CyberShot CyberShot - 2 months ago 19
C++ Question

rand() between 0 and 1

So the following code makes 0 < r < 1

r = ((double) rand() / (RAND_MAX))


Why does having
r = ((double) rand() / (RAND_MAX + 1))
make -1 < r < 0?

Shouldn't adding one to RAND_MAX make 1 < r < 2?

Edit: I was getting a warning: integer overflow in expression

on that line, so that might be the problem. I just did
cout << r << endl
and it definitely gives me values between -1 and 0

Answer

This is entirely implementation specific, but it appears that in the C++ environment you're working in, RAND_MAX is equal to INT_MAX.

Because of this, RAND_MAX + 1 exhibits undefined (overflow) behavior, and becomes INT_MIN. While your initial statement was dividing (random # between 0 and INT_MAX)/(INT_MAX) and generating a value 0 <= r < 1, now it's dividing (random # between 0 and INT_MAX)/(INT_MIN), generating a value -1 < r <= 0

In order to generate a random number 1 <= r < 2, you would want

r = ((double) rand() / (RAND_MAX)) + 1
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