
would use to access and manipulate po_basin[0]? In other words, does the *
function like the & does in C? I assume so, but I don't yet trust my assumptions
w.r.t. IDL.
Also, Pavel mentioned a stylistic preference w.r.t. spaces--is there a good web
resource for beginers that gives some tips on good IDL programming style?
Thanks!
RDH
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| Re: taming the shrew, a.k.a. structure [message #26001 is a reply to message #25990] |
Wed, 01 August 2001 07:35  |
marc schellens[1]
Messages: 183
Registered: January 2000
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Senior Member |
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Using IDL 5.4 I get the following:
IDL> month_struct = {month_struct, name: ptr_new( ), day: ptr_new( ),
temp_c:ptr_new( )}
IDL> station = {station, number:0L, month:{month_struct}}
IDL> po_basin = replicate (station,10)
IDL> help,po_basin
PO_BASIN STRUCT = -> STATION Array[10]
IDL> po_basin.month.name = ptr_new( strarr(2190) )
IDL> help,po_basin.month.name
<Expression> POINTER = Array[10]
IDL> help,po_basin[0].month.name
<Expression> POINTER = <PtrHeapVar1>
IDL> help,po_basin[1].month.name
<Expression> POINTER = <PtrHeapVar1>
note that you set all 10 (or howmany ever) pointer to the SAME heap
variable. I assume somehow later therefore you will get an error.
This is a situation were you definitely (objections?) need a for loop:
for i=0,9 do po_basin[i].month.name= ptr_new( strarr(2190))
Anyway, your snippet works without error.
But you are right,
an object would do the task indeed more elegant/ less error-prone.
hope this helps,
marc
HILBERMAN wrote:
>
> To put it simply, you rock. I have now successfully created a mess: an
> array of a structure
> that contains another embedded structure. Unfortunately, I'm still not
> 'pointed' in the right
> direction. When I try to apply the pointer tip to 'the mess' I get the
> error:
> % Conflicting data structures: <POINTER (<NullPointer>)>,MONTH_STRUCT.
>
> Here's how I have things set up right now.
> month_struct = {month_struct, name: ptr_new( ), day: ptr_new( ), temp_c:
> ptr_new( )}
> station = {station, number:0L, month:{month_struct}}
> po_basin = replicate (station, howmany)
>
> po_basin.month.name = ptr_new( strarr(2190) )
> ...
>
> Any ideas? Since station references a structure with pointers, do I have to
> make a pointer to
> station as well--or something similar? I can't say I know a lick about
> objects, but this is
> kinda seeming like a problem to be solved by an object? Oyvey.
>
> Cheers,
> Davida
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| Re: taming the shrew, a.k.a. structure [message #26020 is a reply to message #26001] |
Wed, 01 August 2001 10:48 |
david[2]
Messages: 100
Registered: June 2001
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Senior Member |
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HILBERMAN writes:
>
> To put it simply, you rock. I have now successfully created a mess: an
> array of a structure
> that contains another embedded structure. Unfortunately, I'm still not
> 'pointed' in the right
> direction. When I try to apply the pointer tip to 'the mess' I get the
> error:
> % Conflicting data structures: <POINTER (<NullPointer>)>,MONTH_STRUCT.
>
> Here's how I have things set up right now.
> month_struct = {month_struct, name: ptr_new( ), day: ptr_new( ), temp_c:
> ptr_new( )}
> station = {station, number:0L, month:{month_struct}}
> po_basin = replicate (station, howmany)
>
> po_basin.month.name = ptr_new( strarr(2190) )
> ...
>
> Any ideas? Since station references a structure with pointers, do I have to
> make a pointer to
> station as well--or something similar? I can't say I know a lick about
> objects, but this is
> kinda seeming like a problem to be solved by an object? Oyvey.
Here is what I tried:
IDL> month_struct = {month_struct, name: ptr_new( ), day: ptr_new(), $
temp_c: Ptr_New()}
IDL> station = {station, number:0L, month:{month_struct}}
IDL> po_basin = replicate (station, 2)
IDL> po_basin.month.name = ptr_new( strarr(2190) )
IDL> *((po_basin)[0].month.name) = 'dad'
IDL> *((po_basin)[1].month.name) = 'mom'
What is the problem!?
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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| Re: taming the shrew, a.k.a. structure [message #26022 is a reply to message #26001] |
Wed, 01 August 2001 10:39 |
Pavel A. Romashkin
Messages: 531
Registered: November 2000
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Senior Member |
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I can't really see why you are getting an error. I copy-pasted your code
onto the command line and it works just fine. I got rid of a few spaces
though :-)
month_struct = {month_struct, name: ptr_new(), day: $
ptr_new(), temp_c:ptr_new()}
station = {station, number:0L, month:{month_struct}}
po_basin = replicate(station, 10)
po_basin.month.name = ptr_new(strarr(2190))
Cheers,
Pavel
BTW, I'd suggest using /Allocate keyword when making those pointers.
This way you can simply say
*(po_basin.month.name)[i] = strarr(2190). Also, looks to me that all
elements of po_basin in your example will have the same strarr(2190) as
their Month.Name. Is this what you wanted? If you want different
contents, you will have to specify pointer contents individually for
every element of the array.
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| Re: taming the shrew, a.k.a. structure [message #26025 is a reply to message #26001] |
Wed, 01 August 2001 10:02 |
HILBERMAN
Messages: 7
Registered: July 2001
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Junior Member |
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To put it simply, you rock. I have now successfully created a mess: an
array of a structure
that contains another embedded structure. Unfortunately, I'm still not
'pointed' in the right
direction. When I try to apply the pointer tip to 'the mess' I get the
error:
% Conflicting data structures: <POINTER (<NullPointer>)>,MONTH_STRUCT.
Here's how I have things set up right now.
month_struct = {month_struct, name: ptr_new( ), day: ptr_new( ), temp_c:
ptr_new( )}
station = {station, number:0L, month:{month_struct}}
po_basin = replicate (station, howmany)
po_basin.month.name = ptr_new( strarr(2190) )
...
Any ideas? Since station references a structure with pointers, do I have to
make a pointer to
station as well--or something similar? I can't say I know a lick about
objects, but this is
kinda seeming like a problem to be solved by an object? Oyvey.
Cheers,
Davida
Todd Clements wrote:
> HILBERMAN <hilberma@colorado.edu> wrote:
>> I'm writing a program that takes in data and places it in a structure.
>> Everything is fine and dandy except that I would like to change the
>> length of the arrays in the structure after the data is read in and the
>> actual lengths (rather than the upper bound) of the arrays are
>> determined. I've tried to use a statement like:
>> po_basin[0].temp = (po_basin[0]).temp[0:1024]
>> but it's not working, and I don't know where to go from here. Any
>> suggestions?
>
> Pointers are going to be your friends here. They are the only way to
> change the size of data in structures at runtime (and really, you aren't
> changing the size of the data structure, but it seems like it). Pointers
> are fun and useful things, but that also means that you have to worry
> about cleaning them up when you're done.
>
> myStruct = {myStruct, array1: ptr_new()}
>
> Then, in your code:
>
> myStruct.array1 = ptr_new( fltarr( startSize ) )
>
> Of course, if you need to shorten or lengthen this later, you have to
> remember to dispose of the pointer that you made AFTER you make the new
> one.
>
> temp = myStruct.array1
> myStruct.array1 = ptr_new( (*myStruct.array1)[0:1024] )
> ptr_free, temp
>
> It's sometimes a lot of work to use pointers, but they do exactly what
> you describe you want to.
>
> Or, if you don't want to use pointers, you can do it the cheating way if
> your arrays aren't going to be too large. Put in the array the maximum
> size that you ever figure you'll use (no one will ever need more than
> 540K, right? =), and also keep an array size indicator in your
> structure, such as:
>
> myStruct = { myStruct, array1: fltarr( 10000 ), maxArray1: 0L }
>
> Then you set maxArray1 to the "size" of the array and make sure to pay
> attention to that when you use the array.
>
>> Also, is there a way to make an array of an array of a structure, i.e.
>> something.something.data?
>> Please say 'yes'
>
> 'yes'
>
> struct1 = {struct1, a: 0, b: 0 }
> struct2 = {struct2, c: {struct1}, d: 0 }
>
> struct2.c.a = 3 ;; this works
>
> Good luck with the program. Hope this helps!
> Todd
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| Re: taming the shrew, a.k.a. structure [message #26045 is a reply to message #26001] |
Tue, 31 July 2001 16:26 |
david[2]
Messages: 100
Registered: June 2001
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Senior Member |
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Todd Clements writes:
> Pointers
> are fun and useful things, but that also means that you have to worry
> about cleaning them up when you're done.
>
> myStruct = {myStruct, array1: ptr_new()}
>
> Then, in your code:
>
> myStruct.array1 = ptr_new( fltarr( startSize ) )
>
Too true. Although one could easily write a little
program to free structure pointers:
PRO Free_Pointers, structure
FOR j=0,N_Tags(structure)-1 DO BEGIN
type = Size(structure.(j), /TName)
IF type EQ 'POINTER' THEN Ptr_Free, structure.(j)
ENDFOR
END
Extra credit for making this recursive. :-)
> Of course, if you need to shorten or lengthen this later, you have to
> remember to dispose of the pointer that you made AFTER you make the new
> one.
Not really. The nice thing about IDL pointers is that
IDL will take care of all the memory manipulation
for you. You don't have to worry about it at all.
> temp = myStruct.array1
> myStruct.array1 = ptr_new( (*myStruct.array1)[0:1024] )
> ptr_free, temp
This really becomes nothing more than this:
*myStruct.array1 = (*myStruct.array1)[0:1024]
There is no need to free the old pointer, make
a new one etc. Pointers are like IDL variables
in this respect. They *always* point to the
current thing you have pointed them too.
> It's sometimes a lot of work to use pointers, but they do exactly what
> you describe you want to.
If by "a lot of work" you mean you have to use
more parentheses than normal, I would agree with
you. Sometimes that syntax drives me crazy! But
the benefits you gain far exceed the cost.
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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| Re: taming the shrew, a.k.a. structure [message #26046 is a reply to message #26045] |
Tue, 31 July 2001 15:35 |
Todd Clements
Messages: 23
Registered: January 2001
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Junior Member |
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HILBERMAN <hilberma@colorado.edu> wrote:
> I'm writing a program that takes in data and places it in a structure.
> Everything is fine and dandy except that I would like to change the
> length of the arrays in the structure after the data is read in and the
> actual lengths (rather than the upper bound) of the arrays are
> determined. I've tried to use a statement like:
> po_basin[0].temp = (po_basin[0]).temp[0:1024]
> but it's not working, and I don't know where to go from here. Any
> suggestions?
Pointers are going to be your friends here. They are the only way to
change the size of data in structures at runtime (and really, you aren't
changing the size of the data structure, but it seems like it). Pointers
are fun and useful things, but that also means that you have to worry
about cleaning them up when you're done.
myStruct = {myStruct, array1: ptr_new()}
Then, in your code:
myStruct.array1 = ptr_new( fltarr( startSize ) )
Of course, if you need to shorten or lengthen this later, you have to
remember to dispose of the pointer that you made AFTER you make the new
one.
temp = myStruct.array1
myStruct.array1 = ptr_new( (*myStruct.array1)[0:1024] )
ptr_free, temp
It's sometimes a lot of work to use pointers, but they do exactly what
you describe you want to.
Or, if you don't want to use pointers, you can do it the cheating way if
your arrays aren't going to be too large. Put in the array the maximum
size that you ever figure you'll use (no one will ever need more than
540K, right? =), and also keep an array size indicator in your
structure, such as:
myStruct = { myStruct, array1: fltarr( 10000 ), maxArray1: 0L }
Then you set maxArray1 to the "size" of the array and make sure to pay
attention to that when you use the array.
> Also, is there a way to make an array of an array of a structure, i.e.
> something.something.data?
> Please say 'yes'
'yes'
struct1 = {struct1, a: 0, b: 0 }
struct2 = {struct2, c: {struct1}, d: 0 }
struct2.c.a = 3 ;; this works
Good luck with the program. Hope this helps!
Todd
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