postfix evaluation algorithm

Question

here is my attempt for evaluation postfix evaluation

#include<iostream>
#include<string>
using namespace std;
template<class T>
class Stack
{
private:
    T *s;int N;
public:
    Stack(int maxn)
    {
        s=new T[maxn];
        N=0;
    }
    int empty()const
    {
    return N==0;
    }
    void push(T k)
    {
        s[N++]=k;
    }
    T pop()
    {
        return s[--N];
    }
    };

int main()
    {
        //postfix evaluation
        char *a="3+4*5";
        int N=strlen(a);
        Stack<int>save(N);
        for(int i=0;i<N;i++)
        {
            if(a[i]=='+')
                save.push(save.pop()+save.pop());
            if(a[i]=='*')
                save.push(save.pop()*save.pop());
            if((a[i]>='0' &&  a[i]<='9'))
                save.push(0);
            while((a[i]>='0' && a[i]<='9'))
                save.push(10*save.pop()+(a[i++]-'0'));
                    }
        cout<<save.pop()<<"  "<<endl;
    return 0;
}

but instead of answer 23 because 4*5+3=23,it gives me answer 5,as i understood,this code gives me this result because,first it checks if there is + mark for i=0,which is not,then it checks if if it is *,this is also not,so it first push 0,then it evaluates 10*0+'3'-'0',which is equal to 3,(it would be pushed in stack),for i=1,a[i] is equal to 3,so it prints 3+, second pop is undefined,so i think it is error,please help me to fix it

Answer 1

This works with a little fix:

#include <iostream>
#include <cstring>

using namespace std;

template<class T>
class Stack
{
private:
    T *s;
    int N;

public:
    Stack(int maxn)
    {
        s = new T[maxn];
        N = 0;
    }
    int empty()const
    {
        return N == 0;
    }
    void push(T k)
    {
        s[N++] = k;
    }
    T pop()
    {
        return s[--N];
    }
};

int main()
{
    //postfix evaluation
    const char *a = "3 4 5*+";
    int N = strlen(a);

    Stack<int>save(N);

    for (int i = 0; i < N; i++)
    {
        if (a[i]=='+')
            save.push(save.pop() + save.pop());

        if (a[i]=='*')
            save.push(save.pop() * save.pop());

        if (a[i] >= '0' && a[i] <= '9')
        {
            save.push(0);
            while (a[i] >= '0' && a[i] <= '9')
                save.push(10 * save.pop() + a[i++] - '0');
            i--;
        }
    }

    cout << save.pop() << "  " << endl;

    return 0;
}

Output (ideone):

23

Now, if you remove that i--; which I added, the code will be skipping characters in a[] because of 2 increments of i, in a[i++] and for (int i = 0; i < N; i++).

Without i--; the output is (ideone):

9