比较浮点数时出现问题，以及如何正确比较它们？

在本文中，我们将看到比较浮点数的问题，并且将讨论比较两个浮点数的正确方法。

通常比较浮点数有什么问题?

让我们首先在关系运算符(==)的帮助下比较两个浮点数。

示例：使用“ ==”进行比较

CPP

// C++ program to compare
// floating point numbers
  
#include 
using namespace std;
  
void compareFloatNum(double a, double b)
{
    if (a == b) {
        cout << "The numbers are equal"
             << endl;
    }
    else {
        cout << "The numbers are not equal"
             << endl;
    }
}
  
// Driver code
int main()
{
    double a = (0.3 * 3) + 0.1;
    double b = 1;
    compareFloatNum(a, b);
}

Java

// Java program to compare
// floating point numbers
class GFG 
{
  
    static void compareFloatNum(double a, double b) 
    {
        if (a == b)
        {
            System.out.print("The numbers are equal" + "\n");
        } 
        else 
        {
            System.out.print("The numbers are not equal" + "\n");
        }
    }
  
    // Driver code
    public static void main(String[] args) 
    {
        double a = (0.3 * 3) + 0.1;
        double b = 1;
        compareFloatNum(a, b);
    }
}
  
// This code is contributed by 29AjayKumar

Python

# Python program to compare
# floating point numbers
def compareFloatNum(a, b):
    if (a == b):
        print("The numbers are equal")
  
    else:
        print("The numbers are not equal")
  
# Driver code
  
a = (0.3 * 3) + 0.1
b = 1
compareFloatNum(a, b)
  
# This code is contributed by mohit kumar 29

C#

// C# program to compare
// floating point numbers
using System;
  
class GFG 
{
  
    static void comparefloatNum(double a, double b) 
    {
        if (a == b)
        {
            Console.Write("The numbers are equal" + "\n");
        } 
        else
        {
            Console.Write("The numbers are not equal" + "\n");
        }
    }
  
    // Driver code
    public static void Main(String[] args) 
    {
        double a = (0.3 * 3) + 0.1;
        double b = 1;
        comparefloatNum(a, b);
    }
}
  
// This code is contributed by PrinciRaj1992

输出：

The numbers are not equal

为什么会出现此问题?

对于浮点数，关系运算符(==)不会产生正确的输出，这是由于在对浮点数进行四舍五入时存在内部精度错误。

在上面的示例中，我们可以看到使用“ ==”运算符比较两个浮点数时的不准确性。两个数字“ a”和“ b”相等(等于(0.3 * 3)+ 0.1 = 1)，但是程序会导致输出错误。

让我们仔细看看下一个片段中的数字。

// C++ program to compare
// floating point numbers
  
#include 
using namespace std;
  
void printFloatNum(double a, double b)
{
    // To print decimal numbers up to 20 digits
    cout << setprecision(20);
  
    cout << "a is : " << a << endl;
    cout << "b is : " << b << endl;
}
  
// Driver code
int main()
{
    double a = (0.3 * 3) + 0.1;
    double b = 1;
    printFloatNum(a, b);
}

输出：

a is : 0.99999999999999988898
b is : 1

现在我们可以看到浮点数内部的舍入误差。数字“ a”没有正确地四舍五入为1，
四舍五入有一个内部错误，一个很小的错误，但是当我们比较这些数字时却有很大的不同。

如何正确比较浮点数?
如果我们必须比较两个浮点数，则不必使用“ ==”运算符，我们将发现数字之间的绝对差(如果正确表示，则差将为0)，并将其与非常小的值进行比较数字1e-9(即10 ^ -9，这个数字很小)，如果相差小于这个数字，我们可以肯定地说两个浮点数相等。

例子：

C++

// C++ program to compare
// floating point numbers correctly
  
#include 
using namespace std;
  
void compareFloatNum(double a, double b)
{
  
    // Correct method to compare
    // floating-point numbers
    if (abs(a - b) < 1e-9) {
        cout << "The numbers are equal "
             << endl;
    }
    else {
        cout << "The numbers are not equal "
             << endl;
    }
}
  
// Driver code
int main()
{
    double a = (0.3 * 3) + 0.1;
    double b = 1;
    compareFloatNum(a, b);
}

Java

// Java program to compare
// floating point numbers correctly
class GFG
{
  
static void compareFloatNum(double a, double b)
{
  
    // Correct method to compare
    // floating-point numbers
    if (Math.abs(a - b) < 1e-9)
    {
        System.out.print("The numbers are equal "
            +"\n");
    }
    else 
    {
        System.out.print("The numbers are not equal "
            +"\n");
    }
}
  
// Driver code
public static void main(String[] args)
{
    double a = (0.3 * 3) + 0.1;
    double b = 1;
    compareFloatNum(a, b);
}
}
  
// This code is contributed by Rajput-Ji

Python3

# Python program to compare
# floating ponumbers correctly
  
def compareFloatNum(a, b):
      
    # Correct method to compare
    # floating-ponumbers
    if (abs(a - b) < 1e-9):
        print("The numbers are equal ");
    else:
        print("The numbers are not equal ");
      
# Driver code
if __name__ == '__main__':
    a = (0.3 * 3) + 0.1;
    b = 1;
    compareFloatNum(a, b);
  
# This code is contributed by PrinciRaj1992

C＃

// C# program to compare
// floating point numbers correctly
using System;
  
class GFG
{
  
static void comparefloatNum(double a, double b)
{
  
    // Correct method to compare
    // floating-point numbers
    if (Math.Abs(a - b) < 1e-9)
    {
        Console.Write("The numbers are equal "
            +"\n");
    }
    else
    {
        Console.Write("The numbers are not equal "
            +"\n");
    }
}
  
// Driver code
public static void Main(String[] args)
{
    double a = (0.3 * 3) + 0.1;
    double b = 1;
    comparefloatNum(a, b);
}
}
  
// This code is contributed by 29AjayKumar

输出：

The numbers are equal

这段代码会产生正确的输出，因此每当比较两个浮点数时，我们将使用上述技术，而不是使用“ ==”运算符。

要从最佳影片策划和实践问题去学习，检查了C++基础课程为基础，以先进的C++和C++ STL课程基础加上STL。要完成从学习语言到DS Algo等的更多准备工作，请参阅“完整面试准备课程” 。