Best way to generate a random float in C

Question

What is the best way to generate a random float in C    Update  I want random floating point numbers from float Minvalue to float Maxvalue  I am using these numbers in unit testing of some mathematical methods

User · Answer

Another solution is to do this   static float NextFloat Random random        float f      do               byte   bytes   new byte 4           random NextBytes bytes           f   BitConverter ToSingle bytes  0             while  float IsInfinity f     float IsNaN f        return f

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Any reason not to use Random NextDouble and then cast to float  That will give you a float between 0 and 1   If you want a different form of  best  you ll need to specify your requirements  Note that Random shouldn t be used for sensitive matters such as finance or security - and you should generally reuse an existing instance throughout your application  or one per thread  as Random isn t thread-safe    EDIT  As suggested in comments  to convert this to a range of float MinValue  float MaxValue      Perform arithmetic in double type to avoid overflowing double range    double  float MaxValue -  double  float MinValue  double sample   rng NextDouble    double scaled    sample   range    float MinValue  float f    float  scaled    EDIT  Now you ve mentioned that this is for unit testing  I m not sure it s an ideal approach  You should probably test with concrete values instead - making sure you test with samples in each of the relevant categories - infinities  NaNs  denormal numbers  very large numbers  zero  etc

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Best approach  no crazed values  distributed with respect to the representable intervals on the floating-point number line  removed  uniform  as with respect to a continuous number line it is decidedly non-uniform    static float NextFloat Random random        double mantissa    random NextDouble     2 0  - 1 0         choose -149 instead of -126 to also generate subnormal floats         double exponent   Math Pow 2 0  random Next -126  128        return  float  mantissa   exponent               check here for subnormal floats  Warning  generates positive infinity as well  Choose exponent of 127 to be on the safe side   Another approach which will give you some crazed values  uniform distribution of bit patterns   potentially useful for fuzzing   static float NextFloat Random random        var buffer   new byte 4       random NextBytes buffer       return BitConverter ToSingle buffer 0       An improvement over the previous version is this one  which does not create  crazed  values  neither infinities nor NaN  and is still fast  also distributed with respect to the representable intervals on the floating-point number line    public static float Generate Random prng        var sign   prng Next 2       var exponent   prng Next  1  lt  lt  8  - 1      do not generate 0xFF  infinities and NaN      var mantissa   prng Next 1  lt  lt  23        var bits    sign  lt  lt  31     exponent  lt  lt  23    mantissa      return IntBitsToFloat bits      private static float IntBitsToFloat int bits        unsafe               return   float    amp bits            Least useful approach   static float NextFloat Random random           Not a uniform distribution w r t  the binary floating-point number line        which makes sense given that NextDouble is uniform from 0 0 to 1 0         Uniform w r t  a continuous number line                The range produced by this method is 6 8e38                Therefore if NextDouble produces values in the range of 0 0 to 0 1        10  of the time  we will only produce numbers less than 1e38 about        10  of the time  which does not make sense      var result    random NextDouble                        Single MaxValue -  double Single MinValue                       Single MinValue      return  float result      Floating point number line from  Intel Architecture Software Developer s Manual Volume 1  Basic Architecture  The Y-axis is logarithmic  base-2  because consecutive binary floating point numbers do not differ linearly

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Here is another way that I came up with  Let s say you want to get a float between 5 5 and 7  with 3 decimals   float myFloat  int myInt  System Random rnd   new System Random     void GenerateFloat     myInt   rnd Next 1  2000   myFloat    myInt   1000    5 5f      That way you will always get a bigger number than 5 5 and a smaller number than 7

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I prefer using the following code to generate a decimal number up to fist decimal point  you can copy paste the 3rd line to add more numbers after decimal point by appending that number in string  combined   You can set the minimum and maximum value by changing the 0 and 9 to your preferred value   Random r   new Random    string beforePoint   r Next 0  9  ToString     number before decimal point string afterPoint   r Next 0 9  ToString     1st decimal point   string secondDP   r Next 0  9  ToString     2nd decimal point string combined   beforePoint     afterPoint  decimalNumber  float Parse combined   Console WriteLine decimalNumber     I hope that it helped you

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I took a slightly different approach than others   static float NextFloat Random random        double val   random NextDouble       range 0 0 to 1 0     val -  0 5     expected range now -0 5 to  0 5     val    2     expected range now -1 0 to  1 0     return float MaxValue    float val      The comments explain what I m doing  Get the next double  convert that number to a value between -1 and 1 and then multiply that with float MaxValue

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One more version     I think this one is pretty good   static float NextFloat Random random         float  float MaxValue   2 0    rand NextDouble  -0 5         inline version float myVal    float  float MaxValue   2 0    rand NextDouble  -0 5      I think this      is the 2nd fastest  see benchmarks  is evenly distributed   And One more version    not as good but posting anyway   static float NextFloat Random random        return float MaxValue     rand Next     1073741824 0f  - 1 0f        inline version float myVal    float MaxValue     rand Next     1073741824 0f  - 1 0f      I think this      is the fastest  see benchmarks  is evenly distributed however because Next   is a 31 bit random value it will only return 2 31 values    50  of the neighbor values will have the same value    Testing of most of the functions on this page   i7  release  without debug  2 28 loops    Sunsetquest1  min  3 402823E 38  max  -3 402823E 38 time  3096ms  SimonMourier  min  3 402823E 38  max  -3 402819E 38 time  14473ms  AnthonyPegram min  3 402823E 38  max  -3 402823E 38 time  3191ms  JonSkeet      min  3 402823E 38  max  -3 402823E 38 time  3186ms  Sixlettervar  min  1 701405E 38  max  -1 701410E 38 time  19653ms  Sunsetquest2  min  3 402823E 38  max  -3 402823E 38 time  2930ms

[c#] Best way to generate a random float in C#

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