Validating a double in c++

Question

I'm completely new to coding, so please keep this in mind before commenting.

So I've been trying to get into coding for a while, and today I went to the library and picked up a book called "programming in c++". I've written some basic programs, but I have gotten stuck at one point, I have no idea how to create a function that makes sure that when the user is prompted for a double, what they enter is valid. (If the user enters a character like 'k', the program just breaks). I searched here and on the net, and there are some answers, but they are more along the line of "here's a line of code that works, insert x, y, z, into it". And I don't have enough experience to know what to do. So here is a sample that is a mix of code from another question, and me failing to try to make it work in my program:

#include <iostream>
#include <stdio.h>


using namespace std;

double GetDouble () {
    double x;
    cin >> x;
    int valid = 0;
    while (valid == 0) {
        if (x == double) {
      return x;
      break;
  } else if (x != double) {
      cout << "Invalid Input! Please input a numerical value." << endl;
      cin.clear();
      while (cin.get() != '\n') ; // empty loop
    }
}
return x;
}

Now what I want it to do is to use "cin << x" to get a user input for x, and then make sure that x is a double, and I'm failing quite hard at this. If someone could explain this to in a manner that clarifies each operation, I would be truly grateful. For example, I don't know what the break function does, and what the cin.clear(), and cin.get() do. (I know this is a duplicate, but the answers on the other question does not at all address what I'm confused about, thank you for taking time to read & answer this! :)

Answer 1

This seems a close analog to your code that (mostly) works:

double GetDouble () {
    double x;
    cin >> x;
    // ver1: while( cin.fail() ) // or !cin.good()   no trailing char check.
    while( cin.fail() || (cin.peek() != '\r' && cin.peek() != '\n'))
    {
        cout << "Invalid Input! Please input a numerical value." << endl;
        cin.clear();
        while( cin.get() != '\n' ); // or cin.ignore(1000, '\n');
        cin >> x;
    }
    return x;
}

Answer 2

First of all, double is a keyword so you don't need to use it in a conditional statement. Secondly, you can use the cin>>x in a while loop and then process x , a sample code is given below:

    while(cin>>x)
     {
       //process
     }

whenever you use anything in a while statement, it first checks it's validity. if for example user enters a character the condition will evaluate to false and you can then tell this to the user. In this way you can run the loop till the time user enters correct input. As for the function of break keyword, it gets you out of a loop, for ex

    for(int i=10; i>0; i++)
    {
       // do something
    }

will keep on running forever, but suppose you rewrite the code as:

    for(int i=10; i>0; i++)
    {
      if(i==15)
      break ;
    }

the loop will end as soon as the value of i reaches 15

Answer 3

Something like so:

double getDouble(const string& askMessage = "Please Enter a number: ",const string& errorMessage = "Please enter a valid number: ", const int numberOfTrials = 10){
 double userInput = 0.0;
 bool isValidInput = false;
 int trials = 0;
 do{
    cout << endl << askMessage;
    if(cin >> input) break;
    else{ 
      cin.clear(); //EDIT thanks ebyrob, forgot about this.
      std::cin.ignore(std::numeric_limits<streamsize>::max(),'\n');
      std::cout << errorMessage;
    }

 }while(!isValidInput || trials < numberOfTrials);

 return userInput;
}

use it like so

const double d1 = getDouble();
const double d2 = getDouble("Please Enter a number: ", "Error! Enter a valid number: ", 1000);

Answer 4

Break breaks out of a loop (in this case, a while) cin.clear() clears the cin buffer of all chars. cin.get() gets a single character from cin (probably standard in).