CODEWITHSUNDEEP
php
Oto Shavadze
Oto Shavadze

Reputation: 42753

Not expected result from mathematical operation

Why this code returns false, can you explain me why this happened?

var_dump( 85.4 + 42.7 == 128.1  );  // bool(false)

Upvotes: 0

Views: 55

Answers (2)

piokuc
piokuc

Reputation: 26164

This is because of floating point numbers imperfect precision, read about it more here: http://php.net/manual/en/language.types.float.php

Upvotes: 1

Alma Do
Alma Do

Reputation: 37365

Yet again, floating point precision. You can't rely on precise comparison when dealing with floats. Instead, you should use precision delta:

$a = 85.4+42.7;
$b = 128.1;
$delta = 1E-13;

if(abs($a-$b)<$delta)
{
   //they are equal
}

-here $delta is precision delta. For PHP, 1E-13 would be quite enough. The manual says:

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.

For a "simple" explanation, see the » floating point guide that's also titled "Why don’t my numbers add up?"

Upvotes: 5

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