Cant get this to update the variables C++

Question

I'm writing a program that is a calculator where you would type in the sum and it would give you the answer. That part works fine. The problem I am having is taking the answer of the previous sum and doing a calculation with that.

Like: 5 + 5 = 10 ans + 10 = 20

When I run the code below it works fine, when doing normal calculations over and over again. However when I type eg. ans*2 it uses the previous values set to operate and numB. So if it was: 5 + 5 and I want to use that result and times it by eg 2 it would do this: 10 + 5 = 15

Here's my code:

#include <iostream>
#include <cmath>
#include <iomanip>
#include <sstream>
#include "bell.h"

using namespace std;

int main()
{
    stringstream ss;
    double numA;
    char operate;
    double numB;
    double ans=0;
    string temp;
    cout<<"input: ";

    getline(cin, temp);
    ss.str(temp);
    ss>>numA>>operate>>numB;

    cout<<setprecision(9);

    while(temp[0] != 'q' && temp[0] != 'Q')
    {
        if(temp[0]=='a' && temp[1]=='n' && temp[2]=='s')
        {
            numA=ans;
        }

        switch(operate)
        {
            case '+':
            {
                ans=numA+numB;
                cout<<numA<<" "<<operate<<" "<<numB <<" = "<< ans<<endl;
                break;
            }
            case '-':
            {
                ans=numA-numB;
                cout<<numA<<" "<<operate<<" "<<numB <<" = "<< ans<<endl;
                break;
            }
            case '*':
            {
                ans=numA*numB;
                cout<<numA<<" "<<operate<<" "<<numB <<" = "<< ans<<endl;
                break;
            }
            case '/':
            {
                ans=numA/numB;
                cout<<numA<<" "<<operate<<" "<<numB <<" = "<< ans<<endl;
                break;
            }
            case '^':
            {
                ans=pow(numA, numB);
                cout<<numA<<" "<<operate<<" "<<numB <<" = "<< ans<<endl;
                break;
            }
            case 'z':
            {
                ans=bell(numA, numB);
                cout<<numA<<" "<<operate<<" "<<numB <<" = "<< ans<<endl;
                break;
            }

            default:
            {
                cout<<"Invalid input. Please try again!"<<endl;
            }
        }
        ss.clear();
        ss.str(" ");
        cout<<"Input: ";
        getline(cin, temp);
        ss.str(temp);
        ss>>numA>>operate>>numB;
    }
    cout<<"Goodbye"<<endl;
    return 0;
}

Could someone please help me get this to work. Why does operate and numB not update?

score 0 · Accepted Answer · answered Apr 14 '14 at 16:30

I suspect that the problem is with this statement:

ss>>numA>>operate>>numB;

numA and numB are of type double. If the input line is for example ans + 10, that statement will try to extract numA as a double from the start of this input, which will fail, without consuming any parts of your input string. As a consequence, extracting the remaining values (operate and numB) will not give you the expected values, because the whole input string is still left to be parsed when it gets to extracting those values.

One approach is that you check for the ans case first, immediately after reading the input line. Then you can make the statement above conditional, parsing the values differently for the ans case.

Another solution is that you first extract the numA and numB values into string variables, and only convert them to doubles after you checked for your ans special case.

Cant get this to update the variables C++

1 Answers1