Question

I am creating a simple CLI calculator tool as an exercise. I need to make sure n1 and n2 are numeric in order for the functions to work; consequently, I would like to make the program quit upon coming across a predetermined non-numeric value.

Can anyone give me some direction?

Additionally, if anyone can offer any general tips as to how I could have done this better, I would appreciate it. I’m just learning c++.

Thank you!

The complete code is included below.

#include <iostream>
#include <new>

using namespace std;

double factorial(double n) { return(n <= 1) ? 1 : n * factorial(n - 1); }

double add(double n1, double n2) { return(n1 + n2); }

double subtract(double n1, double n2) { return(n1 - n2); }

double multiply(double n1, double n2) { return(n1 * n2); }

double divide(double n1, double n2) { return(n1 / n2); }

int modulo(int n1, int n2) { return(n1 % n2); }

double power(double n1, double n2) {
    double n = n1;
    for(int i = 1 ; i < n2 ; i++) {
        n *= n1;
    }
    return(n);
}

void print_problem(double n1, double n2, char operatr) {
    cout<<n1<<flush;
    if(operatr != '!') {
        cout<<" "<<operatr<<" "<<n2<<flush;
    } else {
        cout<<operatr<<flush;
    }
    cout<<" = "<<flush;
}

int main(void) {

double* n1, * n2, * result = NULL;
char* operatr = NULL;

n1 = new (nothrow) double;
n2 = new (nothrow) double;
result = new (nothrow) double;
operatr = new (nothrow) char;

if(n1 == NULL || n2 == NULL || operatr == NULL || result == NULL) {
    cerr<<"\nMemory allocation failure.\n"<<endl;
} else {

    cout<<"\nTo use this calculator, type an expression\n\tex: 3*7 or 7! or \nThen press the return key.\nAvailable operations: (+, -, *, /, %, ^, !)\n"<<endl;

    do {    
        cout<<"calculator>> "<<flush;       
        cin>>*n1;

        cin>>*operatr;

        if(*operatr == '!') {
            print_problem(*n1, *n2, *operatr);
            cout<<factorial(*n1)<<endl;
        } else {

            cin>>*n2;

            switch(*operatr) {
                case '+':
                    print_problem(*n1, *n2, *operatr);
                    cout<<add(*n1, *n2)<<endl;
                    break;
                case '-':
                    print_problem(*n1, *n2, *operatr);
                    cout<<subtract(*n1, *n2)<<endl;
                    break;
                case '*':
                    print_problem(*n1, *n2, *operatr);
                    cout<<multiply(*n1, *n2)<<endl;
                    break;
                case '/':
                    if(*n2 > 0) {
                        print_problem(*n1, *n2, *operatr);
                        cout<<divide(*n1, *n2)<<endl;
                    } else {
                        print_problem(*n1, *n2, *operatr);
                        cout<<" cannot be computed."<<endl;
                    }
                    break;
                case '%':
                    if(*n1 >= 0 && *n2 >= 1) {
                        print_problem(*n1, *n2, *operatr);
                        cout<<modulo(*n1, *n2)<<endl;
                    } else {
                        print_problem(*n1, *n2, *operatr);
                        cout<<" cannot be computed."<<endl;
                    }
                    break;
                case '^':
                    print_problem(*n1, *n2, *operatr);
                    cout<<power(*n1, *n2)<<endl;
                    break;
                default:
                    cout<<"Invalid Operator"<<endl;
            }
        }
    } while(true);
    delete n1, n2, operatr, result;
}
return(0);
}

Answer 1

No need for Boost or writing your own template or forcing yourself to use exceptions vs error codes. cin alone does everything you’re asking for.

You can test if ( cin ) or if ( ! cin ) to determine success or failure. One failure (eg, a letter in numeric input) will stop cin from accepting any more input. Then call cin.clear() to clear the error and resume getting input, starting with whatever text caused the error. Also, you can request that a stream throw exceptions on conversion errors: cin.exceptions( ios::failbit ).

So, you can do this:

for (;;) try {
    double lhs, rhs;
    char oper;
    cin.exceptions( 0 ); // handle errors with "if ( ! cin )"
    cin >> lhs >> oper; // attempt to do "the normal thing"
    if ( ! cin ) { // something went wrong, cin is in error mode
        string command; // did user enter command instead of problem?
        cin.clear(); // tell cin it's again OK to return data,
        cin >> command; // get the command,
        if ( command == "quit" ) break; // handle it.
        else cin.setstate( ios::failbit ); // if command was invalid, 
                                           // tell cin to return to error mode
    }
    cin.exceptions( ios::failbit ); // now errors jump directly to "catch"
        // note that enabling exceptions works retroactively
        // if cin was in error mode, the above line jumps immediately to catch
    if ( oper != '!' ) cin >> rhs;
    // do stuff
} catch ( ios::failure & ) {
    cin.clear();
    cin.ignore( INT_MAX, '\n' ); // skip the rest of the line and continue
}

This is meant as a demonstration of error handling with iostreams. You can choose to use exceptions or manual testing or both.