43 lines
3.9 KiB
Rexx
43 lines
3.9 KiB
Rexx
/*REXX program evaluates a ═════ Reverse Polish notation (RPN) ═════ expression. */
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parse arg x /*obtain optional arguments from the CL*/
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if x='' then x= "3 4 2 * 1 5 - 2 3 ^ ^ / +" /*Not specified? Then use the default.*/
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tokens=words(x) /*save the number of tokens " ". */
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showSteps=1 /*set to 0 if working steps not wanted.*/
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ox=x /*save the original value of X. */
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do i=1 for tokens; @.i=word(x,i) /*assign the input tokens to an array. */
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end /*i*/
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x=space(x) /*remove any superfluous blanks in X. */
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L=max(20, length(x)) /*use 20 for the minimum display width.*/
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numeric digits L /*ensure enough decimal digits for ans.*/
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say center('operand', L, "─") center('stack', L+L, "─") /*display title*/
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Dop= '/ // % ÷'; Bop='& | &&' /*division operators; binary operands.*/
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Aop= '- + * ^ **' Dop Bop; Lop=Aop "||" /*arithmetic operators; legal operands.*/
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$= /*nullify the stack (completely empty).*/
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do k=1 for tokens; ?=@.k; ??=? /*process each token from the @. list.*/
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#=words($); b=word($, max(1, #) ) /*the stack count; the last entry. */
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a=word($, max(1, #-1) ) /*stack's "first" operand. */
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division =wordpos(?, Dop)\==0 /*flag: doing a some kind of division.*/
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arith =wordpos(?, Aop)\==0 /*flag: doing arithmetic. */
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bitOp =wordpos(?, Bop)\==0 /*flag: doing some kind of binary oper*/
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if datatype(?, 'N') then do; $=$ ?; call show "add to───►stack"; iterate; end
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if wordpos(?, Lop)==0 then do; $=e 'illegal operator:' ?; leave; end
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if w<2 then do; $=e 'illegal RPN expression.'; leave; end
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if ?=='^' then ??= "**" /*REXXify ^ ──► ** (make it legal). */
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if ?=='÷' then ??= "/" /*REXXify ÷ ──► / (make it legal). */
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if division & b=0 then do; $=e 'division by zero.' ; leave; end
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if bitOp & \isBit(a) then do; $=e "token isn't logical: " a; leave; end
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if bitOp & \isBit(b) then do; $=e "token isn't logical: " b; leave; end
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interpret 'y=' a ?? b /*compute with two stack operands*/
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if datatype(y, 'W') then y=y/1 /*normalize the number with ÷ by unity.*/
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_=subword($, 1, #-2); $=_ y /*rebuild the stack with the answer. */
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call show ? /*display (possibly) a working step. */
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end /*k*/
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say /*display a blank line, better perusing*/
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if word($,1)==e then $= /*handle the special case of errors. */
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say ' RPN input:' ox; say " answer───►"$ /*display original input; display ans.*/
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parse source upper . y . /*invoked via C.L. or via a REXX pgm?*/
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if y=='COMMAND' | \datatype($,"W") then exit /*stick a fork in it, we're all done. */
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else exit $ /*return the answer ───► the invoker.*/
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/*──────────────────────────────────────────────────────────────────────────────────────*/
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isBit: return arg(1)==0 | arg(1)==1 /*returns 1 if arg1 is a binary bit*/
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show: if showSteps then say center(arg(1), L) left(space($), L); return
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