Pascal表达式求值.doc

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:array1.100 of char; /符号栈shuzi:array1.100 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 si=- then /判断表达式第一个数是否是负数 begin inc(sz_top); shuzisz_top:=0; end; while i=len do begin if si in+,-,*,/, then begin while( (fh_top0)and(jibie(si)=jibie(fuhaofh_top) do begin k:=jisuan(shuzisz_top-1,shuzisz_top,fuhaofh_top); dec(fh_top);dec(sz_top); shuzisz_top:=k; end; inc(fh_top);fuhaofh_top:=si; end; if si=( then begin inc(fh_top);fuhaofh_top:=si; if si+1=- then begin inc(sz_top); shuzisz_top:=0; end; end; if si=) then begin while fuhaofh_top( do begin k:=jisuan(shuzisz_top-1,shuzisz_top,fuhaofh_top); dec(fh_top);dec(sz_top); shuzisz_top:=k; end; dec(fh_top); end; if si in0.9 then begin k:=ord(si)-48;inc(i); while si in 0.9 do begin k:=k*10+ord(si)-48; inc(i); end; inc(sz_top); shuzisz_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(shuzisz_top-1,shuzisz_top,fuhaofh_top); dec(fh_