Why is this condition false? [duplicate] - php

Question

This question already has answers here:
Is floating point math broken?
(31 answers)
Closed 6 years ago.
Some strange things are going on in my code. Could someone explain please why is this condition false?
When I output object's members with php's function var_dump(), I get this:
string(6) "105.63"
float(105.63)
from one object, and this is output from another object:
string(6) "667.69"
float(667.69)
Then I do comparison like this:
if(105.63 == "105.63"){
echo "true";
} else {
echo "false";
}
if(667.69 == "667.69"){
echo "true";
} else {
echo "false";
}
it outputs 2 times true for sure, if we write code like in my example above. But in my class, it behaves differently. In first if I get false and in second if I get true. Then I looked to the data more deeply with var_export() function and it seems like I actually have following data:
'105.63'
105.6300000000000096633812063373625278472900390625
and
'667.69'
667.69000000000005456968210637569427490234375
So I decide to check wether my conditions work with that data... And they dont. Actually, only first one doesn't. Why do I get output false and true in following code?
if(105.6300000000000096633812063373625278472900390625 == "105.63"){
echo "true";
} else {
echo "false";
}
if(667.69000000000005456968210637569427490234375 == "667.69"){
echo "true";
} else {
echo "false";
}
I know how to fix this. But I'm interested why conditions fails in first if.
EDIT
Php does type juggling when we use == comparison operator!
Take a look at this:
var_export((float) "105.63");
var_export((string) 105.6300000000000096633812063373625278472900390625);
var_export((float) "667.69");
var_export((string) 667.69000000000005456968210637569427490234375);
outputs:
105.63
'105.63'
667.69000000000005
'667.69'
Should not this mean that the first condition be true and the second false? But I get false in first and true in second. Sorry if I was unclear.

Answer 1

Check the manual: http://php.net/manual/en/language.types.float.php
There is a big warning section about comparing floating point numbers:
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. For a "simple" explanation, see the » floating point guide
that's also titled "Why don’t my numbers add up?"
Why dont my numbers add up?