Random questions

Daniel_BÃ¼nzli

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Date:  20091203 (14:48) 
From:  Daniel_BÃ¼nzli <daniel.buenzli@e...> 
Subject:  Re: [Camllist] Random questions 
Hello Cedric, Thanks for your comments. Comments on your comments. > let rint () = (Random.bits () lsl 1) lxor (Random.bits ());; That was actually my first version. However I dropped it because I thought that generating a new random number by the interaction of the bits of two successive PRN could somehow change the underlying quality of the generator. Any thought ? > if Random.bool is more efficient than Random.bits, the latter seems the > best; to be benchmarked Random.bool is Random.bits () land 0 = 1. >> 2) Generate an arbitrary int in [0;max] (bounds included) >> >> let random_uint ?(max = max_int) = >> Â if max < 0 then invalid_arg "negative max" else >> Â if max = max_int then Random.bits else >> Â let bound = max + 1 in >> Â fun () > Random.int bound > > Seems correct. > I don't know if use of exception is more efficient or not, it is to try: > > let random_uint ?(max = max_int) () = > Â try if max = max_int then Random.bits () else Random.int (max + 1) > Â with Invalid_argument "Random.int" > invalid_arg "negative max" You lose the ability to partially apply random_uint with the max and avoid the if on each call. > I think it is better to use the bool trick; > simpler and doesn't use Int32: > > let random_int ?(max = max_int) = > Â if max < 0 then invalid_arg "negative max" else > Â if max = 0 then 0 else > Â if Random.bool () > Â then if max = max_int then Random.bits () > Â Â Â Â else Random.int max > Â else negate_minus_1 (Random.int (max  1)) (* inline? *) > Yes, thanks. Your version can also be reordered to be partially applicable. It should also be noted that all our functions for [int]s don't work with the intended semantics on 64 bits platforms. >> 5) Generate an arbitrary float in [0;max] (bounds included) >> >> let after_one = 1. +. epsilon_float >> let random_ufloat ?(max = max_float) = >> Â if max < 0. then invalid_arg "negative max" else >> Â fun () > (Random.float after_one) *. max > > This function is less interesting than Random.float: > only less or as many as values as floats in [0.;1.] can be generated > so you will miss many values only to be able to produce max > (for example, you cannot 1+eps if you try random_ufloat ~max:2. ()) The point you make maybe valid but unfortunately that's also the case for Random.float as this is the way it is implemented (generate a number in [0;1[ and scale it). Best, Daniel