How do you round a double value to 2 decimal places in javascript?
In general, decimal rounding is done by scaling: Show Naive implementation Using the following function with halfway numbers, you will get either the upper rounded value as expected, or the lower rounded value sometimes depending on the input. This
In order to
determine whether a rounding operation involves a midpoint value, the Round function multiplies the original value to be rounded by 10 ** n, where n is the desired number of fractional digits in the return value, and then determines whether the remaining fractional portion of the value is greater than or equal to .5. This In the previous example, Better implementations Exponential notation By converting the number to a string in the exponential notation, positive numbers are rounded as expected. But, be aware that negative numbers round differently than positive numbers. In fact, it performs what is basically equivalent to "round half up" as the rule, you will see that
If you want the usual behavior when rounding negative numbers, you would need to convert negative numbers to positive before calling Math.round(), and then convert them back to negative numbers before returning.
Approximate rounding To correct the rounding problem shown in the previous
Number.EPSILON There is a different purely mathematical technique to perform round-to-nearest (using "round half away from zero"), in which epsilon correction is applied before calling the rounding function. Simply, we add
the smallest possible float value (= 1.0 ulp; unit in the last place) to the product before rounding. This moves to the next representable float value, away from zero, thus it will offset the binary round-off error that may occur during the multiplication by
After adding 1 ulp, the value of 5.015 *
100 which is Note that the size of a unit in last place ("ulp") is determined by (1) the magnitude of the number and (2) the relative machine epsilon (2^-52). Ulps are relatively larger at numbers with bigger magnitudes than they are at numbers with smaller magnitudes. Double rounding Here, we use the toPrecision() method to strip the floating-point round-off errors in the intermediate calculations. Simply, we round to 15 significant figures to strip the round-off error at the 16th significant digit. This technique to preround the result to significant digits is also used by PHP 7 round function. The value of 5.015 * 100 which is
Arbitrary-precision JavaScript library - decimal.js
Solution 1: string in exponential notation Inspired by the solution provided by KFish here: https://stackoverflow.com/a/55521592/4208440 A simple drop in solution that provides accurate decimal rounding, flooring, and ceiling to a specific number of decimal places without adding a whole library. It treats floats more like decimals by fixing the binary rounding issues to avoid unexpected results: for example, floor((0.1+0.7)*10) will return the expected result 8. Numbers are rounded to a specific number of fractional digits. Specifying a negative precision will round to any number of places to the left of the decimal point.
Solution 2: purely mathematical (Number.EPSILON) This solution avoids any string conversion / manipulation of any kind for performance reasons.
Solution 3: double rounding This solution uses the toPrecision() method to strip the floating-point round-off errors.
Solution 4: double rounding v2 This solution is just like Solution 3, however it uses a custom
Benchmarkshttp://jsbench.github.io/#31ec3a8b3d22bd840f8e6822e681a3ac Here is a benchmark comparing the operations per second in the solutions above on Chrome 85.0.4183.83. Obviously all browsers differ, so your mileage may vary. (Note: More is better)Thanks @Mike for adding a screenshot of the benchmark. How do you round a double value in JavaScript?Math.
round() is a function used to return the value of a floating-point number rounded to the nearest whole integer. Depending on the supplied float/double, Math. round() either rounds up or down.
How do you round a double to 2 places?DecimalFormat(“0.00”)
We can use DecimalFormat("0.00") to ensure the number always round to 2 decimal places. For DecimalFormat , the default rounding mode is RoundingMode.
How do you round to a certain decimal places in JavaScript?JavaScript Number toFixed()
The toFixed() method converts a number to a string. The toFixed() method rounds the string to a specified number of decimals.
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