Lodash Rounding Accuracy

Question

I am trying to display the number as a percentage using _.round and then by multiplying the number by 100. For some reason, when I multiply the rounded number, the accuracy gets messed up. Here's what it looks like:

 var num = 0.056789, roundingPrecision = 4, roundedNum = _.round(num, roundingPrecision), percent = (roundedNum * 100) + '%'; console.log(roundedNum); // 0.0568 console.log(percent); // 5.680000000000001% 

fiddle

Why is the value 0.000000000000001 added to the number after multiplying by 100?

Answer 1

This is because numbers are represented internally as binary numbers with limited precision.

See also Is floating point math broken?

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Is floating point math broken?

0.1 + 0.2 = 0.3 → false

0.1 + 0.2 → 0.30000000000000004

Any ideas why this is happening?

What answer did you get:

Binary floating point math like this. In most programming languages, it is based on the IEEE 754 standard . JavaScript uses a 64-bit floating-point representation, which is the same as Java double . The essence of the problem is that the numbers are presented in this format as an integer number twice; rational numbers (such as 0.1 , which is 1/10 ), whose denominator is not a power of two, cannot be accurately represented.

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To get the correct result in your case, you need to round after all arithmetic:

 var num = 0.056789, roundingPrecision = 4, roundedNum = _.round(num * 100, roundingPrecision), percent = roundedNum + '%'; console.log(percent); // 5.0569% 

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