Comparing double values in C#

Question

I've a double variable called x. In the code, x gets assigned a value of 0.1 and I check it in an 'if' statement comparing x and 0.1

if (x==0.1)
{
----
}

Unfortunately it does not enter the if statement

1. Should I use Double or double?

2. What's the reason behind this? Can you suggest a solution for this?

Answer 1

From the documentation:

Precision in Comparisons The Equals method should be used with caution, because two apparently equivalent values can be unequal due to the differing precision of the two values. The following example reports that the Double value .3333 and the Double returned by dividing 1 by 3 are unequal.

...

Rather than comparing for equality, one recommended technique involves defining an acceptable margin of difference between two values (such as .01% of one of the values). If the absolute value of the difference between the two values is less than or equal to that margin, the difference is likely to be due to differences in precision and, therefore, the values are likely to be equal. The following example uses this technique to compare .33333 and 1/3, the two Double values that the previous code example found to be unequal.

So if you really need a double, you should use the techique described on the documentation. If you can, change it to a decimal. It' will be slower, but you won't have this type of problem.

Answer 2

Taking a tip from the Java code base, try using .CompareTo and test for the zero comparison. This assumes the .CompareTo function takes in to account floating point equality in an accurate manner. For instance,

System.Math.PI.CompareTo(System.Math.PI) == 0

This predicate should return true.

Answer 3

floating point number representations are notoriously inaccurate (because of the way floats are stored internally) e.g. x may actually be 0.0999999999 or 0.100000001 and your condition will fail. If you want to determine if floats are equal you need to specific whether they're equal to within a certain tolerance.

i.e.

if(x - 0.1 < tol)

Answer 4

Exact comparison of floating point values is know to not always work due to the rounding and internal representation issue.

Try imprecise comparison:

if (x >= 0.099 && x <= 0.101)
{
}

The other alternative is to use the decimal data type.

Answer 5

1) Should i use Double or double???

Double and double is the same thing. double is just a C# keyword working as alias for the class System.Double The most common thing is to use the aliases! The same for string (System.String), int(System.Int32)

Also see Built-In Types Table (C# Reference)

Answer 6

It's a standard problem due to how the computer stores floating point values. Search here for "floating point problem" and you'll find tons of information.

In short – a float/double can't store 0.1 precisely. It will always be a little off.

You can try using the decimal type which stores numbers in decimal notation. Thus 0.1 will be representable precisely.

---

You wanted to know the reason:

Float/double are stored as binary fractions, not decimal fractions. To illustrate:

12.34 in decimal notation (what we use) means

1 * 101 + 2 * 100 + 3 * 10-1 + 4 * 10-2

The computer stores floating point numbers in the same way, except it uses base 2: 10.01 means

1 * 21 + 0 * 20 + 0 * 2-1 + 1 * 2-2

Now, you probably know that there are some numbers that cannot be represented fully with our decimal notation. For example, 1/3 in decimal notation is 0.3333333…. The same thing happens in binary notation, except that the numbers that cannot be represented precisely are different. Among them is the number 1/10. In binary notation that is 0.000110011001100….

Since the binary notation cannot store it precisely, it is stored in a rounded-off way. Hence your problem.