Knuth's power tree: Difference between revisions

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(Used for computing xⁿ efficiently using Knuth's power tree.)

The requirements of this draft task are to compute and show the list of Knuth's power tree integers necessary for the computation of Xⁿ for any real X and any non-negative integer n.

Then, using those integers, calculate and show the exact (not approximate) value of (at least) the integer powers below:

2ⁿ where n ranges from 0 ──► 17 (inclusive)
3¹⁹¹
1.1⁸¹

A zero power is often handled separately as a special case.

Optionally, support negative integers (for the power).

An example of a small power tree for some low integers:

                                                            1
                                                             \
                                                              2
                  ___________________________________________/ \
                 /                                              \
                3                                                4
               / \____________________________________            \
              /                                       \            \
             5                                         6            8
            / \____________                           / \            \
           /               \                         /   \            \
          7                 10                      9     12           16
         /                 //\\                     │      │           /\
        /            _____//  \\________            │      │          /  \
      14            /     /    \        \           │      │         /    \
     /│ \         11    13      15       20        18     24        17    32
    / │  \         │    /\      /\        │        /\      │        /\     │
   /  │   \        │   /  \    /  \       │       /  \     │       /  \    │
 19  21    28     22 23   26  25   30    40     27   36    48     33 34   64
 │   /\    /│\     │  │   /\   │   /\    /│\     │   /\    /│\     │  │   /\
 │  /  \  / │ \    │  │  /  \  │  /  \  / │ \    │  /  \  / │ \    │  │  /  \
38 35 42 29 31 56 44 46 39 52 50 45 60 41 43 80 54 37 72 49 51 96 66 68 65 128

Where, for the power 43, following the tree "downwards" from 1:

(for 2) compute square of X, store X²
(for 3) compute X * X², store X³
(for 5) compute X³ * X², store X⁵
(for 10) compute square of X⁵, store X¹⁰
(for 20) compute square of X¹⁰, store X²⁰
(for 40) compute square of X²⁰, store X⁴⁰
(for 43) compute X⁴⁰ * X³ (result).

Note that for every even integer (in the power tree), one just squares the previous value.

For an odd integer, multiple the previous value with an appropriate odd power of X (which was previously calculated). For the last multiplication in the above example, it would be (43-40), or 3.

According to Dr. Knuth (see below), computer tests have shown that this power tree gives optimum results for all of the n listed above in the graph.

For n ≤ 100,000, the power tree method:

bests the factor method 88,803 times,
ties 11,191 times,
loses 6 times.

See: Donald E. Knuth's book: The Art of Computer Programming, Vol. 2, Second Edition, Seminumerical Algorithms, section 4.6.3: Evaluation of Powers.

See: link codegolf.stackexchange.com/questions/3177/knuths-power-tree It shows a Haskel, Python, and a Ruby computer program example (but they are mostly code golf).

See: link comeoncodeon.wordpress.com/tag/knuth/ (See the section on Knuth's Power Tree.) It shows a C++ computer program example.

See: link to Rosetta Code addition-chain exponentiation.

J

Generally, this task should be accomplished in J using x^n . Here we take an approach that's more comparable with the other examples on this page.

This task is a bit verbose...

We can represent the tree as a list of indices. Each entry in the list gives the value of n for the index a+n. (We can find a using subtraction.)

<lang J>knuth_power_tree=:3 :0

 L=: P=: %(1+y){._ 1
 findpath=: ]
 while. _ e.P do.
   for_n.(/: findpath&>)I.L=>./L-._ do.
     for_a. findpath n do.
        j=. n+a
        l=. 1+n{L
        if. j>y do. break. end.
        if. l>:j{ L do. continue. end.
        L=: l j} L
        P=: n j} P
     end.
     findpath=: [: |. {&P^:a:
   end.
 end.
 P

)

usepath=:4 :0

 path=. findpath y
 exp=. 1,({:path)#x
 for_ex.(,.~2 -~/\"1])2 ,\path  do.
   'ea eb ec'=. ex
   exp=.((ea{exp)*eb{exp) ec} exp
 end.
 {:exp

)</lang>

Task examples:

<lang J> knuth_power_tree 191 NB. generate sufficiently large tree 0 1 1 2 2 3 3 5 4 6 5 10 6 10 7 10 8 16 9 14 10 14 11 13 12 15 13 18 14 28 15 28 16 17 17 21 18 36 19 26 20 40 21 40 22 30 23 42 24 48 25 48 26 52 27 44 28 38 29 31 30 56 31 42 32 64 33 66 34 46 35 57 36 37 37 50 38 76 39 76 40 41 41 43 42 80 43 84 44 47 45 70 46 62 47 57 48 49 49 51 50 100 51 100 52 70 53 104 54 104 55 108 56 112 57 112 58 61 59 112 60 120 61 120 62 75 63 126 64 65 65 129 66 67 67 90 68 136 69 138 70 140 71 140 72 144 73 144 74 132 75 138 76 144 77 79 78 152 79 152 80 160 81 160 82 85 83 162 84 168 85 114 86 168 87 105 88 118 89 176 90 176 91 122 92 184 93 176 94 126 95 190

  findpath 0

0

  2 usepath 0

1

  findpath 1

1

  2 usepath 1

2

  findpath 2

1 2

  2 usepath 2

4

  findpath 3

1 2 3

  2 usepath 3

8

  findpath 4

1 2 4

  2 usepath 4

16

  findpath 5

1 2 3 5

  2 usepath 5

32

  findpath 6

1 2 3 6

  2 usepath 6

64

  findpath 7

1 2 3 5 7

  2 usepath 7

128

  findpath 8

1 2 4 8

  2 usepath 8

256

  findpath 9

1 2 3 6 9

  2 usepath 9

512

  findpath 10

1 2 3 5 10

  2 usepath 10

1024

  findpath 11

1 2 3 5 10 11

  2 usepath 11

2048

  findpath 12

1 2 3 6 12

  2 usepath 12

4096

  findpath 13

1 2 3 5 10 13

  2 usepath 13

8192

  findpath 14

1 2 3 5 7 14

  2 usepath 14

16384

  findpath 15

1 2 3 5 10 15

  2 usepath 15

32768

  findpath 16

1 2 4 8 16

  2 usepath 16

65536

  findpath 17

1 2 4 8 16 17

  2 usepath 17

131072

  findpath 191

1 2 3 5 7 14 19 38 57 95 190 191

  3x usepath 191

13494588674281093803728157396523884917402502294030101914066705367021922008906273586058258347

  findpath 81

1 2 3 5 10 20 40 41 81

  (x:1.1) usepath 81

2253240236044012487937308538033349567966729852481170503814810577345406584190098644811r1000000000000000000000000000000000000000000000000000000000000000000000000000000000 </lang>

Note that an 'r' in a number indicates a rational number with the numerator to the left of the r and the denominator to the right of the r. We could instead use decimal notation by indicating how many characters of result we want to see, as well as how many characters to the right of the decimal point we want to see.

Thus, for example:

<lang J> 90j83 ": (x:1.1) usepath 81

 2253.24023604401248793730853803334956796672985248117050381481057734540658419009864481100</lang>

Python

<lang python>from __future__ import print_function

remember the tree generation state and expand on demand

def path(n, p = {1:0}, lvl=1): if not n: return [] while n not in p: q = [] for x,y in ((x, x+y) for x in lvl[0] for y in path(x) if not x+y in p): p[y] = x q.append(y) lvl[0] = q

return path(p[n]) + [n]

def tree_pow(x, n):

   r, p = {0:1, 1:x}, 0
   for i in path(n):
       r[i] = r[i-p] * r[p]
       p = i
   return r[n]

def show_pow(x, n):

   fmt = "%d: %s\n" + ["%g^%d = %f", "%d^%d = %d"][x==int(x)] + "\n"
   print(fmt % (n, repr(path(n)), x, n, tree_pow(x, n)))

for x in range(18): show_pow(2, x) show_pow(3, 191) show_pow(1.1, 81)</lang>

Output:

0: []
2^0 = 1

1: [1]
2^1 = 2

2: [1, 2]
2^2 = 4

<... snipped ...>

17: [1, 2, 4, 8, 16, 17]
2^17 = 131072

191: [1, 2, 3, 5, 7, 14, 19, 38, 57, 95, 190, 191]
3^191 = 13494588674281093803728157396523884917402502294030101914066705367021922008906273586058258347

81: [1, 2, 3, 5, 10, 20, 40, 41, 81]
1.1^81 = 2253.240236

REXX

This REXX version supports results up to 1,000 decimal digits (which can be expanded with the numeric digits nnn REXX statement. Also, negative powers are supported. <lang rexx>/*REXX program produces & shows a power tree for P, and calculates & shows X^P*/ numeric digits 1000 /*be able to handle some large numbers.*/ parse arg nums /*get set stuff: sets of three numbers.*/ if nums= then nums='2 -4 17 3 191 191 1.1 81' /*Not specified? Use defaults*/

                                                /*X lowPow highPow ··· repeat*/
 do  until nums=
 parse var nums x pL pH nums;  x=x/1  /*get X, lowP, highP; and normalize X. */
 if pH=  then pH=pL                 /*No highPower?  Then assume lowPower. */

   do e=pL  to pH;    p=abs(e)/1      /*use a range of powers;   use  │E│    */
   $=powerTree(p);    w=length(pH)    /*construct the power tree, (pow list).*/
                                                    /* [↑]  W≡length for show*/
     do i=1  for words($);  @.i=word($,i);  end     /*build a fast pow array.*/

   if p==0  then do; z=1; call show; iterate; end   /*handle case of 0 power.*/
   !.=.;  z=x;  !.1=z;  prv=z         /*define the first power of  X.        */

       do k=2  to words($);  n=@.k    /*obtain the power (number) to be used.*/
       prev=k-1;   diff=n-@.prev      /*these are used for the odd powers.   */
       if n//2==0  then z=prv**2      /*Even power?   Then square the number.*/
                   else z=z*!.diff    /* Odd   "        "  mult. by pow diff.*/
       !.n=z                          /*remember for other multiplications.  */
       prv=z                          /*remember for squaring the numbers.   */
       end   /*k*/
   call show                          /*display the expression and its value.*/
   end       /*e*/
 end         /*until nums ···*/

exit /*stick a fork in it, we're all done. */ /*────────────────────────────────POWERTREE subroutine────────────────────────*/ powerTree: arg y 1 oy; $= /*Z is the result; $ is the power tree.*/ if y=0 | y=1 then return y /*handle special cases for zero & unity*/

.=0; @.=0; #.0=1 /*define default & initial array values*/

                                      /* [↓]  add blank "flag" thingy──►list.*/
       do  while \(y//2);  $=$  ' '   /*reduce "front" even power #s to odd #*/
       if y\==oy  then $=y $          /*(only)  ignore the first power number*/
       y=y%2                          /*integer divide the power (it's even).*/
       end   /*while ···*/

if $\== then $=y $ /*re─introduce the last power number. */ $=$ oy /*insert last power number 1st in list.*/ if y>1 then do while @.y==0; n=#.0; m=0

              do    while  n\==0;             q=0;         s=n
                do  while  s\==0;             _=n+s
                if @._==0  then do;  if q==0  then m_=_;   #._=q;   @._=n;  q=_
                                end
                s=@.s
                end   /*while s¬==0*/
              if q\==0  then do;  #.m=q;  m=m_;  end
              n=#.n
              end     /*while n¬==0*/
            #.m=0
            end       /*while @.y==0*/

z=@.y

                do  while z\==0;  $=z $;  z=@.z;  end     /*build power list.*/

return space($) /*────────────────────────────────SHOW subroutine─────────────────────────────*/ show: if e<0 then z=format(1/z,,40)/1; _=right(e,w) /*use reciprocal ? */ say left('power tree for ' _ " is: " $,60) '═══' x"^"_ ' is: ' z; return</lang> output when using the default inputs:

power tree for  -4  is:  1 2 4                               ═══ 2^-4  is:  0.0625
power tree for  -3  is:  1 2 3                               ═══ 2^-3  is:  0.125
power tree for  -2  is:  1 2                                 ═══ 2^-2  is:  0.25
power tree for  -1  is:  1                                   ═══ 2^-1  is:  0.5
power tree for   0  is:  0                                   ═══ 2^ 0  is:  1
power tree for   1  is:  1                                   ═══ 2^ 1  is:  2
power tree for   2  is:  1 2                                 ═══ 2^ 2  is:  4
power tree for   3  is:  1 2 3                               ═══ 2^ 3  is:  8
power tree for   4  is:  1 2 4                               ═══ 2^ 4  is:  16
power tree for   5  is:  1 2 3 5                             ═══ 2^ 5  is:  32
power tree for   6  is:  1 2 3 6                             ═══ 2^ 6  is:  64
power tree for   7  is:  1 2 3 5 7                           ═══ 2^ 7  is:  128
power tree for   8  is:  1 2 4 8                             ═══ 2^ 8  is:  256
power tree for   9  is:  1 2 3 6 9                           ═══ 2^ 9  is:  512
power tree for  10  is:  1 2 3 5 10                          ═══ 2^10  is:  1024
power tree for  11  is:  1 2 3 5 10 11                       ═══ 2^11  is:  2048
power tree for  12  is:  1 2 3 6 12                          ═══ 2^12  is:  4096
power tree for  13  is:  1 2 3 5 10 13                       ═══ 2^13  is:  8192
power tree for  14  is:  1 2 3 5 7 14                        ═══ 2^14  is:  16384
power tree for  15  is:  1 2 3 5 10 15                       ═══ 2^15  is:  32768
power tree for  16  is:  1 2 4 8 16                          ═══ 2^16  is:  65536
power tree for  17  is:  1 2 4 8 16 17                       ═══ 2^17  is:  131072
power tree for  191  is:  1 2 3 5 7 14 19 38 57 95 190 191   ═══ 3^191  is:  13494588674281093803728157396523884917402502294030101914066705367021922008906273586058258347
power tree for  81  is:  1 2 3 5 10 20 40 41 81              ═══ 1.1^81  is:  2253.240236044012487937308538033349567966729852481170503814810577345406584190098644811