Validating input with do while loop

Question

I am trying to minimize hardcoding numbers into my program and allowing for users to define max and min parameters along with making sure that the input is valid.

#include <iostream>   

int main(){ 
    int max, A=0;
    do
    {
        std::cout << "What is the max?\n";
        std::cin >> max;
        if (std::cin.fail()) {
          std::cin.clear();      
          std::cin.ignore();
          std::cout << "not an integer, try again\n";
          continue;
        }
        if(max < -1000){
            std::cout << "That doesnt make much sense, please enter the max again.\n";
        }
    } while (max <A); \\HERE IS WHERE THE PROBLEM IS.
    std::cout << "The max number of steps are " << max <<std::endl;
    return 0;
    }

If A is 0 or less, the program doesn't ask for user input again. instead the program just exits the loop. If A is 1 or more, then then the program loops until a valid input is provided.

I would like the max number to be any int number, including negatives. This is working for positive numbers, but not for maximums that are 0 or less.

Fixed a missing line from isolating relevant code. I want the code to keep looping until there is valid input. I want to know how to make the code to work for any integer number for -1000, 1000. — Lizzy
– Lizzy, Commented Jul 27, 2020 at 22:30

Usama Tahir · Accepted Answer · 2020-07-27 22:28:07Z

1

 do
{
  //ask for input      
  //input taken

} while (A>=1);

This will the code you have to use for the scenario described at the last line. One more point you just forget to assign any value to A according to your logic.

Thanks!

answered Jul 27, 2020 at 22:28

Usama Tahir

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Comments

AngryParticle · Accepted Answer · 2020-07-27 22:54:26Z

0

If A is 1 or more, then then the program loops until a valid input is provided. - You are saying exactly what the while loop needs to do. Just implement it.

} while (A >= 1); \\If A is greater than or equal to 1 then loop until valid.
    std::cout << "The max number of steps are " << max <<std::endl;
    return 0;
    }

To answer your follow up question:

} while (A >= 1 && max <= 0); \\If A is greater than or equal to 1 then loop until valid.
    std::cout << "The max number of steps are " << max <<std::endl;
    return 0;
    }

edited Jul 27, 2020 at 22:54

answered Jul 27, 2020 at 22:32

AngryParticle

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1 Comment

Lizzy Over a year ago

What if I want to allow the max number to be a negative or 0.

Remy Lebeau · Accepted Answer · 2020-07-27 22:59:14Z

0

I would suggest writing a custom function that takes an acceptable range of min/max values as input parameters, eg:

int promptForInt(const string &prompt, int minAllowed, int maxAllowed)
{
    int value;

    std::cout << prompt << "\n";

    do
    {
        if (!(std::cin >> value)) {
            std::cin.clear();      
            std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
            std::cout << "not an integer, try again\n";
            continue;
        }

        if ((value >= minAllowed) && (value <= maxAllowed)){
            break;
        }

        std::cout << "not an allowed integer, enter a value between " << minAllowed << " and " << maxAllowed << ".\n";
    }
    while (true);

    return value;
}

int main(){ 
    int max = promptForInt("What is the max?", -1000, 1000);
    std::cout << "The max number of steps are " << max << std::endl;
    return 0;
}

answered Jul 27, 2020 at 22:59

Remy Lebeau

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3 Comments

AngryParticle Over a year ago

It might also help to explain what (std::numeric_limits<std::streamsize>::max(), '\n') is doing. She asked a separate question a couple hour ago about that specific if statement.

Remy Lebeau Over a year ago

@GenoC I posted a comment about that on the other question

AngryParticle Over a year ago

Cool, just thought it would help.

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