求前缀表达式的值

  1 /*
  2 ** 求前缀表达式的值
  3 */
  4 #include <stdio.h>
  5 #include <stdlib.h>
  6 #include <string.h>
  7 #include <ctype.h>
  8 
  9 #define  N  30
 10 #define  TRUE   1
 11 #define  FALSE  0
 12 
 13 typedef struct {
 14     double data[N];
 15     int top;
 16 } Stack;
 17 
 18 /*
 19 ** 入栈
 20 */
 21 void Push( Stack *ptrs, double item )
 22 {
 23     if( ptrs->top == N - 1 ){
 24         printf( "Stack is full.\n" );
 25         return;
 26     } else {
 27         ptrs->data[ ++( ptrs->top ) ] = item;
 28         return;
 29     }
 30 }
 31 
 32 /*
 33 ** 岀栈
 34 */
 35 double Pop( Stack * ptrs )
 36 {
 37     if( ptrs->top == -1 ) {
 38         printf( "Stack is empty.\n" );
 39         return;
 40     } else
 41         return ptrs->data[ ( ptrs->top )-- ];
 42 }
 43 
 44 /*
 45 ** 判断是否操作符
 46 */
 47 int IsOperator( char ch )
 48 {
 49     if( ch == ‘+‘ || ch == ‘-‘ || ch == ‘*‘ || ch == ‘/‘ )
 50         return TRUE;
 51     else
 52         return FALSE;
 53 }
 54 
 55 /*
 56 ** 计算
 57 */
 58 double Calculate( double a, double b,char ch )
 59 {
 60     switch( ch ) {
 61         case ‘+‘ : return a + b; break;
 62         case ‘-‘ : return a - b; break;
 63         case ‘*‘ : return a * b; break;
 64         case ‘/‘ : return a / b; 
 65     }
 66 }
 67 
 68 int main()
 69 {
 70     char expr[N];
 71     gets( expr );
 72     int len = strlen( expr );
 73     
 74     Stack ss;
 75     
 76     ss.top = -1;
 77     
 78     double cc = 1;
 79     double tem_sum = 0;
 80     double operand_a;
 81     double operand_b;
 82     double result;
 83     int error = 0; // 记录除数为0的错误情况
 84     int i;
 85     for( i = len - 1; i >= 0; -- i ) {
 86         if( expr[i] >= ‘0‘ && expr[i] <= ‘9‘ ) {
 87             tem_sum += ( expr[i] - ‘0‘ ) * cc;
 88             cc *= 10;
 89             if( expr[i-1] == ‘+‘ ) {
 90                 //printf( "%d\n", tem_sum );
 91                 Push( &ss, tem_sum );
 92                 tem_sum = 0;
 93                 cc = 1;
 94                 i -= 2; // 跳过下一个正号和空格
 95                 continue;
 96             } else if( expr[i-1] == ‘-‘ ) {
 97                 tem_sum = -tem_sum;
 98                 //printf( "%d\n", tem_sum );
 99                 Push( &ss, tem_sum );
100                 tem_sum = 0;
101                 i -= 2; // 跳过下一个负号和空格
102                 continue;
103             } else if( expr[i-1] == ‘ ‘ ) { // 一个数字处理完了
104                 //printf( "%d\n", tem_sum );
105                 Push( &ss, tem_sum );
106                 tem_sum = 0;
107                 cc = 1;
108                 i --;
109                 continue;
110             }
111         } else if( expr[i] == ‘.‘ ) {
112             tem_sum /= cc * 1.0;
113             cc = 1;
114         } else if( IsOperator( expr[i] ) ) {
115             operand_a = Pop( &ss );
116             operand_b = Pop( &ss );
117             if( expr[i] == ‘/‘ && operand_b == 0 ) {
118                 error = 1;
119                 break;
120             } else {
121                 result = Calculate( operand_a, operand_b, expr[i] );
122                 //printf( "result:%.1lf\n", result );
123                 Push( &ss, result );
124                 i--; // 跳过下一个空格
125             }
126         }
127     }
128     if( error != 1 )
129         printf( "%.1lf\n", Pop( &ss ) );
130     else
131         printf( "ERROR\n" );
132     
133     return 0;
134 }