Does mathematics already scare you? Well, if it does take a deep breath and read on and by any chance if it does not, I will try my best to scare you now but ofcourse with a promise that we shall fill all the gaps before ending this post. Doesn't matter what programming langauge you code in, you shall still be able to relate to this post. For my convenience I will kill it with JavaScript.
So long back, I was working on a game that added 1 point to the user's score for every correct answer and deducted 0.1 points for a wrong one. The game starts with 1 point in your balance and then the score gets calculated based on your choices. Prima facie it worked fine (kind of) but then something caught me by dismay. I started the game (score = 1) and submitted three wrong answers back to back. What do you expect? A score of 1  0.1  0.1  0.1 = 0.7 ? Got you! Try that right away in your browser's console. It works alright you say? I bet you did 1  0.3, that indeed shall give you 0.7 but when you do it incrementally like I did, you shall see that
βοΈ 1  0.1 = 0.9
βοΈ 0.9  0.1 = 0.8
β 0.8  0.1 = 0.7000000000000001
Confused? Check out this codepen
Why is 0.8  0.1 not 0.7? Well, it is so in real world mathematics. So, is JavaScript's mathematics broken? Coreaders who also code in python would now tell you that even Python failed in its maths class. What's happening? Well, if you want a short blunt answer its the binary system making floating point calculations unpredictable. So yes its not your favorite programming language. We shall ofcourse discuss how to get around with this limitation but I cannot hold myself from digging a little more into the root cause.
Do we all understand that our computers store all and any kind of information in binary? Assuming you said 'YES', how is a decimal number (which we input) converted into binary before it gets stored? Do you know that after the number is converted into binary, to get stored in the register (memory), the binary should be first arranged in some appropriate format? "Binary Floating Point Representation" is the term we use for those formats. Call it FPR for simplicity.
Floating Point Representation
Binary Floating Point Representation can be of 3 types :

Half Precision Format
 available memory for a given number = 16 bits
 microFloat
 least precise & least wasteful

Single Precision Format
 available memory for a given number = 32 bits
 float datatype in Java

Double Precision Format
 available memory for a given number = 64 bits
 double datatype in Java
 most accurate representation of bigger numbersββ
Taking you back to school? No, please take a quick look (1.5x speed) at this video if you're not sure what did I just say. Now that you know we have limited space in the memory to store the binary representation, what if the binary of some number you input doesn't fit in 64 bits? Well, we round it up and make it fit in 64 bits somehow and hence we introduce the famous Rounding Error. This rounding error is the characteristic feature of floatingpoint computation and obviously when you input some number X, it may or may not stay exactly X after binary round off.
So what could be the examples of numbers whose binary won't fit even in 64 bits? A very obvious case can be of a number whose binary representation is nonterminating. 0.1 ? Seriously? Yes, lets see how this simple small decimal number has got a binary equivalent that never terminates (like the value of Ο in decimal).
Not my best handwriting though
That's how the decimal 0.1 looks in binary
There's a simple rule to find out if the given decimal number will have a non terminating binary or not. A decimal has an equivalent terminating binary if and only if the decimal, written as a proper fraction in lowest terms, has a denominator that is a power of two. Example : 0.1 has an infinite binary : 0.1 = 1/10, and 10 is not a power of two. Also 0.5, on the other hand, terminates: 0.5 = 5/10 = 1/2.
Apart from such numbers with non terminating binaries there can also be numbers with terminating but too big to fit in 64 bits binaries. Such numbers can also result in rounding errors. Now when I ask you to debug my game, you shall be able to atleast say (after looking at the output) that 0.8  0.1 is not 0.7 because somewhere in the binary roundoff 0.8, 0.1 or 0.7 or maybe all of them got introduced to the rounding error. So what do we learn from this? We learn that FPR of the decimal numbers we input can make calculations unpredictable. How do we deal with this? Well, I shall tell you how, atleast how in JavaScript.
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Solution to the roundoff error in JavaScript
 Math.round((0.80.1)*factor)/factor shall give 0.7, where factor = 10 for rounding to single digit, 100 for rounding the result to 2 digits after decimal and so on.
 (0.80.1).toFixed(1) shall give "0.7" but in string format. Mostly irrelevant now but "toFixed" may show inconsistencies amongst older versions of some browsers. Read more.
 There can be many more solutions. For example the "floor" and "ceil" functions of the Math object depending on the usecase or even custom functions like so.
Conclusion
Most decimals have infinite representations in binary. Due to limitation of memory, rounding errors may get introduced in numbers whose binary equivalent does not fit even the Double Precision Format. So do not be surprised the next time you see an anomaly in floating point calculations. Its good to use one of the above mentioned solutions or a custom tailored solution that fits your requirement.
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Originally posted here 
https://mayankav.webflow.io/blog/javascriptsbrokenmathematics
Discussion (10)
Always a good day to remember the amazing 0.30000000000000004.com. There are methods to get the precise result of operations in JS, but they rely on libraries, usually.
Thanks for adding value to the post @lukeshiru
Floating point rounding errors is one of these things you just forget once you've learned them, and whenever someone else stumbles into them for the first time, you're like "Right, this isn't something you'd just know without someone explaining it".
Luckily there are good online resources one can link to that explain. This one is probably my favourite; it's short but explains all the important details.
Math.round((1.7885*10)*100)/100 = 17.88 instead of 17.89. Not working for multiplication?
The IEEE Floating Point standard used by almost all programming languages uses a rounding method known as "rounding ties to even" (also called "round to even" or "Banker's Rounding"):
This method preserves the average of a set of numbers more than the "half round up" method that most people use in daytoday life.
It just skews the numbers towards even values a little.
So, in your case, you are multiplying 1.7885 by 10 and then rounding to 2 decimal places.
The digit to be rounded off is the 5.
Because it is 5, look at the digit on the left, which is 8.
8 is even, so we round down, leaving 17.88.
Hope this makes sense!
Relevant links:
mathsisfun.com/numbers/roundingme... (really nice, and explains many methods)
blogs.sas.com/content/iml/2019/11/...
en.m.wikipedia.org/wiki/IEEE_754 (see "Rounding Rules")
en.m.wikipedia.org/wiki/Rounding#R...
Hey man! Interesting. To answer your question, why don't you type 1.7885*10 in your console and see what result shows up ? ;)
Very nice and clearly expressed!
:)
it is the same case in MongoDB I met last week, so mongoDB use the same way as Javascript, right?
Hey buddy! That's computer arithmetic that shall apply everywhere :)