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 a float and then casted as an integer?

Thanks!

Answer 1

There's a loss in precision when decimals are converted internally to their binary equivalent. The computed value will be something like 7.9+ instead of the expected 8.

If you need a high degree of accuracy, use the GMP family of functions or the bcmath library.

Answer 2

See the manual:

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

It is typical that simple decimal fractions like 0.1 or 0.7 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 7.9.

Answer 3

The other answers explained WHY this happens. This should get you what you want:

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

Just round the float before casting it to an int.

Answer 4

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.

Answer 5

1/10 cannot be represented in a finite number of binary digits, just like 1/3 cannot be represented as a finite number of base-10 digits. Therefore you are actually adding together 0.09999999999999... and 0.69999999999999... -- the sum is almost 8, but not quite.