37 lines
2.8 KiB
Rexx
37 lines
2.8 KiB
Rexx
/*REXX program calculates and displays some values for the Ackermann function. */
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numeric digits 100 /*use up to 100 decimal digit integers.*/
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/*╔═════════════════════════════════════════════════════════════╗
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║ When REXX raises a number to an integer power (via the ** ║
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║ operator, the power can be positive, zero, or negative). ║
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║ Ackermann(5,1) is a bit impractical to calculate. ║
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╚═════════════════════════════════════════════════════════════╝*/
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high=24
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do j=0 to 4; say
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do k=0 to high % (max(1, j))
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call tell_Ack j, k
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if j==4 & k==2 then leave /*there's no sense in going overboard. */
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end /*k*/
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end /*j*/
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exit /*stick a fork in it, we're all done. */
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/*──────────────────────────────────────────────────────────────────────────────────────*/
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tell_Ack: parse arg mm,nn; calls=0 /*display an echo message to terminal. */
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#=right(nn,length(high))
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say 'Ackermann('mm", "#')='right(ackermann(mm, nn), high),
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left('', 12) 'calls='right(calls, high)
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return
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/*──────────────────────────────────────────────────────────────────────────────────────*/
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ackermann: procedure expose calls /*compute value of Ackermann function. */
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parse arg m,n; calls=calls+1
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if m==0 then return n + 1
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if m==1 then return n + 2
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if m==2 then return n + 3 + n
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if m==3 then return 2**(n+3) - 3
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if m==4 then do; #=2 /* [↓] Ugh! ··· and still more ughs.*/
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do (n+3)-1 /*This is where the heavy lifting is. */
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#=2**#
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end
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return #-3
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end
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if n==0 then return ackermann(m-1, 1)
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return ackermann(m-1, ackermann(m, n-1) )
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