Boolean condition not working properly in do-while loop

Question

The loop in the function require() takes 3 conditions, a > b or "a" or "b" aren't digits. Even when I don't satisfy the conditions and put 2 integers in, it just loops once again.

Also when I put in a character then it just endlessly loops "Enter minimum number Enter maximum number" ignoring the cins. Anyone know why? I'm a beginner so this is probably really obvious

#include <iostream>
#include <cmath>
#include <cstdlib>
#include <ctime>

using namespace std;

int random(int minN, int maxN)   //generates random number within specified range
{
    srand (time(NULL));
    int x = (maxN - minN);
    int y = minN + (rand() % (x+1));
    return y;
}

int require()        //makes sure a < b and both are digits
{
    int a,b;
    do {
    cout << "Enter minimum number" << endl;
    cin >> a;
    cout << "Enter maximum number. Note: Has to be greater or equal to minimum." << endl;
    cin >> b;
    } while (a > b || !isdigit(a) || !isdigit(b));

    return random(a,b);
}

int main()
{
    cout << require() << endl;
}

isdigit is for testing characters. If you pass it integers as arguments, it interprets them as character under a certain encoding (depending on locale). — jogojapan
– jogojapan, Commented Feb 15, 2013 at 6:22
Also, what do you mean even when I satisfy the conditions, [...] it loops again. That's how a do-while loop works: It loops as long as the condition is satisfied. It stops when the condition is no longer satisfied. — jogojapan
– jogojapan, Commented Feb 15, 2013 at 6:24
Sorry fixed that too, is there a function that interprets numbers instead of characters? — Seb
– Seb, Commented Feb 15, 2013 at 6:25
You only need to call srand (time(NULL)); once at the start of the program. — Ed Heal
– Ed Heal, Commented Feb 15, 2013 at 6:27

Ed Heal · Accepted Answer · 2013-02-15 09:27:16Z

1

You should not use isdigit as this relates to a particular character is a digiti. Instead the loop should look like this:

int require()        //makes sure a < b and both are digits
{
    validNumbers = true;
    do
    {
       cout << "Enter minimum number" << endl;
       cin.clear();
       cin >> a;
    } while (cin.fail());

    do
    {
       cout << "Enter maximum number. Note: Has to be greater or equal to minimum."
            << endl;
       cin.clear();
       cin >> b;
    } while (cin.fail() || a > b);

    return random(a,b);
}

PS: You only need to call srand (time(NULL)); once at the start of the program.

edited Feb 15, 2013 at 9:27

answered Feb 15, 2013 at 6:35

Ed Heal

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Some programmer dude · Accepted Answer · 2013-02-15 06:31:09Z

1

You are reading the numbers as, well, numbers not as characters as the isdigit function expects. If you are using a C++11 compliant standard library, the values of a and b will actually be zero if the input is not valid integer numbers, which means that e.g. !isdigit(a) will be true. If you are using a non-C++11 library, then the value of a and b will be random, and will most likely cause !isdigit(a) to be true as well as the amount of valid digit ASCII values in a full 32-bit integer range is quite small.

If you read a reference about the input operator, like this one you will see that if extraction fails, then the streams failbit will be set. This can either be tested "inline" like this:

if (!(std::cin >> a))
{
    std::cout << "Not a valid number, try again: ";
    continue;
}

Or it can be tested using the streams fail function.

edited Feb 15, 2013 at 6:31

answered Feb 15, 2013 at 6:23

Some programmer dude

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2 Comments

Seb Over a year ago

I see, what can I do instead? I'm not too knowledgeable but would something like atoi work?

Seb Over a year ago

Thanks, I thought I was stupid for not figuring this out, but I just haven't learned enough syntax

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