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Topic: Loss of Array data (Read 691 times) |
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Matt
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Re: Loss of Array data
« Reply #2 on: Jul 21st, 2013, 05:21am » |
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Hi,
Just noticed my last reply to this seems to have been sucked up into the ether.
on Jul 19th, 2013, 8:47pm, Richard Russell wrote:You mustn't swap a local array with a global array. The BB4W docs call this out:
"LOCAL arrays are stored on the stack and special precautions are required as a result. Firstly, avoid swapping a local array with a global array (i.e. one stored on the heap).... You can safely copy the array, because the data is copied rather than the pointer" |
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I hadn't realised this as I have only read the main help on it. But that's fine. I only need to copy the ndx%() to Index%() so I'll use the loop.
I assume, by trial and error, that Index%() = ndx%() only copies the pointers as well.
I would still be interested to know why the array was cleared before the DIM and then restored after with no apparent movemement of data or pointers, and in this case only during the second pass.
Matt
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admin
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Re: Loss of Array data
« Reply #3 on: Jul 21st, 2013, 06:51am » |
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on Jul 21st, 2013, 05:21am, Matt wrote:| I assume, by trial and error, that Index%() = ndx%() only copies the pointers as well. |
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Huh? My previous reply (second example) shows that the opposite is the case, and the docs also state explicitly "You can safely copy the array, because the data is copied rather than the pointer". Note the comments in this code taken from the docs:
Code: SWAP globalarray(),localarray() : REM Don't do this!
globalarray() = localarray() : REM This is OK
Anyway, think about it. Copying pointers wouldn't copy the array, it would create an alias of the array: modifying the contents of one would cause the contents of the other to change!
Quote:| I would still be interested to know why the array was cleared before the DIM and then restored after with no apparent movemement of data or pointers, and in this case only during the second pass. |
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Clearly what you describe, taken literally, is impossible. Trying to find out what actually did happen could be difficult and ultimately will only confirm that swapping a local and global array is a very bad idea!
Richard.
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