POJ 2106 Boolean Expressions

总时间限制: 1000ms  内存限制: 65536kB

描述

The objective of the program you are going to produce is to evaluate boolean expressions as the one shown next: 

Expression: ( V | V ) & F & ( F | V )

where V is for True, and F is for False. The expressions may include the following operators: ! for not , & for and, | for or , the use of parenthesis for operations grouping is also allowed.

To perform the evaluation of an expression, it will be considered the priority of the operators, the not having the highest, and the or the lowest. The program must yield V or F , as the result for each expression in the input file. 

输入

The expressions are of a variable length, although will never exceed 100 symbols. Symbols may be separated by any number of spaces or no spaces at all, therefore, the total length of an expression, as a number of characters, is unknown. 

The number of expressions in the input file is variable and will never be greater than 20. Each expression is presented in a new line, as shown below. 

输出

For each test expression, print "Expression " followed by its sequence number, ": ", and the resulting value of the corresponding test expression. Separate the output for consecutive test expressions with a new line. 

Use the same format as that shown in the sample output shown below. 

样例输入

( V | V ) & F & ( F| V)
!V | V & V & !F & (F | V ) & (!F | F | !V & V)
(F&F|V|!V&!F&!(F|F&V))

样例输出

Expression 1: F
Expression 2: V
Expression 3: V

解题思路

一开始看到这题就觉得要用栈，毕竟是表达式求值问题，但是自己的中缀表达式求值也没练过，而且这次的课程作业又是练习递归，于是就尝试了递归解法。

但是入门太浅，还是搞不懂递归思路，很难从一个表达式中进行结构的分解，逻辑梳理能力太差。

下面是大佬的代码，真心膜拜，希望我的代码能力能越来越好。

#include<iostream>
using namespace std;

bool f1();
bool f2();

char getc_() //读入一个非空格字符
{
    char c;
    while ((c = cin.get()) == ' ');
    return c;
}
char peekc_()//读到第一个非空格字符前停止
{
    char c;
    while ((c = cin.peek()) == ' ')
    {
        cin.get();
    }
    return c;
}

bool f2()//计算项
{
    char c = peekc_();
    if (c == '(')
    {
        getc_(); //拿走左括号
        bool d = f1();
        getc_(); //拿走右括号 
        return d;
    }
    else
    {
        getc_();
        if (c == '!')    return !f2();//运算符作用于下一个对象
        if (c == 'V')    return true;
        if (c == 'F')    return false;
    }
}

bool f1()//计算整个表达式
{
    char c = peekc_();
    if (c == '\n')   return false;
    bool a = f2();
    while (true)
    {
        c = peekc_();
        if (c == '|')
        {
            getc_();
            bool b = f2();
            a = a || b;
        }
        else if (c == '&')
        {
            getc_();
            bool b = f2();
            a = a && b;
        }
        else    break;
    }
    return a;
}
int main()
{
    int n = 0;
    while (cin.peek() != EOF) {
        bool f = f1();
        if (cin.peek() == '\n' || cin.peek() == EOF) {
            cout << "Expression " << ++n << ": " << (f ? 'V' : 'F') << endl;
            cin.get();//清理换行符和结束符
        }
    }
    return 0;
}

引用网址：

https://blog.csdn.net/chuxin126/article/details/55105076