check if user input a float to a int var?

Question

I have a int var; and using cin, but how to check if user input a float or give me a solution for this problem?

score 2 · Accepted Answer · 2010-08-04 08:17:32Z

2

There is no easy way of doing this. Input via the >> operator is really not intended for interaction with humans who may enter the wrong thing. You will have to read the input as a string and then check it using a function like strtol, or your own hand-rolled code.

Here's an outline of an approach using strtol:

#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;

bool GetInt( istream & is, int & n ) {
    string line;
    if ( ! getline( is, line ) ) {
        return false;
    }
    char * ep;
    n = strtol( line.c_str(), & ep, 10 );
    return * ep == 0;
}


int main() {
    int n;

    while(1) {
        cout << "enter an int: ";
        if ( GetInt( cin, n ) ) {
            cout << "integer" << endl;
        }
        else {
            cout << "not an integer" << endl;
        }
    }
}

edited Aug 4, 2010 at 8:17

answered Aug 4, 2010 at 8:03

anon

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1 Comment

Rajesh Over a year ago

any standard libraries or third party libraries for taking user inputs?

Paul R · Accepted Answer · 2010-08-04 08:01:18Z

1

Read the variable into a temporary string, parse it to see if contains ., ,, E, e, or other non-integer characters, then convert it to int if it's valid.

Alternatively just read the value into a float and convert this to an int (with rounding or truncation as appropriate).

answered Aug 4, 2010 at 8:01

Paul R

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1 Comment

Brian Hooper Over a year ago

I shouldn't do that if I were you, Snoob. Your number will remain a float. Perhaps float a; int b; b = (int) a; would be a better bet.

Clifford · Accepted Answer · 2010-08-04 08:29:10Z

1

Possibly the simplest way is to accept a float/double, cast it to an int, then test to see if the conversion modified the value.

    int i ;
    float f ;
    std::cout << "Input: " ;
    std::cin >> f ;
    i = static_cast<int>(f) ;
    if( static_cast<float>(i) != f )
    {
        std::cout << "Not an integer" ;
    }
    else
    {
        std::cout << "An integer" ;
    }

Note that this will fail for large integers (most values greater than 6 digits) that cannot be exactly represented as floating point but the working range may well suit your needs.

answered Aug 4, 2010 at 8:29

Clifford

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Comments

Kirill V. Lyadvinsky · Accepted Answer · 2010-08-04 08:13:52Z

You could use temporary string and boost::lexical_cast to convert it to an integral or floating point variable as follows:

#include <iostream>
#include <boost/lexical_cast.hpp>

int main() 
{ 
    using namespace std;

    string buf;
    cin >> buf;

    bool int_ok = false, float_ok = false;
    try {
        int x = boost::lexical_cast<int>( buf );
        int_ok = true;
    } catch ( boost::bad_lexical_cast e ) { int_ok = false; }
    if ( !int_ok ) {
        try {
            float x = boost::lexical_cast<float>( buf );
            float_ok = true;
        } catch ( boost::bad_lexical_cast e ) { float_ok = false; }
    }

    return 0; 
}

There is could be a better solution depending on what are you trying to achieve.

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