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!
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i think this happens in any language, and its because the way floating point numbers are handled internallyQuamis– Quamis2010-11-21 07:37:00 +00:00Commented Nov 21, 2010 at 7:37
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5 Answers
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.
Comments
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.
1 Comment
Quamis
also, its recommented you dont actually use floats for currency calculations, because of this loss of precision
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:
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.
Comments
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.
