What is returned ?

[Skybuck Flying]

Hi,

I have a C question.

Let's assume MT is not defined

int holdrand = 1;

int __cdecl rand (void)
{
#ifdef _MT
_ptiddata ptd = _getptd();
return( ((ptd->_holdrand = ptd->_holdrand * 214013L + 2531011L) >> 16)
&
0x7fff );
#else /* _MT */
return(((holdrand = holdrand * 214013L + 2531011L) >> 16) & 0x7fff);
#endif /* _MT */
}

I have trouble understanding what this function returns.

My confusion is with this line:

return(((holdrand = holdrand * 214013L + 2531011L) >> 16) & 0x7fff);

Apperently this line assigns some value to holdrand.

Something like:

holdrand becomes holdrand * blablabla etc.

But what is returned ?

I think it returns holdrand after the calculation.

But.... if that would be true... then this random number generator simply
uses the previous rand value to calculate the next one ?

Weird... I don't understand it... so that's why I ask this newsgroup =D

Bye,
Skybuck

 

[Andrey Tarasevich]

...
My confusion is with this line:

return(((holdrand = holdrand * 214013L + 2531011L) >> 16) & 0x7fff);

Apperently this line assigns some value to holdrand.

This is equivalent to

holdrand = holdrand * 214013L + 2531011L;
return (holdrand >> 16) & 0x7fff;

Something like:

holdrand becomes holdrand * blablabla etc.

Yes, 'holdrand' becomes 'holdrand * 214013L + 2531011L'.

But what is returned ?

I think it returns holdrand after the calculation.

No. It returns '(holdrand >> 16) & 0x7fff', where 'holdrand' holds the
new value, i.e. value assigned by the inner assignment operator.

But.... if that would be true... then this random number generator simply
uses the previous rand value to calculate the next one ?

Of course. That's how oracle-type pseudo-random number generators work.
They are stateful functions, which means that they generate the new
state based on the previous state.

Weird... I don't understand it... so that's why I ask this newsgroup =D

What exactly don't you understand?