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PHP - Floating Number Precision [duplicate]
(8 answers)
Closed 9 years ago.
I just hit an odd issue. I have the following tables in MySQL:
table:deposits
user_id amount
1 0.50
table:withdrawls
user_id amount
1 0.01
1 0.01
1 0.01
To get the balance, I run this:
SELECT (IFNULL((SELECT sum(amount) FROM deposits WHERE user_id = 1),0) - IFNULL((SELECT sum(amount) FROM withdrawls WHERE user_id = 1),0) ) as balance
I then return it as
return (float) $row['balance']
For some weird reason, the result is float(0.47000000067055). Does anyone know why there is bizarre rounding?
Floating-point arithmetic does not represent all real numbers exactly. 0.01 is converted to a representable value, which is, in essence, a binary numeral with a limited number of significant bits. Since 1/100 cannot be represented exactly in binary with a finite number of digits (the same way 1/3 in decimal requires an infinite number of digits: .3333…), the conversion to floating-point rounds the value.
That said, I do not see exactly how 0.47000000067055 is produced. Converting 0.01 to IEEE-754 64-bit binary floating-point with correct rounding to nearest produces 0.01000000000000000020816681711721685132943093776702880859375. Subtracting that three times from .5, rounding each time, produces 0.4699999999999999733546474089962430298328399658203125. Subtracting three times the value from .5 produces the same value.
I suspect that your PHP implementation converted 0.01 to floating-point in a less than optimal way.
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This question already has answers here:
When should I use double instead of decimal?
(12 answers)
Closed 9 years ago.
I keep seeing people using doubles in C#. I know I read somewhere that doubles sometimes lose precision.
My question is when should a use a double and when should I use a decimal type?
Which type is suitable for money computations? (ie. greater than $100 million)
For money, always decimal. It's why it was created.
If numbers must add up correctly or balance, use decimal. This includes any financial storage or calculations, scores, or other numbers that people might do by hand.
If the exact value of numbers is not important, use double for speed. This includes graphics, physics or other physical sciences computations where there is already a "number of significant digits".
My question is when should a use a
double and when should I use a decimal
type?
decimal for when you work with values in the range of 10^(+/-28) and where you have expectations about the behaviour based on base 10 representations - basically money.
double for when you need relative accuracy (i.e. losing precision in the trailing digits on large values is not a problem) across wildly different magnitudes - double covers more than 10^(+/-300). Scientific calculations are the best example here.
which type is suitable for money
computations?
decimal, decimal, decimal
Accept no substitutes.
The most important factor is that double, being implemented as a binary fraction, cannot accurately represent many decimal fractions (like 0.1) at all and its overall number of digits is smaller since it is 64-bit wide vs. 128-bit for decimal. Finally, financial applications often have to follow specific rounding modes (sometimes mandated by law). decimal supports these; double does not.
According to Characteristics of the floating-point types:
.NET Type
C# Keyword
Precision
System.Single
float
~6-9 digits
System.Double
double
~15-17 digits
System.Decimal
decimal
28-29 digits
The way I've been stung by using the wrong type (a good few years ago) is with large amounts:
£520,532.52 - 8 digits
£1,323,523.12 - 9 digits
You run out at 1 million for a float.
A 15 digit monetary value:
£1,234,567,890,123.45
9 trillion with a double. But with division and comparisons it's more complicated (I'm definitely no expert in floating point and irrational numbers - see Marc's point). Mixing decimals and doubles causes issues:
A mathematical or comparison operation
that uses a floating-point number
might not yield the same result if a
decimal number is used because the
floating-point number might not
exactly approximate the decimal
number.
When should I use double instead of decimal? has some similar and more in depth answers.
Using double instead of decimal for monetary applications is a micro-optimization - that's the simplest way I look at it.
Decimal is for exact values. Double is for approximate values.
USD: $12,345.67 USD (Decimal)
CAD: $13,617.27 (Decimal)
Exchange Rate: 1.102932 (Double)
For money: decimal. It costs a little more memory, but doesn't have rounding troubles like double sometimes has.
Definitely use integer types for your money computations.
This cannot be emphasized enough since at first glance it might seem that a floating point type is adequate.
Here an example in python code:
>>> amount = float(100.00) # one hundred dollars
>>> print amount
100.0
>>> new_amount = amount + 1
>>> print new_amount
101.0
>>> print new_amount - amount
>>> 1.0
looks pretty normal.
Now try this again with 10^20 Zimbabwe dollars:
>>> amount = float(1e20)
>>> print amount
1e+20
>>> new_amount = amount + 1
>>> print new_amount
1e+20
>>> print new_amount-amount
0.0
As you can see, the dollar disappeared.
If you use the integer type, it works fine:
>>> amount = int(1e20)
>>> print amount
100000000000000000000
>>> new_amount = amount + 1
>>> print new_amount
100000000000000000001
>>> print new_amount - amount
1
I think that the main difference beside bit width is that decimal has exponent base 10 and double has 2
http://software-product-development.blogspot.com/2008/07/net-double-vs-decimal.html
This question already has answers here:
Is floating point math broken?
(31 answers)
Closed 6 years ago.
In php arithmetic operation
<?php
$lower = floor(63.1);
$value = 63.1;
echo $value - $lower;die; ?>
i get the answer as 0.1, but when i do the same for
64.1-64 i get the result as
0.099999999999994
Why is it so?
It's a problem that many languages, not just PHP have, when running on hardware (like most hardware) that supports floating point in binary or base-2 (base-16 as a shorthand) but not in base-10 that we humans use (and often assume is the only way to represent numbers).
0.1 to us humans is a base-10 notation which means 1 x 10**-1. This cannot be represented accurately in base-16. Closest is 9 x 16**-2 + 9 x 16**-3 ...
Base-10 has this problem in reverse. Base-10 represents the fraction 1/3 as 0.333333... But if we were using base-3, it would be a succinct 0.1 = 1 x 3**-1.
In PHP, if you need better base-10 precision, use the BC Math functions.
(Note some hardware can represent numbers in base-10. Mainframes, for example, for 50+ years have "packed decimal" and the corresponding native opcodes to process this. Not surprising, as they were built as business machines for accurately handling base-10 things like money!).
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
How do you explain floating point inaccuracy to fresh programmers and laymen who still think computers are infinitely wise and accurate?
Do you have a favourite example or anecdote which seems to get the idea across much better than an precise, but dry, explanation?
How is this taught in Computer Science classes?
There are basically two major pitfalls people stumble in with floating-point numbers.
The problem of scale. Each FP number has an exponent which determines the overall “scale” of the number so you can represent either really small values or really larges ones, though the number of digits you can devote for that is limited. Adding two numbers of different scale will sometimes result in the smaller one being “eaten” since there is no way to fit it into the larger scale.
PS> $a = 1; $b = 0.0000000000000000000000001
PS> Write-Host a=$a b=$b
a=1 b=1E-25
PS> $a + $b
1
As an analogy for this case you could picture a large swimming pool and a teaspoon of water. Both are of very different sizes, but individually you can easily grasp how much they roughly are. Pouring the teaspoon into the swimming pool, however, will leave you still with roughly a swimming pool full of water.
(If the people learning this have trouble with exponential notation, one can also use the values 1 and 100000000000000000000 or so.)
Then there is the problem of binary vs. decimal representation. A number like 0.1 can't be represented exactly with a limited amount of binary digits. Some languages mask this, though:
PS> "{0:N50}" -f 0.1
0.10000000000000000000000000000000000000000000000000
But you can “amplify” the representation error by repeatedly adding the numbers together:
PS> $sum = 0; for ($i = 0; $i -lt 100; $i++) { $sum += 0.1 }; $sum
9,99999999999998
I can't think of a nice analogy to properly explain this, though. It's basically the same problem why you can represent 1/3 only approximately in decimal because to get the exact value you need to repeat the 3 indefinitely at the end of the decimal fraction.
Similarly, binary fractions are good for representing halves, quarters, eighths, etc. but things like a tenth will yield an infinitely repeating stream of binary digits.
Then there is another problem, though most people don't stumble into that, unless they're doing huge amounts of numerical stuff. But then, those already know about the problem. Since many floating-point numbers are merely approximations of the exact value this means that for a given approximation f of a real number r there can be infinitely many more real numbers r1, r2, ... which map to exactly the same approximation. Those numbers lie in a certain interval. Let's say that rmin is the minimum possible value of r that results in f and rmax the maximum possible value of r for which this holds, then you got an interval [rmin, rmax] where any number in that interval can be your actual number r.
Now, if you perform calculations on that number—adding, subtracting, multiplying, etc.—you lose precision. Every number is just an approximation, therefore you're actually performing calculations with intervals. The result is an interval too and the approximation error only ever gets larger, thereby widening the interval. You may get back a single number from that calculation. But that's merely one number from the interval of possible results, taking into account precision of your original operands and the precision loss due to the calculation.
That sort of thing is called Interval arithmetic and at least for me it was part of our math course at the university.
Show them that the base-10 system suffers from exactly the same problem.
Try to represent 1/3 as a decimal representation in base 10. You won't be able to do it exactly.
So if you write "0.3333", you will have a reasonably exact representation for many use cases.
But if you move that back to a fraction, you will get "3333/10000", which is not the same as "1/3".
Other fractions, such as 1/2 can easily be represented by a finite decimal representation in base-10: "0.5"
Now base-2 and base-10 suffer from essentially the same problem: both have some numbers that they can't represent exactly.
While base-10 has no problem representing 1/10 as "0.1" in base-2 you'd need an infinite representation starting with "0.000110011..".
How's this for an explantation to the layman. One way computers represent numbers is by counting discrete units. These are digital computers. For whole numbers, those without a fractional part, modern digital computers count powers of two: 1, 2, 4, 8. ,,, Place value, binary digits, blah , blah, blah. For fractions, digital computers count inverse powers of two: 1/2, 1/4, 1/8, ... The problem is that many numbers can't be represented by a sum of a finite number of those inverse powers. Using more place values (more bits) will increase the precision of the representation of those 'problem' numbers, but never get it exactly because it only has a limited number of bits. Some numbers can't be represented with an infinite number of bits.
Snooze...
OK, you want to measure the volume of water in a container, and you only have 3 measuring cups: full cup, half cup, and quarter cup. After counting the last full cup, let's say there is one third of a cup remaining. Yet you can't measure that because it doesn't exactly fill any combination of available cups. It doesn't fill the half cup, and the overflow from the quarter cup is too small to fill anything. So you have an error - the difference between 1/3 and 1/4. This error is compounded when you combine it with errors from other measurements.
In python:
>>> 1.0 / 10
0.10000000000000001
Explain how some fractions cannot be represented precisely in binary. Just like some fractions (like 1/3) cannot be represented precisely in base 10.
Another example, in C
printf (" %.20f \n", 3.6);
incredibly gives
3.60000000000000008882
Here is my simple understanding.
Problem:
The value 0.45 cannot be accurately be represented by a float and is rounded up to 0.450000018. Why is that?
Answer:
An int value of 45 is represented by the binary value 101101.
In order to make the value 0.45 it would be accurate if it you could take 45 x 10^-2 (= 45 / 10^2.)
But that’s impossible because you must use the base 2 instead of 10.
So the closest to 10^2 = 100 would be 128 = 2^7. The total number of bits you need is 9 : 6 for the value 45 (101101) + 3 bits for the value 7 (111).
Then the value 45 x 2^-7 = 0.3515625. Now you have a serious inaccuracy problem. 0.3515625 is not nearly close to 0.45.
How do we improve this inaccuracy? Well we could change the value 45 and 7 to something else.
How about 460 x 2^-10 = 0.44921875. You are now using 9 bits for 460 and 4 bits for 10. Then it’s a bit closer but still not that close. However if your initial desired value was 0.44921875 then you would get an exact match with no approximation.
So the formula for your value would be X = A x 2^B. Where A and B are integer values positive or negative.
Obviously the higher the numbers can be the higher would your accuracy become however as you know the number of bits to represent the values A and B are limited. For float you have a total number of 32. Double has 64 and Decimal has 128.
A cute piece of numerical weirdness may be observed if one converts 9999999.4999999999 to a float and back to a double. The result is reported as 10000000, even though that value is obviously closer to 9999999, and even though 9999999.499999999 correctly rounds to 9999999.
Can php handle extremely small numbers without rounding them? For example, when calculating exp(-99) + 1/2, php compute this to be 0.5. This is problematic if later I want to multiply the given result, instead of an extremely small number, it just gives half the number.
echo exp(-99) + 1/2 // Outputs 0.5
You're out of the range supported by floating point numbers.
On my platform (PHP floats are 64 bit) echo exp(-99); returns currectly 1.0112214926104E-43
That's because the exponential part is wide enought to represent e^-99
But as I add... echo exp(-99)+0.5; I get 0.5
The result you expect would be something with more than 50 decimal digits.
Double floats doesn't have a such large mantissa (usually the limit is around 18-20 decimals).
To answer your question, If you really need to do such math (handle extremely small numbers without rounding them) you could use PHP's arbitrary precision math extension:
http://php.net/manual/en/book.bc.php
Apologies for my poor maths skills, I've tried to understand this to answer my own query but I'm not convinced.
We all know that PHP doesn't store Floats in base 10 but base 2.
I have a series of calculations that are using 0.5 as the only float, and in trying to understand if they will be stored as 0.500001 or 0.4999999 (for rounding purposes there is a big difference!!!) I have come to understand that 0.5 will be stored precisely in base2.
My queries are
A Have I understood this correctly?
B What other floats are stored precisely in base2? eg 0.25?
Any multiple of 1/pow(x, 2) can be precisely represented as a float.
That means x/2, x/4, x/8, x/16 ...ect. can be accurately represented.
For more information on how floating point numbers are store see http://kipirvine.com/asm/workbook/floating_tut.htm
Gmp is a good library for high precision math.
PHP is not required to use binary floating-point. It depends on the system.
Many systems use IEEE-754 binary floating-point (sometimes incompletely or with modifications, such as flushing subnormal numbers to zero).
In IEEE-754 64-bit binary floating point, a number is exactly representable if and only if it is representable as an integer F times a power of two, 2E, such that:
The magnitude of F is less than 253.
–1074 ≤ E < 972.
For example, ½ equals 1•2–1. 1 is an integer under the integer limit, and –1 is an exponent within the exponent limits. So ½ is representable.
253+1 is not representable. As it is, it is an integer outside the integer limit. If you try to scale it by a power of two to bring it within the limit, you get a number that is not an integer. So there is no way to represent this value exactly in IEEE-754 64-bit binary floating-point.
1/3 and 1/10 are also not representable because no matter what power of two you scale them by, you will not produce an integer.