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Re: Array Subscripting Memory Usage (watch out!) [message #32378 is a reply to message #32328]

Fri, 04 October 2002 13:58

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Fri, 04 Oct 2002 13:13:14 -0700, Dick Jackson wrote:

>> STRING:
>> .reset
>> s=memory()&x=indgen(20,/string)&e=memory()& print,e[3]-s[0]
>> 748
>> .reset
>> s=memory()&x=indgen(10,/string)&e=memory()& print,e[3]-s[0]
>> 458
>> The difference is 290. How to explain, the string length is 12 of each
>> element. Normally I believe 1 char is 1 byte. So first result should
> be
>> 12 byte * 20 = 240 and the second one 120. The differnce should be
> 120.
>> So I have no explanation why it needs so much more memory. Any ideas?
>
> I think each string would be terminated by a null character, making it
> 13 bytes each, then increase that to 16 because of 4-byte words, but
> that still doesn't match... I give up at this point!

Actually, if you look through export.h, you can see that IDL strings are
not only null-terminated (like C), but also have other information saved
in the IDL_STRING structure, like the length and type of the string. You
typically need 12 bytes just for an empty string! Also of interest in
solving the puzzle:

IDL> print,'*'+indgen(1,/string)+'*'
* 0*

These strings are 12 characters long. That's 25 bytes so far, counting
the terminating null. One more 4-byte word will do it. I'll leave that
one as an exercise for the reader ;)

JD

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[Message index]

		Array Subscripting Memory Usage (watch out!) By: Dick Jackson on Wed, 02 October 2002 14:28
		Re: Array Subscripting Memory Usage (watch out!) By: JD Smith on Fri, 04 October 2002 13:58
		Re: Array Subscripting Memory Usage (watch out!) By: Dick Jackson on Fri, 04 October 2002 13:13
		Re: Array Subscripting Memory Usage (watch out!) By: R.Bauer on Fri, 04 October 2002 12:40
		Re: Array Subscripting Memory Usage (watch out!) By: R.Bauer on Thu, 03 October 2002 03:55
		Re: Array Subscripting Memory Usage (watch out!) By: R.Bauer on Thu, 03 October 2002 03:49

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