#include <iostream>

#include <cstdlib>

#include <cctype> //字符串判定

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using namespace std;

const int MAX = 1024;

double operation(char *str);

char * extract(char *str, int &index)

{

??? char *pstr(nullptr);//處理字符串

??? int num(0);//記錄一下多少對括號

??? int bufindex(index);//記錄下標

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??? do

??? {

??????? switch (*(str + index))

??????? {

??????? case ')':

??????????? if (0 == num)

??????????? {

??????????????? ++index;

??????????????? pstr = new char[index - bufindex];

??????????????? if (!pstr)

??????????????? {

??????????????????? throw? "malloc fail";

??????????????? }

??????????????? //拷貝字符串

??????????????? strncpy_s(pstr, index - bufindex, str + bufindex, index - bufindex - 1);

??????????????? return pstr;

??????????? }

??????????? else

??????????? {

??????????????? num--;

??????????? }

??????????? break;

??????? case '(':

??????????? num++;

??????????? break;

??????? }

??? } while (*(str + index++) != '\0');

??? throw? "errorfail";

}

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//獲得計算表達是中字符串的數字

double getNum(char *str, int &index)

{

??? double value(0.0);

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??? if (*(str + index) == '(')

??? {

??????? char *substr(nullptr);

??????? substr = extract(str, ++index);

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??????? value = operation(substr);

??????? delete[] substr;

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??????? return value;

??? }

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??? if (!isdigit(*(str + index)))

??? {

??????? char error[30] = "geterror";

??????? throw error;

??? }

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??? //判斷數值是否是數值

??? while (isdigit(*(str + index)))

??? {

??????? value = 10 * value + (*(str + index++) - '0');

??? }

??? //帶有小數點時不做處理

??? if (*(str + index) != '.')

??? {

??????? return value;

??? }

??? else

??? {

??????? double decimals(1.0); //定義一個小數

??????? while (isdigit(*(str + (++index))))

??????? {

??????????? decimals /= 10;

??????????? value = value + (*(str + index) - '0') * decimals;

??????? }

??????? return value;

??? }

}

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double term(char *str, int & index)

{

??? double value(0.0);

??? value = getNum(str, index);//獲取數據

??? while (1)

??? {

??????? if (*(str+index) == '*')

??????? {

??????????? //乘除法

??????????? value *= getNum(str, ++index);

??????? }

??????? else if (*(str + index) == '/')

??????? {

??????????? value /= getNum(str, ++index);

??????? }

??????? else

??????? {

??????????? break;

??????? }

??? }

??? return value;

}

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double operation(char *str)

{

??? double value(0.0);

??? int index(0);

??? value += term(str, index);

??? for (;;)

??? {

??????? switch (*(str + (index++)))

??????? {

??????? case '\0':

??????????? return value;

??????????? break;

??????? case '+':

??????????? value += term(str,index);

??????????? break;

??????? case '-':

??????????? value -= term(str, index);

??????????? break;

??????? default:

??????????? break;

??????? }

??? }

}

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void removeBlankSpace(char *str)

{

??? int i(0);

??? int j(0);

??? //*(str + i) = *(str + j++) :通過這種方式循環獲得字符串中的每個字符

??? //下面的意思是獲得字符不是'\0',也就是說沒有到達字符串末尾

??? //下面的方式是把不是空格的字符賦值給*(str+i)

??? while ((*(str + i) = *(str + j++))!= '\0')

??? {

??????? if (*(str+i) != ' ')

??????? {

??????????? i++;

??????? }

??? }

??? //兩個下標輪替，往前移動，鏈表的算法一樣，循環向前挖

}

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void main()

{

??? char str[MAX] = { 0 };

??? cout << "請輸入表達式";

??? cin.getline(str, MAX);

??? cout << "\n" << str << endl;

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??? //去除空格

??? removeBlankSpace(str);

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??? cout << str << endl;

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??? cout << operation(str) << endl;

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??? cin.get();

}

運行結果：

總結

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