Cpp程序例子

console版计算器：

#define _CRT_SECURE_NO_WARNINGS // for strncpy , disable error C4996
#include <iostream>
//#include <cstring>
using std::cout;
using std::cin;

//function prototype
char* extract(const char* str, size_t& index);//extract()提取圆括号包括的子字符串
double number(const char* str, size_t& index);//number()计算数值或括号中的表达式
double term(const char* str, size_t& index);//一项，考虑 * /
double expr(char* str);//expr()计算表达式的值，考虑 + -
void eatspaces(char* str);//eliminate blanks

const size_t MAX = 100;

//extract()提取圆括号包括的子字符串
char* extract(const char* str, size_t& index)
{
    char* pstr = 0; //point a new string for return
    size_t numL = 0; //count of left parentheses
    size_t bufIndex = index; //save old index
    do
    {
        switch (str[index])
        {
        case ')':
            if (0 == numL)
            {
                ++index;
                pstr = new char[index - bufIndex]();
                if (!pstr)
                    throw "Memory alloction is failed.";
                strncpy(pstr, str + bufIndex, index - bufIndex - 1);
                return pstr;
            }
            else
                --numL;
            break;
        case '(':
            ++numL;
            break;
        }
    } while (str[index++] != '\0');
    throw "Ran off the end of expression, must be bad input.";
}

/////////////////////////////////////////////////////////////////
// To recognize an expression in parentheses or a number in a string.
// return: the value of this expession or number.
// @Param 
//    str : the expression of input
//    index : the index of str
/////////////////////////////////////////////////////////////////
double number(const char* str, size_t& index)
{
    double value = 0.0;
    if (str[index] == '(')
    {
        char* psubs = 0;
        psubs = extract(str, ++index);
        value = expr(psubs);
        delete[] psubs;
        return value;
    }
    if (!isdigit(str[index]))
    {
        char szMsg[31] = "Invalid character in number: ";
        strncat(szMsg, str + index, 1);
        throw szMsg;
    }
    while (isdigit(str[index]))
    {
        value = value * 10 + (str[index++] - '0');
    }
    if ('.' != str[index])
        return value;
    double factor = 1.0;
    while (isdigit(str[++index]))
    {
        factor *= 0.1;
        value = value + (str[index] - '0')*factor;
    }
    return value;
}

// To get the value of term(项)
double term(const char* str, size_t& index)
{
    double value = 0.0;
    value = number(str, index);
    while (true)
    {
        if ('*' == str[index])
            value *= number(str, ++index);
        else if ('/' == str[index])
            value /= number(str, ++index);
        else
            break;
    }
    return value;
}

// To calculate an arithmetic expression
double expr(char* str)
{
    double value = 0.0;
    size_t index = 0;
    value = term(str, index);//Get first term
    for (;;)
    {
        switch (str[index++])
        {
        case '\0':
            return value;
        case '+':
            value += term(str, index);
            break;
        case '-':
            value -= term(str, index);
            break;
        default:
            char szMsg[38] = "Expression evaluate error. Find: ";
            strncat(szMsg, str + index - 1, 1);
            throw szMsg;
            break;
        }
    }
    return value;
}
// 去掉空格
void eatspaces(char* str)
{
    int i, j;
    i = j = 0;
    while ((str[i] = str[j]) != '\0')
    {
        ++j;
        if (str[i] != ' ' && str[i] != '\t' )
            ++i;
    }
}
int main()
{
    try
    {
        char buffer[MAX] = { 0 };
        std::cout << "Welcome to your friendly calcultor.\n"
            << "Please enter an expression, or a empty line for quit.\n";
        for (;;)
        {
            cin.getline(buffer, sizeof(buffer));
            eatspaces(buffer);
            if (!buffer[0])
            {
                cout << "You quitted!\n";
                return 0;
            }
            cout << "\t= " << expr(buffer) << '\n';
        }
        

    }
    catch (std::string& s)
    {
        std::cerr << s.c_str() << '\n';
    }
    catch (const char* s)
    {
        std::cerr << s << '\n';
    }
    catch (...)
    {
        std::cerr << "Unknown error.\n";
    }
    system("pause");
    return 0;
}