Creating Random hex numbers

I'm trying to create a random product ID for a project I'm developing. I'm just looking to create some random hex values. However, I noticed that when I used %x with rand(), it generated 8 digit hex values when I know the maximum value of rand() is 32767.

I have a code snippet...am I doing anything wrong?

Code:

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srand(time(NULL));
_tcscpy(ProductID, _T("{%8X-%4X-%4X-%4X-%12X}", rand(), rand(), rand(), rand(), rand()));
_tprintf(ProductID);

Output: {2102E416-3388587B-1C7A6E8-7FFDD000-    CCCCCCCC}

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If you only want the first four digits, you can do it via a bitwise AND operation.

Code:

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_tcscpy(ProductID, _T("{%8X-%4X-%4X-%4X-%12X}", rand(), rand() & 0xFFFF, rand() & 0xFFFF, rand() & 0xFFFF, rand()));

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Why do I get a 8 digit hex value when the maximum value is only 0x7FFF?

In all honesty, I don't think you should if you're using the rand() from stdlib.h/cstdlib.

Just out of curiosity, what does this print?

Code:

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_tprintf("%d\n", RAND_MAX);

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Code:

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_tprintf(_T("%d",RAND_MAX));

Output: 2020380200

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??? This doesn't make much sense to me...

What about:

Code:

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std::cout << RAND_MAX << std::endl;

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See what that prints.

You know what.... I wonder.....

Could you be using the _T macro all wrong, and those large digits are the address of a new string it's creating? Those addresses possibly point to your random numbers.

what is _T()?

Match parens.

Code:

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_tcscpy(ProductID, _T("{%8X-%4X-%4X-%4X-%12X}", rand(), rand(), rand(), rand(), rand()));

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Are you missing some intervening call in haste?

>> what is _T()?
It's a macro that converts the string inside to either a wide character string or a regular string depending on whether UNICODE is defined for your project or not.

AFAIK it does not take format parameters like that, although I might be wrong.

Quote:

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Originally Posted by swoopy

What about:

Code:

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std::cout << RAND_MAX << std::endl;

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See what that prints.

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7fff

Quote:

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Originally Posted by MacGyver

You know what.... I wonder.....

Could you be using the _T macro all wrong, and those large digits are the address of a new string it's creating? Those addresses possibly point to your random numbers.

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I believe you're right MacGyver...

Code:

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srand(time(NULL));
_tcscpy(ProductID, _T("{%8X-%4X-%4X-%4X-%12X}\n", 12345678, rand(), rand(), rand(), rand()));
_tprintf(ProductID);

Output: {7DDC7E09-3459C061-1C7A77E-7FFDB000-    CCCCCCCC}

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What would be a better way to do this then?

You shouldn't create GUIDs with such a poor RNG as the stdlib rand() anyway. The WinAPI provides a CreateGUID() function that creates a good GUID.

Even if the rand was good - the output cannot be named GUID... GUID creation algorithm uses some data about creation date, computer hardware and maybe something else to ensure the uniqueness of the return value during the Universe life... So the algorithm using only some pseudo-random generator will be wrong in any case, because it cannot guarantee such a uniqueness

Quote:

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Originally Posted by disruptor108

7fff

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Ok well that confirms your assumption about the value of RAND_MAX, and the print function is the problem. I would try some version of sprintf(). So instead of:

Code:

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>_tcscpy(ProductID, _T("{%8X-%4X-%4X-%4X-%12X}", rand(), rand(), rand(), rand(), rand()));

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Try:

Code:

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_tsprintf(ProductID, _T("{%8X-%4X-%4X-%4X-%12X}", rand(), rand(), rand(), rand(), rand()));

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And if that doesn't work or there is no such animal, then try sprintf():

Code:

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_sprintf(ProductID, "{%8X-%4X-%4X-%4X-%12X}", rand(), rand(), rand(), rand(), rand()));

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And if you're creating a GUID, use the WinAPI function CornedBee suggests.

Personally I'm very much not a fan of CoCreateGuid, instead, i initialize a variable to the size of _GUID and fill it with CryptGenRandom. Advertising my MAC address is not something I'm willing to do in the name of uniqueness.

Ehtyar.

Quote:

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Originally Posted by Ehtyar

Personally I'm very much not a fan of CoCreateGuid, instead, i initialize a variable to the size of _GUID and fill it with CryptGenRandom. Advertising my MAC address is not something I'm willing to do in the name of uniqueness.

Ehtyar.

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Then don't call it GUID

I didn't. I also believe the topic for this thread is "Creating Random hex numbers", to which my suggestion is perfectly suited.

Ehtyar.

Quote:

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Originally Posted by Ehtyar

Personally I'm very much not a fan of CoCreateGuid, instead, i initialize a variable to the size of _GUID and fill it with CryptGenRandom. Advertising my MAC address is not something I'm willing to do in the name of uniqueness.

Ehtyar.

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I strongly suggest you rethink that strategy. Using a different algorithm to everyone else gives a possibility that the GUIDs you pick will clash with GUIDs created by others, and even potentially with your own.
The idea you speak of means that you can no longer call them GUIDs, as they are not GLOBALLY unique. They are in fact only LOCALLY unique, with very high probability.

Your escuse about not advertising your MAC address is absurdly paranoid.

Can i remind you again that i made no mention of GUID, other than using the structure, because i like the format nor did i ever claim that my idea would generate "globally uniqe" anything. Can i also remind you again that this topic is called "Creating Random hex numbers", and that the code put forth in the initial post is even more likely to generate collisions than my own.

Ehtyar.

OK, true...I said I was trying to create random hex numbers, but yes...more specifically I was trying to create a random GUID for an installation program that I had created.

I was wondering...what header file needs to be included to use CoCreateGUID?

I think <ole2.h>, but MSDN will tell you for sure. Just search for the function and look at the bottom of its page. It tells you what header file to include and what library to link against.

Thanks a lot. You guys answered my problem :D