int((0.1+0.7)*10) = 7 in several languages. How to prevent this?

Question

Recently I came across a bug/feature in several languages. I have a very basic knowledge about how it's caused (and I'd like some detailed explanation), but when I think of all the bugs I must have made over the years, the question is how can I determine "Hey, this might cause a riddiculous bug, I'd better use arbitrary precision functions", what other languages do have this bug (and those who don't, why). Also, why 0.1+0.7 does this and i.e. 0.1+0.3 doesn't, are there any other well-known examples?

PHP

//the first one actually doesn't make any sense to me,
//why 7 after typecast if it's represented internally as 8?
debug_zval_dump((0.1+0.7)*10); //double(8) refcount(1)
debug_zval_dump((int)((0.1+0.7)*10)); //long(7) refcount(1)
debug_zval_dump((float)((0.1+0.7)*10)); //double(8) refcount(1)

Python:

>>> ((0.1+0.7)*10)
7.9999999999999991
>>> int((0.1+0.7)*10)
7

Javascript:

alert((0.1+0.7)*10); //7.999999999999999
alert(parseInt((0.7+0.1)*10)); //7

Ruby:

>> ((0.1+0.7)*10).to_i                                                  
=> 7                                                                    
>>((0.1+0.7)*10)                                                       
=> 7.999999999999999                                                    

Answer 1

What Every Computer Scientist Should Know About Floating-Point Arithmetic

Answer 2

Stop using floats. No, really.

Answer 3

It's not a language issue. It's general issue with float point arithmetic.

Answer 4

The floating point representation of numbers is not exact.

In Python, int truncates floats towards zero to the nearest integer. (int) in PHP, parseInt in Javascript, and to_i in Ruby do the same thing.

This is not a bug; it's just how these functions work.

For example, from the docs for Python's int:

Conversion of floating point numbers to integers truncates (towards zero).

Answer 5

PHP uses floating point numbers by default, you need to manually cast to integers.

You should be aware of floating point arithmetic. The other posts here provide enough links about that.

Personally I use round/ ceil/ float depending on what I expect as opposed to int

$a = (int) round((0.7 + 0.1) * 10);

Answer 6

This is a known problem that has to do with floating point representation, from which you can find more information here:

http://en.wikipedia.org/wiki/IEEE_754-2008

The specific problem is that 7.9 will be directly converted (trunc) to 7 while transforming it to an int. In Python you can solve this with:

int( round(((0.1+0.7)*10)) )

... and similarly in other languages.

But yes, this can be a problem in many situations. Floating point numbers are not reliable enough for payroll programs, for example.

Maybe others can give you other hints. Hpe this helps, anywway.

Answer 7

Use the decimal module:

>>> int((decimal.Decimal('0.1')+decimal.Decimal('0.7'))*10)
8