PHP integer rounding problems

Question

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

Why does the above output 7? I understand how PHP rounds towards 0, but isn\'t (0.1+0.7) * 10 evaluated as

Answer 1

I don't have php installed, but in python:

$ python
>>> 0.1+0.7
0.79999999999999993
>>> 

Not all numbers in base 10 can be represented precisely in base 2 system. Check Wikipedia article:

• http://en.wikipedia.org/wiki/Binary_numeral_system

section Fractions in Binary. In particular, this line:

Fraction    Decimal     Binary  Fractional Approx.
1/10    0.1     0.000110011...  1/16+1/32+1/256...

1/10 cannot be represented in a finite way in base 2. Thus, 0.1 + 0.7 cannot be precisely calculated in base 2.

Never assume floating-point calculations are precise, it will bite you sooner or later.