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Full Discussion: Floating point error in C
Top Forums Programming Floating point error in C Post 302100690 by Perderabo on Thursday 21st of December 2006 01:48:14 PM
Old 12-21-2006
Floating point numbers have a complex representation using base 2 rather than base 10. Scientific notation is close enough to understand what is happening. With scientific notation, we want a number between 1 and 10 multiplied by a power of 10: 8080.9940 = 8.0809940 * 10^3 and we can check that out with bc:
Code:
$ bc
bc 1.03 (Nov 2, 1994)
Copyright (C) 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
8.0809940 * 10^3
8080.9940000
^D$

But now we need to do that in binary... We want a number between 1 and 10 in binary (which means 1 and 2 in decimal) that can be multiplied by a power of 10 in binary to yield the decimal number 8080.9940. Fortunately, you picked an easy number! It's is obvious that dividing it by 4096 will yield a number between 1 and 2. So let's use bc to do this...
Code:
$ bc -l
bc 1.03 (Nov 2, 1994)
Copyright (C) 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
obase=2
scale=40
8080.9940/4096
1.111110010000111111100111011011001000101101000011100101011000000100\
0001100010010011011101001011110001101010011111101111100111011011001
l(4096)/l(2)
1100.000000000000000000000000000000000000000000000000000000000000000\
00000000000000000000000000000000000000000000000000000000000000000000\
00
obase=10
ibase=2
1.1111100100001111111001110110110010001011010000 * 10^1100
8080.9939999999478459358215332031250000000000000000
1.11111001000011111110011101101100100 * 10^1100
8080.99399995803833007812500000000000000

Now repeat the last operation, but replace 1.11111001000011111110011101101100100 with some other close-by binary number. No matter what you try, you cannot get exactly 8080.9940 Floating point numbers are bit more complex than this, but the exact same issue arises with them. There is no way to exactly represent 8080.9940 as a floating point number.
 

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Num(3o)                                                            OCaml library                                                           Num(3o)

NAME
Num - Operation on arbitrary-precision numbers. Module Module Num Documentation Module Num : sig end Operation on arbitrary-precision numbers. Numbers (type num ) are arbitrary-precision rational numbers, plus the special elements 1/0 (infinity) and 0/0 (undefined). type num = | Int of int | Big_int of Big_int.big_int | Ratio of Ratio.ratio The type of numbers. === Arithmetic operations === val (+/) : num -> num -> num Same as Num.add_num . val add_num : num -> num -> num Addition val minus_num : num -> num Unary negation. val (-/) : num -> num -> num Same as Num.sub_num . val sub_num : num -> num -> num Subtraction val ( */ ) : num -> num -> num Same as Num.mult_num . val mult_num : num -> num -> num Multiplication val square_num : num -> num Squaring val (//) : num -> num -> num Same as Num.div_num . val div_num : num -> num -> num Division val quo_num : num -> num -> num Euclidean division: quotient. val mod_num : num -> num -> num Euclidean division: remainder. val ( **/ ) : num -> num -> num Same as Num.power_num . val power_num : num -> num -> num Exponentiation val abs_num : num -> num Absolute value. val succ_num : num -> num succ n is n+1 val pred_num : num -> num pred n is n-1 val incr_num : num Pervasives.ref -> unit incr r is r:=!r+1 , where r is a reference to a number. val decr_num : num Pervasives.ref -> unit decr r is r:=!r-1 , where r is a reference to a number. val is_integer_num : num -> bool Test if a number is an integer === The four following functions approximate a number by an integer : === val integer_num : num -> num integer_num n returns the integer closest to n . In case of ties, rounds towards zero. val floor_num : num -> num floor_num n returns the largest integer smaller or equal to n . val round_num : num -> num round_num n returns the integer closest to n . In case of ties, rounds off zero. val ceiling_num : num -> num ceiling_num n returns the smallest integer bigger or equal to n . val sign_num : num -> int Return -1 , 0 or 1 according to the sign of the argument. === Comparisons between numbers === val (=/) : num -> num -> bool val (</) : num -> num -> bool val (>/) : num -> num -> bool val (<=/) : num -> num -> bool val (>=/) : num -> num -> bool val (<>/) : num -> num -> bool val eq_num : num -> num -> bool val lt_num : num -> num -> bool val le_num : num -> num -> bool val gt_num : num -> num -> bool val ge_num : num -> num -> bool val compare_num : num -> num -> int Return -1 , 0 or 1 if the first argument is less than, equal to, or greater than the second argument. val max_num : num -> num -> num Return the greater of the two arguments. val min_num : num -> num -> num Return the smaller of the two arguments. === Coercions with strings === val string_of_num : num -> string Convert a number to a string, using fractional notation. val approx_num_fix : int -> num -> string See Num.approx_num_exp . val approx_num_exp : int -> num -> string Approximate a number by a decimal. The first argument is the required precision. The second argument is the number to approximate. Num.approx_num_fix uses decimal notation; the first argument is the number of digits after the decimal point. approx_num_exp uses scien- tific (exponential) notation; the first argument is the number of digits in the mantissa. val num_of_string : string -> num Convert a string to a number. === Coercions between numerical types === val int_of_num : num -> int val num_of_int : int -> num val nat_of_num : num -> Nat.nat val num_of_nat : Nat.nat -> num val num_of_big_int : Big_int.big_int -> num val big_int_of_num : num -> Big_int.big_int val ratio_of_num : num -> Ratio.ratio val num_of_ratio : Ratio.ratio -> num val float_of_num : num -> float OCamldoc 2012-06-26 Num(3o)
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