Question

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How to determine if a string is a number with C++?

I have written a very simple calculator program in C++. Here it is:

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

int main()
{
   double num1;
   double num2;
   string op;
   double num3;
   int x;
   bool y = false;

   do
   {
      cout<<"Press t to terminate the application"<<endl;

      cout<<"Enter the first number"<<endl;
      cin>>num1;

      cout<<"Enter the operator"<<endl;
      cin>>op;
      cout<<"Enter the next number"<<endl;
      cin>>num2;

      if(op=="/"&&num2==0)
      {
         cout<<"You are attempting to divide by 0. This is impossible and causes the destruction of the universe. However, the answer is infinity"<<endl;
         y = true;
      }

      if(y==false)
      {
         if(op=="+") {
            num3 = num1+num2;
         }
         else if(op=="-") {
            num3 = num1-num2;
         }
         else if(op=="*"||op=="x"||op=="X") {
            num3 = num1*num2;
         }
         else {
            num3 = num1/num2;
         }
         cout<<endl;
         cout<<endl;
         cout<<"Answer:"<<num3<<endl<<endl;
      }
   } while(x!=12);

   return 0;
}

As you can see, I want to allow people to terminate the application by pressing ‘t’. This obviously won’t work because cin will try and assign a letter to a double (if I do press ‘t’ the application crashes). I am planning to use strings instead to get the input, but how would I test if the string is a letter or a number?

Answer 1

Here is sample and working code, just change it so it suits your needs

#include <iostream>
#include <string>
#include <cctype>
#include <stdlib.h>

using namespace std;

bool isNum(char *s) {
    int i = 0,  flag;

    while(s[i]){
            //if there is a letter in a string then string is not a number
        if(isalpha(s[i])){
            flag = 0;
            break;
        }
        else flag = 1;
        i++;
        }
    if (flag == 1) return true;
    else return false;
}


int main(){
    char stingnum1[80], stringnum2[80];
    double doublenum1, doublenum2;
    cin>>stingnum1>>stringnum2;
    if(isNum(stingnum1) && isNum(stringnum2)){
        doublenum1 = atof(stingnum1);
        doublenum2 = atof(stringnum2);
        cout<<doublenum1 + doublenum2 << endl;
    } 
    else cout<<"error";

   return 0;
}