CODEWITHSUNDEEP
phpif-statementbehavior
Žan Kusterle
Žan Kusterle

Reputation: 570

Strange behaviour of conditions in PHP

I'm wondering why is output of the following code:

$a = log(5, 5);
$b = round(log(5, 5));
echo 'a: ';
var_dump($a);
echo '<br>';
echo 'b: ';
var_dump($b);
echo '<br>';
echo ($a == $b) ? 'equal' : 'not equal';

this

a: float(1) 
b: float(1) 
not equal

Thanks to anyone who can explain this to me.

Upvotes: 1

Views: 82

Answers (1)

Stefan Gehrig
Stefan Gehrig

Reputation: 83622

Just to quote the chapter Floating point numbers from the PHP manual:

Warning Floating point precision

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.

Upvotes: 7

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