Pascal表达式求值.docVIP

Pascal表达式求值: 负数能够参与运算。其中负数（-m）的解决办法是通过在数字栈中构建0-m方式，把单目的负数符号变成减法运算（负数主要出现在表达式的开头以及“（”后面，程序默认输入的表达式格式正确。 program express; var s:string; //输入字符串 fuhao:array[1..100] of char; //符号栈 shuzi:array[1..100] of longint; //数字栈 fh_top,sz_top:integer; //栈指针 i,j,len,k:longint; c:char; function jibie(c:char):integer; //定义运算符的优先级 begin case c of (:exit(0); +,-:exit(1); *,/:exit(2); ^:exit(3); end; end; function jisuan(x,y:longint;c:char):longint; //计算函数 var r,t:longint; begin case c of +:exit(x+y); -:exit(x-y); *:exit(x*y); /:exit(x div y); ^:begin t:=1; for r:=1 to y do t:=t*x; end; end; jisuan:=t; end; begin readln(s); fh_top:=0;sz_top:=0; len:=length(s); i:=1; if s[i]=- then //判断表达式第一个数是否是负数 begin inc(sz_top); shuzi[sz_top]:=0; end; while i=len do begin if s[i] in[+,-,*,/,^] then begin while( (fh_top0)and(jibie(s[i])=jibie(fuhao[fh_top]))) do begin k:=jisuan(shuzi[sz_top-1],shuzi[sz_top],fuhao[fh_top]); dec(fh_top);dec(sz_top); shuzi[sz_top]:=k; end; inc(fh_top);fuhao[fh_top]:=s[i]; end; if s[i]=( then begin inc(fh_top);fuhao[fh_top]:=s[i]; if s[i+1]=- then begin inc(sz_top); shuzi[sz_top]:=0; end; end; if s[i]=) then begin while fuhao[fh_top]( do begin k:=jisuan(shuzi[sz_top-1],shuzi[sz_top],fuhao[fh_top]); dec(fh_top);dec(sz_top); shuzi[sz_top]:=k; end; dec(fh_top); end; if s[i] in[0..9] then begin k:=ord(s[i])-48;inc(i); while s[i] in [0..9] do begin k:=k*10+ord(s[i])-48; inc(i); end; inc(sz_top); shuzi[sz_top]:=k; dec(i); end; inc(i); end; while fh_top0 do //符号栈中的元素全部运算，并弹栈 begin k:=jisuan(shuzi[sz_top-1],shuzi[sz