计算器 表达式求值

整数小数均可，支持加减乘除乘方，表达式中可加空格，最后输不输入等号都行。

#include <cstdio>#include <cstring>#include <cmath>#include <stack>using namespace std;#define N 100char s[N];int length, i;stack<double> num;stack<char> op;double calc(double a, char o, double b){switch (o){case '+': return a + b;case '-': return a - b;case '*': return a * b;case '/': return a / b;case '^': return pow(a, b);}}int pro(char o){switch (o){case '#': return -1;case '+':case '-': return 1;case '*':case '/': return 2;case '^': return 3;case '(': return 0;}}double getNumber(){char number[N];int j = 0;while ((i < length) && (s[i] >= '0' && s[i] <= '9' || s[i] == '.')) number[j++] = s[i++];number[j] = '\0';return atof(number);}void back(){double b = num.top(); num.pop();double a = num.top(); num.pop();char o = op.top(); op.pop();num.push(calc(a, o, b));}double calculate(){op.push('#');length = strlen(s);i = 0;while (i < length){if (s[i] >= '0' && s[i] <= '9'){num.push(getNumber());continue;}switch (s[i]){case ' ': i++; break;case '(': op.push(s[i++]); break;case ')': while (op.top() != '(')back();op.pop();i++;break;case '=':case '\n':while (op.top() != '#') back();return num.top();default:while (pro(op.top()) >= pro(s[i])) back();op.push(s[i++]);}}}int main(){fgets(s, sizeof(s), stdin);printf("%lf\n", calculate());return 0;}