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Re: random integers between 0 and 1,000,000 [message #45960 is a reply to message #45956]

Mon, 24 October 2005 07:04

James Kuyper
Messages: 425
Registered: March 2000

Senior Member

Norbert Hahn wrote:
> "Peter Albert" <peter.albert@gmx.de> wrote:
>
>> Hi Mike,
>>
>> I'd guess you got the negative numbers because you defined the array
>
> Unfortunately your guess does not correspond to the docu:

I don't follow you. How does his guess fail to correspond to the
docu[mentation?]?

>> weights beforehand to be of integer type. Assigning it with values of
>> type long will lead to negative numbers if the values are larger than
>> 2^15-1.
>
> fix uses 15+1 bits for integer numbers (one bit is used for sign)
> long uses 31+1 bits for integer numbers ...
> ulong uses 32 bits for unsigned numbers. There is no sign.
> long64...

Agreed. Therefore, if the code the original poster showed us were
preceeded by the following statement:

weights = intarr(29)

then the expression:

weights[i] = ULONG(1000000 * RANDOMU( seed, 1 ))

takes a 32 bit unsigned long with a value somewhere in the range from 0
to 1000000, and converts it into a 16 bit signed int, with a range from
-32768 to 32767. This is the only way that weights[i] could ever gain a
negative value from that expression. For any other data types, all of
the values would be positive.

> So the original problem comes from interpreting the internal bits of
> unsigned numbers. The preferable function for transforming 0...1 float
> to integer without sign would be long.

Using 'long' rather than 'ulong' wouldn't do any good if the type of
"weights" itself is INT.

Note to original poster: loops are very inefficient in IDL. You'd be
better off writing:

weights[*] = LONG(1000000*RANDOMU(seed, 29))

If the length of "weights" happens to be 29, and not something longer,
you're even better off using:

weights = LONG(1000000*RANDOMU(seed,29))

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[Message index]

		Re: random integers between 0 and 1,000,000 By: Norbert Hahn on Mon, 24 October 2005 09:38
		Re: random integers between 0 and 1,000,000 By: Norbert Hahn on Mon, 24 October 2005 09:36
		Re: random integers between 0 and 1,000,000 By: James Kuyper on Mon, 24 October 2005 07:04
		Re: random integers between 0 and 1,000,000 By: peter.albert@gmx.de on Mon, 24 October 2005 06:23
		Re: random integers between 0 and 1,000,000 By: Norbert Hahn on Mon, 24 October 2005 01:59
		Re: random integers between 0 and 1,000,000 By: peter.albert@gmx.de on Sun, 23 October 2005 23:12
		Re: random integers between 0 and 1,000,000 By: steven.leopardi on Sat, 22 October 2005 19:44
		Re: random integers between 0 and 1,000,000 By: James Kuyper on Mon, 24 October 2005 12:06

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