Rounding a double to turn it into an int (java)
What is the return type of the round()
method in the snippet?
If this is the Math.round()
method, it returns a Long when the input param is Double.
So, you will have to cast the return value:
int a = (int) Math.round(doubleVar);
How to cast a double to an int in Java by rounding it down?
Casting to an int implicitly drops any decimal. No need to call Math.floor() (assuming positive numbers)
Simply typecast with (int), e.g.:
System.out.println((int)(99.9999)); // Prints 99
This being said, it does have a different behavior from Math.floor
which rounds towards negative infinity (@Chris Wong)
Rounding double to int always to the smaller
Use
double d = ...
int n = (int)Math.floor(d);
Convert double to Int, rounded down
If you explicitly cast double
to int
, the decimal part will be truncated. For example:
int x = (int) 4.97542; //gives 4 only
int x = (int) 4.23544; //gives 4 only
Moreover, you may also use Math.floor()
method to round values in case you want double
value in return.
Converting double to integer in Java
is there a possibility that casting a double created via
Math.round()
will still result in a truncated down number
No, round()
will always round your double to the correct value, and then, it will be cast to an long
which will truncate any decimal places. But after rounding, there will not be any fractional parts remaining.
Here are the docs from Math.round(double)
:
Returns the closest long to the argument. The result is rounded to an integer by adding 1/2, taking the floor of the result, and casting the result to type long. In other words, the result is equal to the value of the expression:
(long)Math.floor(a + 0.5d)
How to round a number to n decimal places in Java
Use setRoundingMode
, set the RoundingMode
explicitly to handle your issue with the half-even round, then use the format pattern for your required output.
Example:
DecimalFormat df = new DecimalFormat("#.####");
df.setRoundingMode(RoundingMode.CEILING);
for (Number n : Arrays.asList(12, 123.12345, 0.23, 0.1, 2341234.212431324)) {
Double d = n.doubleValue();
System.out.println(df.format(d));
}
gives the output:
12
123.1235
0.23
0.1
2341234.2125
EDIT: The original answer does not address the accuracy of the double values. That is fine if you don't care much whether it rounds up or down. But if you want accurate rounding, then you need to take the expected accuracy of the values into account. Floating point values have a binary representation internally. That means that a value like 2.7735 does not actually have that exact value internally. It can be slightly larger or slightly smaller. If the internal value is slightly smaller, then it will not round up to 2.7740. To remedy that situation, you need to be aware of the accuracy of the values that you are working with, and add or subtract that value before rounding. For example, when you know that your values are accurate up to 6 digits, then to round half-way values up, add that accuracy to the value:
Double d = n.doubleValue() + 1e-6;
To round down, subtract the accuracy.
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