中序運算

gistfile1.txt

#include <bits/stdc++.h>
using namespace std;

double Eval2(char *, char *&);
double Eval1(char *, char *&);
double Eval0(char *, char *&);

double Eval2(char *expr, char *&end) {
    double res=0;
    if (*(end=expr) == '(') {
        res = Eval0(end+1, end);
        if (*end == ')') ++end;
    } else { res=strtod(end, &end); }
    return res;
}

double Eval1(char* expr, char *&end) {
    double res = Eval2(expr, end);
    while (*end=='*' || *end=='/')
        (*end=='*')? (res*=Eval2(end+1, end)): (res/=Eval2(end+1, end));
    return res;
}

double Eval0(char* expr, char *&end) {
    double res = Eval1(expr, end);
    while (*end=='+' || *end=='-')
        res +=(*end=='+')? Eval1(end+1, end): -Eval1(end+1, end);
    return res;
}

int main() {
    char expr[128]={0}, *pos=0;
    printf("Exp : ");
    scanf("%s", expr);
    printf("Ans : %f\n", Eval0(expr, pos));
}