Question

Possible Duplicate:
Is double Multiplication Broken in .NET?
PHP unexpected result of float to int type cast

echo (int) ( (0.1+0.7) * 10); 

Gives me answer 7 but not 8.

echo (int) ( (0.1+0.8) * 10); 

Gives answer 9 which is right

whats wrong? can anybody explain

thanx,
-Navi

Answer 1

It’s normal – There’s a thing called precision while working with floating point numbers. It’s present in most of the modern languages. See here: http://www.mredkj.com/javascript/nfbasic2.html for more information.

((0.1+0.7) * 10) is probably something like 7.9999999 and (int) 7.9999999 = 7

On the other hand ((0.1+0.8) * 10) is probably 9.00000001 and (int)9.00000001 = 9

Floating point numbers have limited precision. Although it depends on the system, PHP typically uses the IEEE 754 double precision format, which will give a maximum relative error due to rounding in the order of 1.11e-16. Non elementary arithmetic operations may give larger errors, and, of course, error propagation must be considered when several operations are compounded.

Additionally, rational numbers that are exactly representable as
floating point numbers in base 10, like 0.1 or 0.7, do not have an
exact representation as floating point numbers in base 2, which is
used internally, no matter the size of the mantissa. Hence, they
cannot be converted into their internal binary counterparts without a
small loss of precision. This can lead to confusing results: for
example, floor((0.1+0.7)*10) will usually return 7 instead of the
expected 8, since the internal representation will be something like
7.9999999999999991118….

So never trust floating number results to the last digit, and do not
compare floating point numbers directly for equality. If higher
precision is necessary, the arbitrary precision math functions and gmp
functions are available.

Source: http://php.net/manual/en/language.types.float.php