快速调试的老秘诀：动画源代码

program EightQueens(output); { Place 8 hostile queens on a chessboard, such that none can be captured. } { From Wirth: Algorithms + Data Structures = Programs, page 146. } var i: integer; RowFree: array [1..8] of boolean; UpDiag: array [2..16] of boolean; { diagonal to upper right } DownDiag: array [-7..7] of boolean; { diagonal to lower right } QueenIn: array [1..8] of integer; procedure print; { Write out one solution } var k: integer; begin { print } for k := 1 to 8 do write(QueenIn[k]: 4); writeln; end { print }; procedure try(col: integer); { Try to place a queen in this column } var row: integer; begin { try } for row := 1 to 8 do if RowFree[row] and UpDiag[col+row] and DownDiag[col-row] then begin QueenIn[col] := row; RowFree[row] := false; UpDiag[col+row] := false; DownDiag[col-row] := false; if col < 8 then try(col+1) else print; RowFree[row] := true; UpDiag[col+row] := true; DownDiag[col-row] := true; end; end { try }; begin { EightQueens } for i := 1 to 8 do RowFree[i] := true; for i := 2 to 16 do UpDiag[i] := true; for i := -7 to 7 do DownDiag[i] := true; try(1) end { EightQueens }. 

 {%F- no reformatting } {%O+} program Pas1 (input,output,paskey,pasksy,spsfile,enterf,symsetf{,textf}); const nkw = 27; (*no. of key words*) alng = 10; (*no. of significant chars in identifiers*) type symbol = (intcon,realcon,charcon,string, notsy,plus,minus,times,idiv,rdiv,imod,andsy,orsy, eql,neq,gtr,geq,lss,leq, lparent,rparent,lbrack,rbrack,comma,semicolon,period, colon,becomes,constsy,typesy,varsy,functionsy, proceduresy,arraysy,recordsy,programsy,ident, beginsy,ifsy,casesy,repeatsy,whilesy,forsy, endsy,elsesy,untilsy,ofsy,dosy,tosy,downtosy,thensy); alfa = packed array [1..alng] of char; object = (konstant,variable,type1,prozedure,funktion); types = (notyp,ints,reals,bools,chars,arrays,records); keytype = array [1..nkw] of alfa; ksytype = array [1..nkw] of symbol; spstype = array [char] of symbol; symset = set of symbol; entertype = record fx0: alfa; fx1: object; fx2: types; fx3: integer; end; var key: keytype; ksy: ksytype; sps: spstype; (*special symbols*) syset : symset; pasksy : file of ksytype; paskey : file of keytype; spsfile : file of spstype; enterf : file of entertype; symsetf : file of symset; { textf : text;} procedure enter(x0: alfa; x1: object; x2: types; x3: integer); var EnterRec : EnterType; begin with EnterRec do begin fx0 := x0; fx1 := x1; fx2 := x2; fx3 := x3 end; write ( enterf, EnterRec ); end (*enter*) ; begin {main program} key[ 1] := 'and '; key[ 2] := 'array '; key[ 3] := 'begin '; key[ 4] := 'case '; key[ 5] := 'const '; key[ 6] := 'div '; key[ 7] := 'do '; key[ 8] := 'downto '; key[ 9] := 'else '; key[10] := 'end '; key[11] := 'for '; key[12] := 'function '; key[13] := 'if '; key[14] := 'mod '; key[15] := 'not '; key[16] := 'of '; key[17] := 'or '; key[18] := 'procedure '; key[19] := 'program '; key[20] := 'record '; key[21] := 'repeat '; key[22] := 'then '; key[23] := 'to '; key[24] := 'type '; key[25] := 'until '; key[26] := 'var '; key[27] := 'while '; ksy[ 1] := andsy; ksy[ 2] := arraysy; ksy[ 3] := beginsy; ksy[ 4] := casesy; ksy[ 5] := constsy; ksy[ 6] := idiv; ksy[ 7] := dosy; ksy[ 8] := downtosy; ksy[ 9] := elsesy; ksy[10] := endsy; ksy[11] := forsy; ksy[12] := functionsy; ksy[13] := ifsy; ksy[14] := imod; ksy[15] := notsy; ksy[16] := ofsy; ksy[17] := orsy; ksy[18] := proceduresy; ksy[19] := programsy; ksy[20] := recordsy; ksy[21] := repeatsy; ksy[22] := thensy; ksy[23] := tosy; ksy[24] := typesy; ksy[25] := untilsy; ksy[26] := varsy; ksy[27] := whilesy; rewrite (paskey); write (paskey, key); ksy[ 1] := andsy; ksy[ 2] := arraysy; ksy[ 3] := beginsy; ksy[ 4] := casesy; ksy[ 5] := constsy; ksy[ 6] := idiv; ksy[ 7] := dosy; ksy[ 8] := downtosy; ksy[ 9] := elsesy; ksy[10] := endsy; ksy[11] := forsy; ksy[12] := functionsy; ksy[13] := ifsy; ksy[14] := imod; ksy[15] := notsy; ksy[16] := ofsy; ksy[17] := orsy; ksy[18] := proceduresy; ksy[19] := programsy; ksy[20] := recordsy; ksy[21] := repeatsy; ksy[22] := thensy; ksy[23] := tosy; ksy[24] := typesy; ksy[25] := untilsy; ksy[26] := varsy; ksy[27] := whilesy; rewrite (pasksy); write (pasksy, ksy); sps['+'] := plus; sps['-'] := minus; sps['*'] := times; sps['/'] := rdiv; sps['('] := lparent; sps[')'] := rparent; sps['='] := eql; sps[','] := comma; sps['['] := lbrack; sps[']'] := rbrack; sps['#'] := neq; sps['&'] := andsy; sps[';'] := semicolon; rewrite (spsfile); write (spsfile, sps); rewrite (enterf); enter(' ', variable, notyp, 0); (*sentinel*) enter('false ', konstant, bools, 0); enter('true ', konstant, bools, 1); enter('real ', type1, reals, 1); enter('char ', type1, chars, 1); enter('boolean ', type1, bools, 1); enter('integer ', type1, ints , 1); enter('abs ', funktion, reals,0); enter('sqr ', funktion, reals,2); enter('odd ', funktion, bools,4); enter('chr ', funktion, chars,5); enter('ord ', funktion, ints, 6); enter('succ ', funktion, chars,7); enter('pred ', funktion, chars,8); enter('round ', funktion, ints, 9); enter('trunc ', funktion, ints, 10); enter('sin ', funktion, reals, 11); enter('cos ', funktion, reals, 12); enter('exp ', funktion, reals, 13); enter('ln ', funktion, reals, 14); enter('sqrt ', funktion, reals, 15); enter('arctan ', funktion, reals, 16); enter('eof ', funktion, bools, 17); enter('eoln ', funktion, bools, 18); enter('read ', prozedure, notyp, 1); enter('readln ', prozedure, notyp, 2); enter('write ', prozedure, notyp, 3); enter('writeln ', prozedure, notyp, 4); enter(' ', prozedure, notyp, 0); rewrite (symsetf); syset := [plus,minus,intcon,realcon,charcon,ident]; write ( symsetf, syset ); syset := [ident,arraysy,recordsy]; write ( symsetf, syset ); syset := [constsy,typesy,varsy,proceduresy,functionsy,beginsy]; write ( symsetf, syset ); syset := [intcon,realcon,charcon,ident,lparent,notsy]; write ( symsetf, syset ); syset := [beginsy,ifsy,whilesy,repeatsy,forsy,casesy]; write ( symsetf, syset ); end. 

 procedure init; {%s-} var i : integer; EnterRec : EnterType; begin writeln; reset (paskey); read (paskey, key); reset (pasksy); read (pasksy, ksy); reset (spsfile); read (spsfile, sps); reset (symsetf); read (symsetf,constbegsys,typebegsys,blockbegsys,facbegsys,statbegsys); stantyps := [notyp,ints,reals,bools,chars]; lc := 0; ll := 0; cc := 0; ch := ' '; errpos := 0; errs := []; insymbol; t := -1; a := 0; b := 1; sx := 0; c2 := 0; display[0] := 1; iflag := false; oflag := false; if sy <> programsy then freeze{3} else begin insymbol; if sy <> ident then freeze{2} else begin progname := id; insymbol; if sy <> lparent then freeze{9} else repeat insymbol; if sy <> ident then freeze{2} else begin if id = 'input ' then iflag := true else if id = 'output ' then oflag := true else freeze{0}; insymbol; end until sy <> comma; if sy = rparent then insymbol else freeze{4}; if not oflag then freeze{20}; end end ; reset (enterf); while not eof (enterf) do begin read (enterf,EnterRec ); with EnterRec do enter (fx0,fx1,fx2,fx3); end; with btab[1] do begin last := t; lastpar := 1; psize := 0; vsize := 0 end ; end {init}; procedure finish; {%s-} begin if sy <> period then freeze{22}; {emit(31)}; {halt} if btab[2].vsize > stacksize then freeze{49}; end {finish};