c - Getting a random real number in a certain range using WELL512 -

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i'm using well512 pseudorandom number generator function described in this paper. function returns random unsigned long value.

how use return value produce random real number within range - float between 340.92491 , 859812.53198 inclusive.

the documentation c rand() function seems warn against using mod.

well, mathematically it's just:

min_value + (max_value - min_value) * (my_random() / (long double)ulong_max)

(assuming my_random() returns uniformly distributed number between 0 , ulong_max)

however, depending on exact values of min_value, max_value, , ulong_max, floating point numbers more others.

each possible random unsigned long maps float formula. since number of distinct floating point numbers between min_value , max_value not ulong_max, unsigned longs map same floating point number or floating point numbers have no unsigned long map them or both.

fixing make result uniform is... non-trivial, think. maybe better read can cite paper.

[edit]

or see answer question:

generating random floating-point values based on random bit stream

that answer depends on internals of ieee double representation. not sure understand how works.

[edit 2]

ok understand how works. idea pick random floating point representation between min , max, , throw out probability inversely proportional scale represented exponent. because uniform distribution, numbers between (say) 1/2 , 1 need half between 1 , 2, number of floating point representations in ranges same.

i think make code more efficient first picking exponent on logarithmic scale -- say, using ffs on randomly-chosen integer -- , picking mantissa @ random. hm...


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