double or float datatype doesn't addup properly in a loop?

Question

In a loop I am adding 0.10 till I reach the desired #, and getting the index. This is my code :

    private static int getIndexOfUnits(float units) {
    int         


        

Answer 1

Exact representation is not available with Float datatype.

BigDecimal will help you.Below example might be helpful for you

BigDecimal decimal=new BigDecimal(10.0);
    for (int i = 0; i < 10; i++) {
        decimal=decimal.add(new BigDecimal(.1));
        System.out.println(decimal.floatValue());

    }

Answer 2

I think this may be what you're looking for:

Java provides a class from the import package: import java.text.DecimalFormat called DecimalFormat. Its signature is:

DecimalFormat myFormat = new DecimalFormat("0.0");

It takes a String argument where you specify how you want the formatting to be displayed.

Here's how you can apply it to your code:

DecimalFormat myFormat;
private static int getIndexOfUnits(float units) {
myFormat = new DecimalFormat("0.0");
int index = -1;
float addup = 0.10f;
for(float i = 1.00f; i < units; i=(float)i+addup) {
    index++;
    System.out.println("I = " + myFormat.format(i) + " Index = " + index);
}

return index;

}

In your println, you can see that the DecimalFormat's class format() method is called on float i using the myFormat object reference to DecimalFormat - this is where the formatting takes place.

Answer 3

The problem is that the format that float uses can't represent all decimal numbers, so precision is lost sometimes.

Use BigDecimal instead.

Answer 4

From the Floating-Point Guide:

Why don’t my numbers, like 0.1 + 0.2 add up to a nice round 0.3, and instead I get a weird result like 0.30000000000000004?

Because internally, computers use a format (binary floating-point) that cannot accurately represent a number like 0.1, 0.2 or 0.3 at all.

When the code is compiled or interpreted, your “0.1” is already rounded to the nearest number in that format, which results in a small rounding error even before the calculation happens.

You cnnot use float or double if you need numbers to add up exaclty. Use BigDecimal instead.

Answer 5

You shouldn't use float's for this kind of thing (indexing / iterating).

Try calculating I every time:

I = 1.0f + (i*addup);

And you'll not accumulate floating-point errors.