将浮点数float / double的余数转换为int

Question

I have a double (or float) number x:

x = 1234.5678;

Now, the question is, how do break down the number into 2 int's whereas int1 would get the number before the point, and int2 is the number after the point.

The first part is easy, which we can either cast, or take a round or ceiling to get the first part into an int, I am looking for the second part to become int2=5678 without any floating points there.

ie to to extend the above example:

float x = 1234.5678;
int x1 = (int) x;   // which would return 1234 great.
int x2 = SomeFunction????(x);            // where I need x2 to become = 5678

Notice the 5678 should not have any points there.

It would be nice to hear from you.

Thanks Heider

Answer 1

Here are two ways of doing it.

The first one uses std::stringstream , std::string and std::strtol and is sorta hacky. It is also not very efficient, but it does the job.

The second one needs to know the number of decimals and uses simple multiplication. NOTE: This method will not do any kind of checking on whether the float you pass in actually has that number of decimals.

None of these methods are particularly elegant, but they worked well for the numbers I tested ( both positive and negative. ) Feel free to comment if you find bugs/errors or if you have suggestions for improvement.

EDIT : As @dan04 pointed out, this method will return the same value for 0.4 as for 0.04 . If you want do distinguish these, you'd need a second int for storing the number of zeros after the decimal point.

#include <iostream>
#include <sstream>
#include <math.h>

int GetDecimalsUsingString( float number );
int GetDecimals( float number, int num_decimals );

int main() {
    float x = 1234.5678;
    int x1 = (int) x;   // which would return 1234 great.
    float remainder = x - static_cast< float > ( x1 );
    std::cout << "Original : " << x << std::endl;
    std::cout << "Before comma : " << x1 << std::endl;
    std::cout << "Remainder : " << remainder << std::endl;

    // "Ugly" way using std::stringstream and std::string
    int res_string = GetDecimalsUsingString( remainder );

    // Nicer, but requires that you specify number of decimals
    int res_num_decimals = GetDecimals( remainder, 5 );

    std::cout << "Result using string : " << res_string << std::endl;
    std::cout << "Result using known number of decimals : " << res_num_decimals << std::endl;
    return 0;
}

int GetDecimalsUsingString( float number )
{
    // Put number in a stringstream
    std::stringstream ss;
    ss << number;

    // Put content of stringstream into a string
    std::string str = ss.str();

    // Remove the first part of the string ( minus symbol, 0 and decimal point)
    if ( number < 0.0 )
        str = str.substr( 3, str.length() - 1);
    else
        str = str.substr( 2, str.length() - 1);

    // Convert string back to int
    int ret =  std::strtol( str.c_str(), NULL, 10 );

    /// Preserve sign
    if ( number < 0 )
        ret *= -1;

    return ret;
}
int GetDecimals( float number, int num_decimals )
{
    int decimal_multiplier = pow( 10, num_decimals );
    int result = number *  decimal_multiplier;

    return result;
}

Output :

Original : 1234.57
Before comma : 1234
Remainder : 0.567749
Result using string : 567749
Result using known number of decimals : 56774

Ideone

Answer 2

I guess there are no built in C/C++ commands to do this, other than the 2 methods of:

1) Using the above to convert into string and then scan back into 2 ints. 2) Accessing the memory contents of the memory variable and then decoding manually.