C++ How to check if double number is not entered as an alphabet

Question

This is a program where a user enters 5 marks, and the program calculates the average mark and grade. However, it should show an error when the user enters a string (alphabet) instead of a number. How do I do this?

#include <iostream>

using namespace std;

int main()
{
double dblMarkOne;
double dblMarkTwo;
double dblMarkThree;
double dblMarkFour;
double dblMarkFive;
double dblAverage;
string strGrade;

cout<<"Enter your first mark: ";
cin>>dblMarkOne;

while (dblMarkOne < 0 || dblMarkOne > 100)
{ 
    cout << "Enter a valid test score within 1 to 100. ";
    cout << "Enter your first mark: ";
    cin >> dblMarkOne;
}

analogous to this one http://stackoverflow.com/questions/18728754/checking-input-value-is-an-integer — brunocodutra, Sep 11 '13 at 00:54

score 0 · Answer 1 · answered Sep 11 '13 at 01:25

Here's a simple solution. I'm sure you can tweak it a bit to improve speed.

#include <iostream>
#include <stdlib.h>     /* atoi */

using namespace std;

bool isDouble(char a[]);

int main()
{
  double dblMarkOne;    
  char a[10];

  cout<<"Enter your first mark: ";
  cin >> a;  /* read it as a char array */

  if(isDouble(a)){    
    dblMarkOne = atof(a);
    cout << dblMarkOne;
  }
  else
  {
    cout << "not a double";
  }
}

bool isDouble(char a[])
{
  int i = 0;

  while(a[i] != 0)
  {
    if(!(isdigit(a[i]) || a[i] == '.'))
    {
      return false;    
    }
    i++;
  }
  return true;
}

What if the input is `1.1.1`? – thefourtheye Sep 11 '13 at 02:10 — thefourtheye, Sep 11 '13 at 02:10

score 0 · Answer 2 · edited May 23 '17 at 12:28

0

Based on this answer, https://stackoverflow.com/a/3274025/1903116

do
{ 
    cout << "Enter a valid test score within 1 to 100. ";
    cout << "Enter your first mark: ";
} while (! (cin >> dblMarkOne) );

edited May 23 '17 at 12:28

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answered Sep 11 '13 at 02:20

thefourtheye

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C++ How to check if double number is not entered as an alphabet

2 Answers2