| Re: Copying a hash [message #81001] |
Mon, 06 August 2012 14:44  |
![Paul Van Delst[1] is currently offline](theme/default/images/xoffline.png.pagespeed.ic.XRkd1fkXye.png "Paul Van Delst[1] is currently offline")
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
|
Senior Member |
|
|
On 08/06/12 16:54, Matt wrote:
> Hi All,
>
> Does anyone know if there's a simple way that I can make a copy of a hash, which I can then edit independently of the
> original? For example, it seems that, like a pointer, changes that I make to the copy are also applied to the
> original:
>
> IDL> original=hash('A', [1, 2]) IDL> copy=original IDL> copy['A', 1]=10 IDL> print, copy A: 1 10 IDL>
> print, original A: 1 10
>
> I can copy to a new hash key-by-key:
>
> copy=hash() foreach variable, original, key do copy[key]=original[key]
>
> Which works fine, unless one of the elements in the hash is itself a hash, then I end up with the same problem one
> level down.
>
> Is there something simple I'm missing here?
This is what the documentation says:
-----%<-----
To create a new hash variable whose elements are copies of the values in the original hash, you could use the following:
newHash = HASH(origHash.Keys(), origHash.Values())
Another method to copy a hash is to use array syntax to copy all of the elements:
newHash = origHash[*]
This is equivalent to using HASH(origHash.Keys(), origHash.Values()) and is provided as a programming shortcut.
For example:
hash1 = HASH('key1', 1, 'key2', 2)
hash2 = hash1[*]
hash2['key1'] = 'hello'
HELP, hash1['key1'], hash2['key2']
IDL Prints:
<Expression> STRING = 1
<Expression> STRING = 'hello'
Note that the value in hash1 remains unchanged.
-----%<-----
There's no mention of what happens if a hash value is itself a hash though.
cheers,
paulv
|
|
|
|
| Re: Copying a hash [message #81100 is a reply to message #81001] |
Mon, 06 August 2012 15:56  |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
|
Senior Member |
|
|
With apologies for replying to my own email, but I had to try it out....
On 08/06/12 17:44, Paul van Delst wrote:
> On 08/06/12 16:54, Matt wrote:
>> Hi All,
>>
>> Does anyone know if there's a simple way that I can make a copy of a hash, which I can then edit independently of the
>> original? For example, it seems that, like a pointer, changes that I make to the copy are also applied to the
>> original:
>>
>> IDL> original=hash('A', [1, 2]) IDL> copy=original IDL> copy['A', 1]=10 IDL> print, copy A: 1 10 IDL>
>> print, original A: 1 10
>>
>> I can copy to a new hash key-by-key:
>>
>> copy=hash() foreach variable, original, key do copy[key]=original[key]
>>
>> Which works fine, unless one of the elements in the hash is itself a hash, then I end up with the same problem one
>> level down.
>>
>> Is there something simple I'm missing here?
>
> This is what the documentation says:
>
> -----%<-----
> To create a new hash variable whose elements are copies of the values in the original hash, you could use the following:
>
> newHash = HASH(origHash.Keys(), origHash.Values())
>
> Another method to copy a hash is to use array syntax to copy all of the elements:
>
> newHash = origHash[*]
>
> This is equivalent to using HASH(origHash.Keys(), origHash.Values()) and is provided as a programming shortcut.
>
> For example:
>
> hash1 = HASH('key1', 1, 'key2', 2)
> hash2 = hash1[*]
> hash2['key1'] = 'hello'
> HELP, hash1['key1'], hash2['key2']
>
> IDL Prints:
>
> <Expression> STRING = 1
> <Expression> STRING = 'hello'
>
> Note that the value in hash1 remains unchanged.
> -----%<-----
>
> There's no mention of what happens if a hash value is itself a hash though.
IDL> o=hash('a',[1,2],7,'a string',5.0,hash('b',indgen(10)))
IDL> print, o
5.00000: <ObjHeapVar1(HASH)>
a: 1 2
7: a string
IDL> c=o[*]
IDL> print, c
5.00000: <ObjHeapVar1(HASH)>
a: 1 2
7: a string
IDL> c['a',1]=10
IDL> print, c
5.00000: <ObjHeapVar1(HASH)>
a: 1 10
7: a string
IDL> print, o
5.00000: <ObjHeapVar1(HASH)>
a: 1 2
7: a string
But, as you can see, the hash in each is the same object reference.
IDL> print, c[5.0,'b']
0 1 2 3 4 5 6 7 8 9
IDL> print, o[5.0,'b']
0 1 2 3 4 5 6 7 8 9
IDL> c[5.0,'b',4]=100
IDL> print, o[5.0,'b']
0 1 2 3 100 5 6 7 8 9
Bummer. To be honest, I'm not sure what the correct behaviour should be. Recursively copy all the components? I guess if
we think of the numbers and strings as objects also, then the answer should probably be yes.... ? Why duplicate one type
of object (int, float, or string) but not another (hash or list)? Still... it just doesn't seem right.
cheers,
paulv
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
