There is some problem, i can't understand anyway.
look at this code please
<script type="text/javascript">
function math(x)
{
var y;
y = x*10;
alert(y);
}
</script>
<input type="button" onclick="math(0.011)">
What must be alerted after i click on button?
i think 0.11, but no, it alerts
0.10999999999999999
explain please this behavior.
thanks in advance
Ok. Let me try to explain it.
The basic thing to remember with floating point numbers is this: They occupy a limited amount of bits and try to represent the original number using base-2 arithmetic.
As you know, in base-2 arithmetic integers are represented by the powers of 2 that they contain. Thus, 6 would be represented as 4 + 2, ie. in binary as 110.
In order to understand how fractional numbers are represented, you have to think about how we represent fractional numbers in our decimal system. The fractional part of numbers (for example 0.11) is represented as multiples of inverse powers of 10 (since the base is 10). Thus 0.11 is actually 1/10 + 1/100. As you can appreciate, this is not powerful enough to represent all fractional numbers in a limited number of digits. For example, 1/3 would be 0.333333.... in a never ending fashion. If we had only 32 digits of space to write the number down, we would end up having only an approximation to the original number, 0.33333333333333333333333333333333. This number, for example, would give 0.99999999999999999999999999999999 if it was multiplied by 3 and not 1 as you would have expected.
The situation is similar in base-2. Each fractional number would be represented as multiples of inverse powers of 2. Thus 0.75 (in decimal) (ie 3/4) would be represented as 1/2 + 1/4, which would mean 0.11 (in base-2). Just as base 10 is not capable enough to represent every fractional number in a finite manner, base-2 cannot represent all fractional numbers given a limited amount of space.
Now, try to represent 0.11 in base-2; you start with 11/100 and try to find an inverse power of 2 that is just less than this number. 1/2 doesn't work, 1/4 neither, nor does 1/8. 1/16 fits the bill, so you mark a 1 in the 4th place after the decimal point and subtract 1/16 from 11/100. You are left with 19/400. Now try to find the next power of 2 that fits the description. 1/32 seems to be that one, mark the 5th place after the point and subtract 1/32 from 19/400, you get 13/800. Next one is 1/64 and you are left with 1/1600 thus the next one is all the way up at 1/2048, etc. etc. Thus we got as far as 0.00011100001 but it goes on and on; and you will see that there always is a fraction remaining. Now, I didn't go through the whole calculation, but after you have put in 32 binary digits after the dot you will still probably have some fraction left (and this is assuming that all of the 32 bits of space is spent representing the decimal part, which it is not). Thus, I am sure you can appreciate that the resulting number might differ from its actual value by some amount.
In your case, the difference is 0.00000000000000001 which is 1/100000000000000000 = 1/10^17 and I am sure that you can see why you might have that.
this is because you are dealing with floating point, and this is the expected behavior of floating point math.
what you need to do is format that number.
see this java explanation which also applies here if you want to know why this is happening.
in javascript all numbers are represented as 64bit floats, so you will run into this sort of thing often.
the quick overview of that article is that floating point tries to represent a range of values larger then would fit in 64bits, therefor there is going to be some imprecise representation, and this is what you are seeing.
With floating point number you get a representation of the number you try to encode. Mostly it is a number that is very close the the original number. More information on encoding/storing floating point numbers can be found here.
Note:
If you show the value of x, it still shows 0.011 because JavaScript has not yet decided what variable type x has. But after multiplying it with 10 the type got set to floating point (it is the only possibility) and the round error shows.
You can try to fix the nr of decimals with this one:
// fl is a float number with some nr of decimals
// d is how many decimals you want
function dec(fl, d) {
var p = Math.pow(10, d);
return Math.round(fl*p)/p;
}
Ex:
var n = 0.0012345;
console.log(dec(n,6)); // 0.001235
console.log(dec(n,5)); // 0.00123
console.log(dec(n,4)); // 0.0012
console.log(dec(n,3)); // 0.001
It works by first multiplying the float with 10^3 (1000) for three decimals, or 10^2 (100) for two decimals. Then do round on that and divide it back to original size.
Math.pow(10, d) makes 10^d (means that d will give us 1000).
In your case, do alert(dec(y,2));, it should work.
Related
This question already has answers here:
Is floating point math broken?
(31 answers)
Closed 6 years ago.
Why 2.65 + 2.66 = 5.3100000000000005 in Javascript?
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.\
The basic thing to remember with floating point numbers is this: They occupy a limited amount of bits and try to represent the original number using base-2 arithmetic.
As you know, in base-2 arithmetic integers are represented by the powers of 2 that they contain. Thus, 6 would be represented as 4 + 2, ie. in binary as 110.
In order to understand how fractional numbers are represented, you have to think about how we represent fractional numbers in our decimal system. The fractional part of numbers (for example 0.11) is represented as multiples of inverse powers of 10 (since the base is 10). Thus 0.11 is actually 1/10 + 1/100. As you can appreciate, this is not powerful enough to represent all fractional numbers in a limited number of digits. For example, 1/3 would be 0.333333.... in a never ending fashion. If we had only 32 digits of space to write the number down, we would end up having only an approximation to the original number, 0.33333333333333333333333333333333. This number, for example, would give 0.99999999999999999999999999999999 if it was multiplied by 3 and not 1 as you would have expected.
The situation is similar in base-2. Each fractional number would be represented as multiples of inverse powers of 2. Thus 0.75 (in decimal) (ie 3/4) would be represented as 1/2 + 1/4, which would mean 0.11 (in base-2). Just as base 10 is not capable enough to represent every fractional number in a finite manner, base-2 cannot represent all fractional numbers given a limited amount of space.
Now, try to represent 0.11 in base-2; you start with 11/100 and try to find an inverse power of 2 that is just less than this number. 1/2 doesn't work, 1/4 neither, nor does 1/8. 1/16 fits the bill, so you mark a 1 in the 4th place after the decimal point and subtract 1/16 from 11/100. You are left with 19/400. Now try to find the next power of 2 that fits the description. 1/32 seems to be that one, mark the 5th place after the point and subtract 1/32 from 19/400, you get 13/800. Next one is 1/64 and you are left with 1/1600 thus the next one is all the way up at 1/2048, etc. etc. Thus we got as far as 0.00011100001 but it goes on and on; and you will see that there always is a fraction remaining. Now, I didn't go through the whole calculation, but after you have put in 32 binary digits after the dot you will still probably have some fraction left (and this is assuming that all of the 32 bits of space is spent representing the decimal part, which it is not). Thus, I am sure you can appreciate that the resulting number might differ from its actual value by some amount.
See:
Weird Javascript Behaviour: Floating Point Addition giving the wrong answer
Is JavaScript's Math broken?
Understanding floating point variables
I know a little bit about how floating-point numbers are represented, but not enough, I'm afraid.
The general question is:
For a given precision (for my purposes, the number of accurate decimal places in base 10), what range of numbers can be represented for 16-, 32- and 64-bit IEEE-754 systems?
Specifically, I'm only interested in the range of 16-bit and 32-bit numbers accurate to +/-0.5 (the ones place) or +/- 0.0005 (the thousandths place).
For a given IEEE-754 floating point number X, if
2^E <= abs(X) < 2^(E+1)
then the distance from X to the next largest representable floating point number (epsilon) is:
epsilon = 2^(E-52) % For a 64-bit float (double precision)
epsilon = 2^(E-23) % For a 32-bit float (single precision)
epsilon = 2^(E-10) % For a 16-bit float (half precision)
The above equations allow us to compute the following:
For half precision...
If you want an accuracy of +/-0.5 (or 2^-1), the maximum size that the number can be is 2^10. Any X larger than this limit leads to the distance between floating point numbers greater than 0.5.
If you want an accuracy of +/-0.0005 (about 2^-11), the maximum size that the number can be is 1. Any X larger than this maximum limit leads to the distance between floating point numbers greater than 0.0005.
For single precision...
If you want an accuracy of +/-0.5 (or 2^-1), the maximum size that the number can be is 2^23. Any X larger than this limit leads to the distance between floating point numbers being greater than 0.5.
If you want an accuracy of +/-0.0005 (about 2^-11), the maximum size that the number can be is 2^13. Any X larger than this lmit leads to the distance between floating point numbers being greater than 0.0005.
For double precision...
If you want an accuracy of +/-0.5 (or 2^-1), the maximum size that the number can be is 2^52. Any X larger than this limit leads to the distance between floating point numbers being greater than 0.5.
If you want an accuracy of +/-0.0005 (about 2^-11), the maximum size that the number can be is 2^42. Any X larger than this limit leads to the distance between floating point numbers being greater than 0.0005.
For floating-point integers (I'll give my answer in terms of IEEE double-precision), every integer between 1 and 2^53 is exactly representable. Beyond 2^53, integers that are exactly representable are spaced apart by increasing powers of two. For example:
Every 2nd integer between 2^53 + 2 and 2^54 can be represented exactly.
Every 4th integer between 2^54 + 4 and 2^55 can be represented exactly.
Every 8th integer between 2^55 + 8 and 2^56 can be represented exactly.
Every 16th integer between 2^56 + 16 and 2^57 can be represented exactly.
Every 32nd integer between 2^57 + 32 and 2^58 can be represented exactly.
Every 64th integer between 2^58 + 64 and 2^59 can be represented exactly.
Every 128th integer between 2^59 + 128 and 2^60 can be represented exactly.
Every 256th integer between 2^60 + 256 and 2^61 can be represented exactly.
Every 512th integer between 2^61 + 512 and 2^62 can be represented exactly.
.
.
.
Integers that are not exactly representable are rounded to the nearest representable integer, so the worst case rounding is 1/2 the spacing between representable integers.
The precision quoted form Peter R's link to the MSDN ref is probably a good rule of thumb, but of course reality is more complicated.
The fact that the "point" in "floating point" is a binary point and not decimal point has a way of defeating our intuitions. The classic example is 0.1, which needs a precision of only one digit in decimal but isn't representable exactly in binary at all.
If you have a weekend to kill, have a look at What Every Computer Scientist Should Know About Floating-Point Arithmetic. You'll probably be particularly interested in the sections on Precision and Binary to Decimal Conversion.
First off, neither IEEE-754-2008 nor -1985 have 16-bit floats; but it is a proposed addition with a 5-bit exponent and 10-bit fraction. IEE-754 uses a dedicated sign bit, so the positive and negative range is the same. Also, the fraction has an implied 1 in front, so you get an extra bit.
If you want accuracy to the ones place, as in you can represent each integer, the answer is fairly simple: The exponent shifts the decimal point to the right-end of the fraction. So, a 10-bit fraction gets you ±211.
If you want one bit after the decimal point, you give up one bit before it, so you have ±210.
Single-precision has a 23-bit fraction, so you'd have ±224 integers.
How many bits of precision you need after the decimal point depends entirely on the calculations you're doing, and how many you're doing.
210 = 1,024
211 = 2,048
223 = 8,388,608
224 = 16,777,216
253 = 9,007,199,254,740,992 (double-precision)
2113 = 10,384,593,717,069,655,257,060,992,658,440,192 (quad-precision)
See also
Double-precision
Half-precision
See IEEE 754-1985:
Note (1 + fraction). As #bendin point out, using binary floating point, you cannot express simple decimal values such as 0.1. The implication is that you can introduce rounding errors by doing simple additions many many times or calling things like truncation. If you are interested in any sort of precision whatsoever, the only way to achieve it is to use a fixed-point decimal, which basically is a scaled integer.
If I understand your question correctly, it depends on your language.
For C#, check out the MSDN ref. Float has a 7 digit precision and double 15-16 digit precision.
It took me quite a while to figure out that when using doubles in Java, I wasn't losing significant precision in calculations. floating point actually has a very good ability to represent numbers to quite reasonable precision. The precision I was losing was immediately upon converting decimal numbers typed by users to the binary floating point representation that is natively supported. I've recently started converting all my numbers to BigDecimal. BigDecimal is much more work to deal with in the code than floats or doubles, since it's not one of the primitive types. But on the other hand, I'll be able to exactly represent the numbers that users type in.
I have been trying to figure this floating-point problem out in javascript.
This is an example of what I want to do:
var x1 = 0
for(i=0; i<10; i++)
{
x1+= 0.2
}
However in this form I will get a rounding error, 0.2 -> 0.4 -> 0.600...001 doing that.
I have tried parseFloat, toFixed and Math.round suggested in other threads but none of it have worked for me. So are there anyone who could make this work, because I feel that I have run out of options.
You can almost always ignore the floating point "errors" while you're performing calculations - they won't make any difference to the end result unless you really care about the 17th significant digit or so.
You normally only need to worry about rounding when you display those values, for which .toFixed(1) would do perfectly well.
Whatever happens you simply cannot coerce the number 0.6 into exactly that value. The closest IEEE 754 double precision is exactly 0.59999999999999997779553950749686919152736663818359375, which when displayed within typical precision limits in JS is displayed as 0.5999999999999999778
Indeed JS can't even tell that 0.5999999999999999778 !== (e.g) 0.5999999999999999300 since their binary representation is the same.
To better understand how the rounding errors are accumulating, and get more insight on what is happenning at lower level, here is a small explanantion:
I will assume that IEEE 754 double precision standard is used by underlying software/hardware, with default rounding mode (round to nearest even).
1/5 could be written in base 2 with a pattern repeating infinitely
0.00110011001100110011001100110011001100110011001100110011...
But in floating point, the significand - starting at most significant 1 bit - has to be rounded to a finite number of bits (53)
So there is a small rounding error when representing 0.2 in binary:
0.0011001100110011001100110011001100110011001100110011010
Back to decimal representation, this rounding error corresponds to a small excess 0.000000000000000011102230246251565404236316680908203125 above 1/5
The first operation is then exact because 0.2+0.2 is like 2*0.2 and thus does not introduce any additional error, it's like shifting the fraction point:
0.0011001100110011001100110011001100110011001100110011010
+ 0.0011001100110011001100110011001100110011001100110011010
---------------------------------------------------------
0.0110011001100110011001100110011001100110011001100110100
But of course, the excess above 2/5 is doubled 0.00000000000000002220446049250313080847263336181640625
The third operation 0.2+0.2+0.2 will result in this binary number
0.011001100110011001100110011001100110011001100110011010
+ 0.0011001100110011001100110011001100110011001100110011010
---------------------------------------------------------
0.1001100110011001100110011001100110011001100110011001110
But unfortunately, it requires 54 bits of significand (the span between leading 1 and trailing 1), so another rounding error is necessary to represent the result as a double:
0.10011001100110011001100110011001100110011001100110100
Notice that the number was rounded upper, because by default floats are rounded to nearest even in case of perfect tie. We already had an error by excess, so bad luck, successive errors did cumulate rather than annihilate...
So the excess above 3/5 is now 0.000000000000000088817841970012523233890533447265625
You could reduce a bit this accumulation of errors by using
x1 = i / 5.0
Since 5 is represented exactly in float (101.0 in binary, 3 significand bits are enough), and since that will also be the case of i (up to 2^53), there is a single rounding error when performing the division, and IEEE 754 then guarantees that you get the nearest possible representation.
For example 3/5.0 is represented as:
0.10011001100110011001100110011001100110011001100110011
Back to decimal, the value is represented by default 0.00000000000000002220446049250313080847263336181640625 under 3/5
Note that both errors are very tiny, but in second case 3/5.0, four times smaller in magnitude than 0.2+0.2+0.2.
Depending on what you're doing, you may want to do fixed-point arithmetic instead of floating point. For example, if you are doing financial calculations in dollars with amounts that are always multiples of $0.01, you can switch to using cents internally, and then convert to (and from) dollars only when displaying values to the user (or reading input from the user). For more complicated scenarios, you can use a fixed-point arithmetic library.
There is some problem, i can't understand anyway.
look at this code please
<script type="text/javascript">
function math(x)
{
var y;
y = x*10;
alert(y);
}
</script>
<input type="button" onclick="math(0.011)">
What must be alerted after i click on button?
i think 0.11, but no, it alerts
0.10999999999999999
explain please this behavior.
thanks in advance
Ok. Let me try to explain it.
The basic thing to remember with floating point numbers is this: They occupy a limited amount of bits and try to represent the original number using base-2 arithmetic.
As you know, in base-2 arithmetic integers are represented by the powers of 2 that they contain. Thus, 6 would be represented as 4 + 2, ie. in binary as 110.
In order to understand how fractional numbers are represented, you have to think about how we represent fractional numbers in our decimal system. The fractional part of numbers (for example 0.11) is represented as multiples of inverse powers of 10 (since the base is 10). Thus 0.11 is actually 1/10 + 1/100. As you can appreciate, this is not powerful enough to represent all fractional numbers in a limited number of digits. For example, 1/3 would be 0.333333.... in a never ending fashion. If we had only 32 digits of space to write the number down, we would end up having only an approximation to the original number, 0.33333333333333333333333333333333. This number, for example, would give 0.99999999999999999999999999999999 if it was multiplied by 3 and not 1 as you would have expected.
The situation is similar in base-2. Each fractional number would be represented as multiples of inverse powers of 2. Thus 0.75 (in decimal) (ie 3/4) would be represented as 1/2 + 1/4, which would mean 0.11 (in base-2). Just as base 10 is not capable enough to represent every fractional number in a finite manner, base-2 cannot represent all fractional numbers given a limited amount of space.
Now, try to represent 0.11 in base-2; you start with 11/100 and try to find an inverse power of 2 that is just less than this number. 1/2 doesn't work, 1/4 neither, nor does 1/8. 1/16 fits the bill, so you mark a 1 in the 4th place after the decimal point and subtract 1/16 from 11/100. You are left with 19/400. Now try to find the next power of 2 that fits the description. 1/32 seems to be that one, mark the 5th place after the point and subtract 1/32 from 19/400, you get 13/800. Next one is 1/64 and you are left with 1/1600 thus the next one is all the way up at 1/2048, etc. etc. Thus we got as far as 0.00011100001 but it goes on and on; and you will see that there always is a fraction remaining. Now, I didn't go through the whole calculation, but after you have put in 32 binary digits after the dot you will still probably have some fraction left (and this is assuming that all of the 32 bits of space is spent representing the decimal part, which it is not). Thus, I am sure you can appreciate that the resulting number might differ from its actual value by some amount.
In your case, the difference is 0.00000000000000001 which is 1/100000000000000000 = 1/10^17 and I am sure that you can see why you might have that.
this is because you are dealing with floating point, and this is the expected behavior of floating point math.
what you need to do is format that number.
see this java explanation which also applies here if you want to know why this is happening.
in javascript all numbers are represented as 64bit floats, so you will run into this sort of thing often.
the quick overview of that article is that floating point tries to represent a range of values larger then would fit in 64bits, therefor there is going to be some imprecise representation, and this is what you are seeing.
With floating point number you get a representation of the number you try to encode. Mostly it is a number that is very close the the original number. More information on encoding/storing floating point numbers can be found here.
Note:
If you show the value of x, it still shows 0.011 because JavaScript has not yet decided what variable type x has. But after multiplying it with 10 the type got set to floating point (it is the only possibility) and the round error shows.
You can try to fix the nr of decimals with this one:
// fl is a float number with some nr of decimals
// d is how many decimals you want
function dec(fl, d) {
var p = Math.pow(10, d);
return Math.round(fl*p)/p;
}
Ex:
var n = 0.0012345;
console.log(dec(n,6)); // 0.001235
console.log(dec(n,5)); // 0.00123
console.log(dec(n,4)); // 0.0012
console.log(dec(n,3)); // 0.001
It works by first multiplying the float with 10^3 (1000) for three decimals, or 10^2 (100) for two decimals. Then do round on that and divide it back to original size.
Math.pow(10, d) makes 10^d (means that d will give us 1000).
In your case, do alert(dec(y,2));, it should work.
This question already has answers here:
Closed 12 years ago.
Possible Duplicates:
Is JavaScript’s Math broken?
How is floating point stored? When does it matter?
Code:
var tax= 14900*(0.108);
alert(tax);
The above gives an answer of 1609.2
var tax1= 14900*(10.8/100);
alert(tax1);
The above gives an answer of 1609.200000000003
why? i guess i can round up the values, but why is this happening?
UPDATE:
Found a temp solution for the problem.
Multiply first:
(14900*10.8)/100 = 1609.2
However
(14898*10.8)/100 = 1608.9840000000002
For this multiply the 10.8 by a factor(100 in this case) and adjust the denominator:
(14898*(10.8*100))/10000 = 1608.984
I guess if one can do a preg_match for the extra 000s and then adjust the factor accordingly, the float error can be avoided.
The final solution would however be a math library.
Floating point value is inexact.
This is pretty much the answer to the question. There is finite precision, which means that some numbers can not be represented exactly.
Some languages support arbitrary precision numeric types/rational/complex numbers at the language level, etc, but not Javascript. Neither does C nor Java.
The IEEE 754 standard floating point value can not represent e.g. 0.1 exactly. This is why numerical calculations with cents etc must be done very carefully. Sometimes the solution is to store values in cents as integers instead of in dollars as floating point values.
"Floating" point concept, analog in base 10
To see why floating point values are imprecise, consider the following analog:
You only have enough memory to remember 5 digits
You want to be able to represent values in as wide range as practically possible
In representing integers, you can represent values in the range of -99999 to +99999. Values outside of those range would require you to remember more than 5 digits, which (for the sake of this example) you can't do.
Now you may consider a fixed-point representation, something like abc.de. Now you can represent values in the range of -999.99 to +999.99, up to 2 digits of precision, e.g. 3.14, -456.78, etc.
Now consider a floating point version. In your resourcefulness, you came up with the following scheme:
n = abc x 10de
Now you can still remember only 5 digits a, b, c, d, e, but you can now represent much wider range of numbers, even non-integers. For example:
123 x 100 = 123.0
123 x 103 = 123,000.0
123 x 106 = 123,000,000.0
123 x 10-3 = 0.123
123 x 10-6 = 0.000123
This is how the name "floating point" came into being: the decimal point "floats around" in the above examples.
Now you can represent a wide range of numbers, but note that you can't represent 0.1234. Neither can you represent 123,001.0. In fact, there's a lot of values that you can't represent.
This is pretty much why floating point values are inexact. They can represent a wide range of values, but since you are limited to a fixed amount of memory, you must sacrifice precision for magnitude.
More technicalities
The abc is called the significand, aka coefficient/mantissa. The de is the exponent, aka scale/characteristics. As usual, the computer uses base 2 instead 10. In addition to remembering the "digits" (bits, really), it must also remember the signs of the significand and exponent.
A single precision floating point type usually uses 32 bits. A double precision usually uses 64 bits.
See also
What Every Computer Scientist Should Know About Floating-Point Arithmetic
Wikipedia/IEEE 754
That behavior is inherent to floating point arithmic. That is why floating point arithmic is not suitable for dealing with money issues, which need to be exact.
There exist libraries, like this one, which help you limit rounding errors to the point where you actually need them (to represent as text). Those libraries don't really deal with floating point values, but with fractions (of integer values). So no 0.25, but 1/4 and so on.
Floating point values can be used for efficiently representing values in a much wide range than integer values could. However, it comes at a price: some values cannot be represented exactly (because they are stored binary) Every negative power of 10 for example (0.1, 0.01, etc.)
If you want exact results, try not to use floating point arithmetic.
Of course sometimes you can't avoid them. In that case, a few simple guidelines may help you minimize roundoff errors:
Don't subtract nearly equal values. (0.1-0.0999)
Add or multiply the biggest values first. (100*10)* 0.1 instead of 100*(10*0.1)
Multiply first, then divide. (14900*10.8)/100 instead of 14900*(10.8/100)
If exact values are available, use them instead of calculating them to get 'prettier' code
Also,
let JavaScript figure out math precedence, there is no reason to use parentheses:
var tax1 = 14900 * 10.8 / 100
1609.2
It's magic. Just remember to avoid useless parentheses.