Input validation C++

Dev Zone Code
1

Hello!
My question is if I only want the user to input a int, how do I create a loop that will continuously ask the user to try again if they input the wrong data type or if they don't enter anything at all. Thanks.

ps: I'm not asking for the syntax of any kind of loop just want to know what to set the condition to in order to check that the input is valid.

2

Depends what's your definition of "valid input" and "invalid input".

If by valid you mean that the int value is a number, you can do a do-while loop that loops if the input value is not valid.
In C I would do:

do {
    printf("Insert number: ");
    scanf("%s", s);
} while (!isNumber(n));
int n = atoi(s);

bool isnumber(const char *s) {
   char* e = NULL;
   (void) strtol(s, &e, 0);
   return e != NULL && *e == (char) 0;
}
3

This one is a little cumbersome but I learned this from my professor. You are going to need to use the preprocessor directives #include<limits> and of course #include<iostream>. You can use using namespace std; on a new line if you are using the standard name space.

The process uses a boolean and a do while loop.

while inside the main:
of course instantiate your integer as
int x;
and your boolean as
bool nFail = false;

now that you have those instantiated, you can set up your do while loop
I set it up as
do {
cout << "enter and integer" << endl;
cin >> int x;
nFail = cin.fail(); //we set our bool to cin.fail() to see if it fits the parameters of the entered value
cin.clear(); //clears the input the user entered while not setting the value
cin.ignore(numeric_limits<streamsize>::max(), '\n'); //forgive me as I am not entirely sure how this one works.
} while (nFail == truie);

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4

cool thanks Theonewhoisdrunk. I'm curious about the ignore function, going to look that guy up. Oh and btw the "::" what exactly is that ?

5

The :: operator is called the scope-resolution operator and does just that, it resolves scope. So, by prefixing a type-name with this, it tells your compiler to look in the global namespace for the type.

1 Like
7

What about if the user enters a double? Also how would you handle input validation for a string (only letters) using getline()?

8

Most of the code here is so... cumbersome....

In (more or less pure) C-like C++ :

#include <cstdio>
using namespace std;
int main()
{
    do {
       int temp = 0;
       if (scanf("%d", &temp) != 1) 
         printf("Try again.\n");
      else {
          // do something with number.
         break; // eventually
       }
   } while(true);   
   return 0;
}

Or correct C++ way:

#include <iostream>
using namespace std;
int main()
{
    int temp = 0;
   while (true) {
       if (!(cin >> temp)) {
         cout << "Try again." << endl; 
      } else {          
         // do something with number.
         break; // eventually
     }
   } 
   return 0;
}

Or even more paranoic way: 

#include <iostream>
#include <limits>
using namespace std;
int main()
{
    int temp = 0;
   while (true) {
       if (!(cin >> temp)) {
         cout << "Try again." << endl; 
         cin.clear();
         cin.ignore(numeric_limits<streamsize>::max(), ' '); // skipping
      } else {          
         // do something with number.
         break; // eventually
     }
   } 
   cout << temp << endl;
   return 0;
}

But the best answer is not to handle data that are corrupted in any way and just quit after detecting the bad input.

2 Likes
9

or even like this...

#include <iostream>
using namespace std;
int main()
{
    int temp = 0;
    while (!(cin >> temp)) cout << "Try again." << endl; 
    // do something with number.
    return 0;
}
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