How to check if the input number integer not float?

Question

I want to check if the input is valid, but when i do run this code I see that it checks only input for charcters. If i input a float number it will take it and going to use like integer without fractional part.

#inclide <iostream>
using namespace std;
...
int n;
cout << "Your input is: "<<endl;
cin >> n;
while (cin.fail()) {
    cout << "Error. Number of elements must be integer. Try again: " << endl;
    cin.clear();
    cin.ignore(256, '\n');  
    cin >> n;
}
...        
      `

So, how to make this code see if the input is float?

You are asking the wrong question, the question that has a follow-up question: "And how do I observe any non-number input?" The question you should be asking is this: *"How do I determine, whether the entire input is part of an integer, and this answer perfectly addressed that. — IInspectable
– IInspectable, Commented Oct 19, 2016 at 0:33

Galik · Accepted Answer · 2021-03-07 23:50:16Z

2

You can try to convert the input string to a int using a std::istringstream. If it succeeds then check for eof() (after ignoring blank spaces) to see if the whole input was consumed while converting to int. If the whole input was consumed then it was a valid int.

Something a bit like this:

int input_int()
{
    int i;

   // get the input
    for(std::string line; std::getline(std::cin, line);)
    {
        // try to convert the input to an int
        // if at eof() all of the input was converted - must be an int
        if(!line.empty() && (std::istringstream(line) >> i >> std::ws).eof())
            break;

        // try again
        std::cout << "Not an integer please try again: " << std::flush;
    }

    return i;
}

int main()
{
    std::cout << "Enter an integer: " << std::flush;

    std::cout << "i: " << input_int() << '\n';
}

edited Mar 7, 2021 at 23:50

answered Oct 18, 2016 at 23:29

Galik

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Comments

Community · Accepted Answer · 2017-05-23 12:32:20Z

0

Building on Raindrop7's solution, here's the full code to do what you need:

#include <cstdio>
#include <iostream>
#include <cmath>
using namespace std;

int main()
{
    double m;
    cout << "Your input is: "<<endl;
    cin >> m;
    while (cin.fail() || (m-floor(m)))
    {
        cout << "Error. Nubmer of elements has to be integer. Try again: " << endl;
        cin.clear();
        cin.ignore(256, '\n');  
        cin >> m;
    }
    int n = (int)m;
    return 0;
}

Here's a sample output:

Your input is: 
2.7
Error. Nubmer of elements has to be integer. Try again: 
erer
Error. Nubmer of elements has to be integer. Try again: 
2

edited May 23, 2017 at 12:32

CommunityBot

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answered Oct 18, 2016 at 23:31

space_voyager

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4 Comments

IInspectable Over a year ago

This will, by sheer coincidence, produce false positives. Floating point representation is inexact, and operator>> loses information. If things go bad, it turns a floating point value into something that is representable by an integer value. And there goes your test...

Raindrop7 Over a year ago

@IInspectable give an example proving what you say

IInspectable Over a year ago

@Raindrop7: Have a look at std::nextafter, std::nexttoward, and produce arbitrary examples that prove this fact yourself. (Hint: Start with any integer, call std::next towards a higher value, and enter the value that's just below the midpoint of those two values.)

Raindrop7 Over a year ago

thanx! yes that is good. but we for practice we create our own try. in a real program we use these new facilities

Anthony L. · Accepted Answer · 2017-04-09 11:17:44Z

The code below should be able to do what you are hoping to achieve:

#inclide <iostream>
using namespace std;
int n;
cout << "Your input is: "<<endl;
while (!(cin >> n) || cin.get() != '\n') {
    cout << "Error. Number of elements must be integer. Try again: " << endl;
    cin.clear();
    cin.ignore(256, '\n');  
}

The program asks the user to re-enter an integer if either of the following happens:

If the program is unable to extract an integer from the std::cin stream. (For example, when a character or string is entered by the user)
If, after an integer is extracted successfully, the next character in std::cin is not the new line '\n' character. (For example, when a number with a decimal point like 1.1 is entered, or when an integer followed by a character like 1a is entered.)

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