What do I push onto the stack?

Write a program in C++ that evaluates portfix strings (one per line in a text file) with the following restrictions on the input: Each character is either an operand- if it is a digit and the floating point value associated with the digit, or it is one of the 5 binary operators: +, -, *, / (floating point), ^ (exponentiation). You must use a stack, in particular, the linked list implementation of the stack.

I want to push the return value from my function Eval. How would I say that?

Here's my code:

Code:

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#include<iostream>
#include<cstdio>
#include<cmath>
#include<string>
using namespace std;

const int DefaultListSize =50;
typedef float Elem;

class Link {
public:
        Elem element;        /*Value for this node*/
        Link* next;                /*Pointer to the next node in list*/
        Link(const Elem& elemval, Link* nextval =NULL)
        { element = elemval; next = nextval; }
        Link(Link* nextval =NULL) { next = nextval; }
};

class Lstack {
private:
        Link*top;        /*Pointer to first element*/
        int size;        /*Count number of elements*/

public:
        Lstack(int sz =DefaultListSize) {top = NULL; size = 0;}
        ~Lstack() {clear();}        /*Destructor*/
        void clear() {
                while (top != NULL) { /*Delete link nodes*/
                        Link*temp = top;
                        top = top->next;
                        delete temp;
                }
                size = 0;
        }
        bool push(const Elem& item) {
                top = new Link(item,top);
                size++;
                return true;
        }
        bool pop(Elem& item) {
                if(size == 0) return false;
                item = top->element;
                Link*ltemp = top->next;
                delete top;
                top = ltemp;
                size--;
                return true;
        }
        bool topValue(Elem& it) const {
                if (size ==0) return false;
                it = top->element;
                return true;
        }
        int length() const {return size;}
        bool isEmpty() const {
                if (top == NULL) return true;
                else return false;
        }
};
        bool Operand(float ch){
                if(('0' <= ch) && (ch <= '9'))
                        return true;
                else return false;
        }
        bool Operator(char ch){
                if(ch == '+' || '-' || '*' || '/' || '^')
                        return true;
                else return false;
        }

        float Eval(float oprndL, char oper, float oprndR);
       
int main(){

        Lstack S;
        string s;
        float a;
        char b;
        float c;
        float answer;
        unsigned int i;
       
        cout << "Enter Postfix string:\n";
        getline(cin, s);

        for(i=0; i < s.length(); i++ ){
                if(Operand(s.at(i)))
                        S.push(s.at(i));

                if(Operator(s.at(i)))
                        b = Operator(s.at(i));
                }
        if(!S.isEmpty()){
                S.pop(a);
                S.pop(c);
                Eval(Operand(a), Operator(b) , Operand(c));
                S.push();               
                S.pop(answer);
                cout << answer;
        }
        return 0;
}

float Eval(float oprndL, char oper, float oprndR){
                if(oper == '+') return oprndL + oprndR;
                else if(oper == '-') return oprndL - oprndR;
                else if(oper == '*') return oprndL * oprndR;
                else if(oper == '/') return oprndL / oprndR;
                else if(oper == '^') return pow(oprndL,oprndR);
                else return 0.0;
}

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You need to put the item you want to push inside the () behind the word "push". Eval(this, that, theother) is a perfectly good item.

oh ok! That makes sense. Now My program runs but when I input any postfix strings I keep getting 0. Like I entered 23+ the first time and it return 0. Then I tried 42- and it return 0 as well.

The program should convert 23+ to 2+3 and the same with 42-, it should become 4-2.

Operator(b) is a boolean value, not a char. Same with operand(a) and operand(c). You need to pass the floats/characters.

Code:

---

        Eval(a, b, c);
          S.push(Eval(a, b, c));

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I tried this but I still get 0.

You were intended to fix it everywhere:

Code:

---

b = Operator(s.at(i));

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ok so could I instead assign b to any of the 5 operators then and take out my bool operator?

You should assign b to just be the character. You should still do the check to see whether it is an operator, but that's taken care of in the if statement.

ok I did that but I still get 0 for my answers. Here's my main program:

Code:

---

int main(){

        Lstack S;
        string s;
        float a = 0;
        char b = 0;
        float c = 0;
        float answer;
        unsigned int i;
       
        cout << "Enter Postfix string:\n";
        getline(cin, s);

        for(i=0; i < s.length(); i++ ){
                if(Operand(s.at(i)))
                        S.push(s.at(i));

                if(Operator(b)){
                        Eval(a, b, c);
                }
        }

        if(!S.isEmpty()){
                S.pop(a);
                S.pop(c);
                Eval(a, b, c);
                cout << Eval(a, b, c);
                S.push(Eval(a, b, c));
                S.pop(answer);
                cout << answer;
        }
        return 0;
}

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You should assign b to just be the character, as opposed to just ignoring it completely.