Reading 'unsigned int' using 'cin'

Question

I am trying to read an unsigned int using cin as follows:

#include <limits.h>
#include <iostream>

using namespace std;

int main(int argc, char* argv[])
{
    unsigned int number;

    // UINT_MAX = 4294967295
    cout << "Please enter a number between 0 and " << UINT_MAX << ":" << endl;

    cin >> number;

    // Check if the number is a valid unsigned integer
    if ((number < 0) || ((unsigned int)number > UINT_MAX))
    {
        cout << "Invalid number." << endl;
        return -1;
    }
    return 0;
}

However, whenever I enter a value greater than the upper limit of unsigned integer (UINT_MAX), the program displays 3435973836. How do I check if the input given by user falls between 0 to UINT_MAX?

`if ((number < 0) || ((unsigned int)number > UINT_MAX))` I cannot even describe how misguided this is. — ildjarn, Mar 05 '12 at 22:09
Maybe start with something simpler, and definitely a good book. — Kerrek SB, Mar 05 '12 at 22:10
Fairly basic yet reasonable question to ask. But very long to explain. — Mysticial, Mar 05 '12 at 22:10
If it doesn't work with Long i would suggest that you try to use a char array and convert it after to an unsigned int. — Oliver, Mar 05 '12 at 22:17

score 5 · Accepted Answer · edited Mar 05 '12 at 22:20

Two things:

Checking if an unsigned integer is < 0 or > UINT_MAX is pointless, since it can never reach that value! Your compiler probably already complains with a warning like "comparison is always false due to limited range of type".
The only solution I can think of is catching the input in a string, then use old-fashioned strtoul() which sets errno in case of overflow.

I.e.:

#include <stdlib.h>

unsigned long number;
std::string numbuf;
cin >> numbuf;
number = strtoul(numbuf.c_str(), 0, 10);
if (ULONG_MAX == number && ERANGE == errno)
{
    std::cerr << "Number too big!" << std::endl;
}

Note: strtoul returns an unsigned long; there's no function strtou(), returning an unsigned int.

Probably better to set errno to 0 before the call and then check for any non-zero value. It appears at least EINVAL can also happen. — edA-qa mort-ora-y, Mar 05 '12 at 22:29

PlasmaHH · Answer 2 · 2012-03-05T22:21:02.880

3

Your check makes no sense (which a compiler with properly enabled warnings would tell you) as your value is never under 0 and never over UINT_MAX, since those are the smallest and biggest value a variable of the type unsigned int (which number is) can hold.

Use the stream state to determine if reading into the integer worked properly.

edited Mar 05 '12 at 22:21

answered Mar 05 '12 at 22:10

PlasmaHH

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score 2 · Answer 3 · answered Mar 05 '12 at 22:10

2

You could read into an unsigned long long and test that against the unsigned int limit.

answered Mar 05 '12 at 22:10

AShelly

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@BoPersson, are you saying the `>>` automatically promotes to long long? Even if it does, it would need to truncate when storing in an unsigned int. – AShelly Mar 05 '12 at 22:21
I'm saying that `operator>>` is using `num_get` which collects characters, converts to the largest unsigned integer type and then verifies the target range before assigning the result. Just like if you had done that yourself. – Bo Persson Mar 05 '12 at 22:30

score 2 · Answer 4 · answered Mar 05 '12 at 22:10

When users enter a number higher than UINT_MAX, cin caps it at UINT_MAX anyway. The value cannot be negative, either.

If you need to extend the range, use unsigned long long for input, and cast to unsigned int after the check. This will not guard against numbers that are outside of range of unsigned long long, though.

For a general-purpose solution, you can read a string, and do a conversion yourself using unsigned long long as your result.

andand · Answer 5 · 2012-03-05T22:17:21.667

1

If you try to read it into an unsigned int you are going to have to limit yourself to the constraints of an unsigned int.

The most general way to do what you're asking is to read the input as a string and parse it to make sure it's in the proper range. Once you have validated it, you can convert it to an unsigned int.

edited Mar 05 '12 at 22:17

answered Mar 05 '12 at 22:12

andand

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Reading 'unsigned int' using 'cin'

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