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How would you make an input validation to allow only positive numbers?

Austinomical (67)
I am making a program to tell the user what kind of triangle the sides entered make. How can I ensure they can only enter positive values. Using a loop or functions or something? All I can think of is to make separate do while loops for every variable. I also want the program to repeat again if the user wants to.

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  #include <iostream>
#include <math.h>
using namespace std;

int main ()
{



    int a, b, c;

    cout<<"Enter the length of side a:" << endl;
    cin>>a;

    cout<<"Enter the length of side b:" << endl;
    cin>>b;

    cout<<"Enter the length of side c:"<< endl;
    cin>>c;






    if(a==b && b==c)
    cout << "Equilateral triangle (all sides are equal)\n";

    else if(a==b || a==c || b==c)
    cout << "Isosceles triangle (only two sides are equal)\n";

    else if (pow(a,2) == pow(b,2) + pow(c,2) || pow(b,2) == pow(a,2) + pow(c,2) || pow(c,2) == pow(a,2) + pow(b,2))
    cout << "This is a RIGHT triangle";

    else
    cout << "Other type of triangle (Not ISOSCELES, NOT RIGHT, and NOT EQUILATERAL)\n";

    return 0;
}
Naughty Albatross (1283)
Hi,
> How can I ensure they can only enter positive values.
Use an if-statement and a while-statement : 

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int a = 0, b = 0, c = 0;
while(1)
{
    cout << "Enter the length of side a: " << endl;
    cin >> a;
    cout << "Enter the length of side b: " << endl;
    cin >> b;
    cout << "Enter the length of side c: " << endl;
    cin >> c;

    if(a <= 0 || b <= 0 || c <= 0)
    {
        cout << "Invalid lengths for the triangle. Try again.\n\n";
        continue;
    }
    break;
}
Naughty Albatross (1283)
Does that help you? : )
Austinomical (67)
It does, however, I want it to say that the value entered invalid right after the one entered. I don't want it to say it after all 3 are entered.
Austinomical (67)
Basically, I want something like so:

Please enter side A: 1

Please enter Side B: -10

Value entered must be positive!

Please enter side B: -100

Value entered must be positive!

Please enter side B: 1

Please enter side C: 1

This is an EQUILATERAL triangle! (all sides are equal)

Would you like to repeat (1-Yes, 2-No): 1
closed account (E0p9LyTq)
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#include <iostream>
#include <cmath>

int input(char*);

int main()
{
   while (true)
   {
      int a = input("Enter the length of side a: ");

      int b = input("Enter the length of side b: ");

      int c = input("Enter the length of side c: ");

      if (a == b && b == c)
      {
         std::cout << "Equilateral triangle (all sides are equal)\n";
      }
      else if (a == b || a == c || b == c)
      {
         std::cout << "Isosceles triangle (only two sides are equal)\n";
      }
      else if (std::pow(a, 2) == (std::pow(b, 2) + std::pow(c, 2)) ||
               std::pow(b, 2) == (std::pow(a, 2) + std::pow(c, 2)) ||
               std::pow(c, 2) == (std::pow(a, 2) + std::pow(b, 2)))
      {
         std::cout << "This is a RIGHT triangle\n";
      }
      else
      {
         std::cout << "Other type of triangle (Not ISOSCELES, NOT RIGHT, and NOT EQUILATERAL)\n";
      }

      std::cout << "\nWant to try again? (y or n): ";
      char again;
      std::cin >> again;

      if (again != 'y' && again != 'Y')
      {
         break;
      }
      std::cout << "\n";
   }
}


int input(char* question)
{
   int in = 0;
   
   while (true)
   {
      std::cout << question;
      std::cin >> in;

      if (in < 0)
      {
         std::cout << "Invalid input, number must be positive!\n";
      }
      else
      {
         break;
      }
   }
   return in;
}


Enter the length of side a: 5
Enter the length of side b: 5
Enter the length of side c: 5
Equilateral triangle (all sides are equal)

Want to try again? (y or n): y

Enter the length of side a: -15
Invalid input, number must be positive!
Enter the length of side a: 25
Enter the length of side b: 5
Enter the length of side c: 25
Isosceles triangle (only two sides are equal)

Want to try again? (y or n): n
Last edited on
Naughty Albatross (1283)
> I want it to say that the value entered invalid right after the one entered.

Then you use three copies of while if-statements : 

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int a = 0, b = 0, c = 0;
while(1)
{
    cout << "Enter the length of side a: " << endl;
    cin >> a;
   if(a <= 0)
    {
        cout << "Value entered must be positive!\n\n";
        continue;
    }
    break;
}

while(1)
{
    cout << "Enter the length of side b: " << endl;
    cin >> b;
   if(b <= 0)
    {
        cout << "Value entered must be positive!\n\n";
        continue;
    }
    break;
}

while(1)
{
    cout << "Enter the length of side c: " << endl;
    cin >> c;
   if(c <= 0)
    {
        cout << "Value entered must be positive!\n\n";
        continue;
    }
    break;
}
Naughty Albatross (1283)
Does that help you? :)
Austinomical (67)
Thank you all, especially Furry guy!
Naughty Albatross (1283)
Glad it helped :)
closed account (E0p9LyTq)
Then you use three copies of while if-statements

Why not just write one function and call that function three times?
closed account (E0p9LyTq)
@Austinomical,

If your question was answered to your satisfaction, check-mark it as solved, please.

And you are welcome. :)
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