Question

okay. So I wrote a formula calculator a while ago that does many different functions. I opened it today and noticed that every time I accessed a certain function and then finished that function, the program went back to the main menu. Of course this is fine and I programmed it to do this, but I got annoyed when I accessed the calculator function (simple math) and I finished an equation, I couldn’t do another right away. I want to be able to stay in a certain function until I press ‘q’, and then it will go back to the main menu.

The real problem, is that my function only accepts doubles, so if I put in a string (‘q’), the program will just crash. I need a way to have the user enter either a string or a double so I can check if it’s a ‘q’ and the user wants to quit.

I want to eventually do this with all of my functions, but here’s just the “calc” function (the simplest one):

    int calculation()
{
  double x=0, y=0, answer=0;
  char o;//operator variable
  cout<<"calculation: ";
  cin>>x>>o>>y; //I just don't know what to use here so that the user can enter a  
  cin.ignore(); //double or a string.
  if (o=='+') answer=x+y;
  else if (o=='-') answer=x-y;
  else if (o=='*') answer=x*y;
  else if (o=='/') answer=x/y;
  else if (o=='^') answer= pow(x, y);
  else if (o=='%') {
        answer= (100/x)*y; 
        cout<<"answer= "<<answer<<"%";
  }
  if (o!='%') cout<<"answer= "<<answer;
  cin.get();
  return 0;
}

I need the function to keep repeating until the user enters a single “q”.
Sorry for all the words.

Answer 1

Solve it in two steps:

1. Write a loop that accepts any string and only quits when you enter a ‘q’.
2. Pass that string to a new function that will only perform a calculation if the string can be decomposed into the two operands and an operator (you can use stringstream for this).

After that, you can customize your functions however you want. One possibility would be mapping strings for operators to callbacks that take two operands and print a result. You would then pass that mapping in to your calculation function so that the number of operators you support can increase with ease.

Here’s a very rough example that only works for addition, but demonstrates the concept.

#include <iostream>
#include <cmath>
#include <sstream>
#include <map>
using namespace std;

typedef double (*p_fn)(double,double);

double add(double x, double y)
{
    return x + y;
}

typedef map<string,p_fn> operators;

double execute( const operators &op, double x, double y, const string& o )
{
    operators::const_iterator i = op.find(o);
    if( i != op.end())
    {
        p_fn f = i->second;
        double const result = f(x,y);
        return result;
    }
    cout<<"unknown operator\n";
    return 0;
}

bool get_data( double& x, double&y, string& o )
{
    string s1,s2,s3;
    cin>>s1;
    if(s1=="q")
        return false;
    cin>>s2>>s3;
    stringstream sstr;
    sstr<<s1<<" "<<s2<<" "<<s3;
    sstr>>x>>o>>y;
    stringstream sstr2;
    sstr2<<x<<" "<<o<<" "<<y;
    return sstr.str() == sstr2.str();
}

double calculation2( const operators& op )
{
    double x,y;
    string o;
    while(get_data(x,y,o))
        cout<<execute(op, x, y, o)<<"\n";
    return 0;
}

int main(int argc, char* argv[])
{
    operators o;
    o["+"]=add;
    calculation2(o);
    return 0;
}

This example uses pointers to functions to map the string “+” to the function add(x,y). It also uses stringstreams to perform very basic input validation.