nan/"not a number" is not meant to see if the data type is numeric/textual/etc..
NaN is actually a set of values which can be stored in floating-point variables, but dont actually evaluate to a proper floating point number.
The floating point system has three sections: 1 bit for the sign (+/-), an 8 bit exponent, and a 23 bit fractional part.
There are rules governing which combinations of values can be placed into each section, and some values are reserved for numbers such as infinity. This leads to certain combinations being invalid, or in other words, not a number.
is_nan
(PHP 4 >= 4.2.0, PHP 5, PHP 7, PHP 8)
is_nan — Checks whether a float is NAN
Description
Returns whether the given num is NAN (Not A Number
).
NAN is returned from mathematical operations that are undefined,
for example when passing parameters outside of function’s input domain. The square root
(sqrt()) is only defined for positive numbers, passing a negative number
will result in NAN. Other examples of operations returning NAN
are dividing INF by INF and any operation involving an
existing NAN value.
Note:
Despite its name of
Not A Number,NANis a valid value of type float.
Caution
NAN does not compare equal to NAN. To check whether
a float is NAN, is_nan() must be used. Checking
$float === NAN will not work.
Return Values
true if num is NAN, else false.
Examples
Example #1 is_nan() example
<?php
$nan = sqrt(-1);
var_dump($nan, is_nan($nan));
?>The above example will output:
float(NAN) bool(true)
See Also
- is_finite() - Checks whether a float is finite
- is_infinite() - Checks whether a float is infinite
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User Contributed Notes 7 notes
darkangel at moveinmod dot net ¶
18 years ago
10basetom ¶
9 years ago
I would use is_numeric() instead of ctype_digit() if you cannot be 100% sure what data type the string will be. Example from the docs:
<?php
$numeric_string = '42';
$integer = 42;
ctype_digit($numeric_string); // true
ctype_digit($integer); // false (ASCII 42 is the * character)
is_numeric($numeric_string); // true
is_numeric($integer); // true
?>
php at darkain dot com ¶
8 years ago
Starting with PHP 7, the string 'NaN' evaluates to the NaN value as well.
Example:
var_dump( (float) 'NaN' );
PHP 5.x and HHVM:
float(0)
PHP 7.0:
float(NAN)
modern dot apocalypse at gmail dot com ¶
12 years ago
I have decided to do some testing because I am getting unusual results with the is_nan function and here are the results of my tests:
<?php
var_dump(NAN); // float NAN
var_dump(NAN == NAN); // boolean true
var_dump(NAN === NAN); // boolean true
var_dump(is_nan(NAN)); // boolean true
var_dump(NAN == 12); // boolean true
var_dump(NAN === 12); // boolean false
var_dump(is_nan(12)); // boolean false
var_dump(NAN == 12.4); // boolean true
var_dump(NAN === 12.4); // boolean true
var_dump(is_nan(12.4)); // boolean false
var_dump(NAN == NULL); // boolean true
var_dump(NAN === NULL); // boolean false
var_dump(is_nan(NULL)); // boolean false
var_dump(NAN == 'K<WNPO'); // boolean true
var_dump(NAN === 'K<WNPO'); // boolean false
var_dump(is_nan('K<WNPO')); // null and throws a warning "Warning: is_nan() expects parameter 1 to be double, string given in NANTest.php on line 13"
php at keith tyler dot com ¶
14 years ago
It seems odd to me, but in boolean context, NAN evalutes to true.
<?php
var_dump(acos(8));
var_dump((bool)acos(8));
?>
Returns:
float(NAN)
bool(true)
Incidentally INF and -INF also evaluate to true.
