Test if number is odd or even

Question

What is the simplest most basic way to find out if a number/variable is odd or even in PHP? Is it something to do with mod?

I've tried a few scripts but.. google isn't delivering at the moment.

Answer 1

You were right in thinking mod was a good place to start. Here is an expression which will return true if $number is even, false if odd:

$number % 2 == 0

Works for every integer^PHP value, see as well Arithmetic Operators^PHP.

Example:

$number = 20;
if ($number % 2 == 0) {
  print "It's even";
}

Output:

It's even

Answer 2

Another option is a simple bit checking.

n & 1

for example:

if ( $num & 1 ) {
  //odd
} else {
  //even
}

Answer 3

Yes using the mod

$even = ($num % 2 == 0);
$odd = ($num % 2 != 0);

Answer 4

While all of the answers are good and correct, simple solution in one line is:

$check = 9;

either:

echo ($check & 1 ? 'Odd' : 'Even');

or:

echo ($check % 2 ? 'Odd' : 'Even');

works very well.

Answer 5

(bool)($number & 1)

or

(bool)(~ $number & 1)

Answer 6

I did a bit of testing, and found that between mod, is_int and the &-operator, mod is the fastest, followed closely by the &-operator. is_int is nearly 4 times slower than mod.

I used the following code for testing purposes:

$number = 13;

$before = microtime(true);
for ($i=0; $i<100000; $i++) {
    $test = ($number%2?true:false);
}
$after = microtime(true);

echo $after-$before." seconds mod<br>";

$before = microtime(true);
for ($i=0; $i<100000; $i++) {
    $test = (!is_int($number/2)?true:false);
}
$after = microtime(true);

echo $after-$before." seconds is_int<br>";

$before = microtime(true);
for ($i=0; $i<100000; $i++) {
    $test = ($number&1?true:false);
}
$after = microtime(true);

echo $after-$before." seconds & operator<br>";

The results I got were pretty consistent. Here's a sample:

0.041879177093506 seconds mod
0.15969395637512 seconds is_int
0.044223070144653 seconds & operator

Answer 7

Another option is to check if the last digit is an even number :

$value = "1024";// A Number
$even = array(0, 2, 4, 6, 8);
if(in_array(substr($value, -1),$even)){
  // Even Number
}else{
  // Odd Number
}

Or to make it faster, use isset() instead of array_search :

$value = "1024";// A Number
$even = array(0 => 1, 2 => 1, 4 => 1, 6 => 1, 8 => 1);
if(isset($even[substr($value, -1)]){
  // Even Number
}else{
  // Odd Number
}

Or to make it more faster (beats mod operator at times) :

$even = array(0, 2, 4, 6, 8);
if(in_array(substr($number, -1),$even)){
  // Even Number
}else{
  // Odd Number
}

Here is the time test as a proof to my findings.

Answer 8

PHP is converting null and an empty string automatically to a zero. That happens with modulo as well. Therefor will the code

$number % 2 == 0 or !($number & 1)

with value $number = '' or $number = null result in true. I test it therefor somewhat more extended:

function testEven($pArg){
    if(is_int($pArg) === true){
        $p = ($pArg % 2);
        if($p === 0){
            print "The input '".$pArg."' is even.<br>";
        }else{
            print "The input '".$pArg."' is odd.<br>";
        }
    }else{
        print "The input '".$pArg."' is not a number.<br>";
    }
}

The print is there for testing purposes, hence in practice it becomes:
function testEven($pArg){
    if(is_int($pArg)=== true){
        return $pArg%2;
    }
    return false;
}

This function returns 1 for any odd number, 0 for any even number and false when it is not a number. I always write === true or === false to let myself (and other programmers) know that the test is as intended.

Answer 9

All even numbers divided by 2 will result in an integer

$number = 4;
if(is_int($number/2))
{
   echo("Integer");
}
else
{
   echo("Not Integer");
}

Answer 10

I am making an assumption that there is a counter already in place. in $i which is incremented at the end of a loop, This works for me using a shorthand query.

$row_pos = ($i & 1) ? 'odd' : 'even';

So what does this do, well it queries the statement we are making in essence $i is odd, depending whether its true or false will decide what gets returned. The returned value populates our variable $row_pos

My use of this is to place it inside the foreach loop, right before i need it, This makes it a very efficient one liner to give me the appropriate class names, this is because i already have a counter for the id's to make use of later in the program. This is a brief example of how i will use this part.

<div class='row-{$row_pos}'> random data <div>

This gives me odd and even classes on each row so i can use the correct class and stripe my printed results down the page.

The full example of what i use note the id has the counter applied to it and the class has my odd/even result applied to it.:

$i=0;
foreach ($a as $k => $v) {

    $row_pos = ($i & 1) ? 'odd' : 'even';
    echo "<div id='A{$i}' class='row-{$row_pos}'>{$v['f_name']} {$v['l_name']} - {$v['amount']} - {$v['date']}</div>\n";

$i++;
}

in summary, this gives me a very simple way to create a pretty table.

Answer 11

Try this,

$number = 10;
 switch ($number%2)
 {
 case 0:
 echo "It's even";
 break;
 default:
 echo "It's odd";
 }

Answer 12

$before = microtime(true);

$n = 1000;  
$numbers = range(1,$n);

$cube_numbers = array_map('cube',$numbers);

function cube($n){      
    $msg ='even';       
    if($n%2 !=0){
        $msg = 'odd';
    }               
    return "The Number is $n is ".$msg;
}

foreach($cube_numbers as $cube){
    echo $cube . "<br/>";
}

$after = microtime(true);

echo $after-$before. 'seconds';

Answer 13

$number %2 = 1 if odd... so don't have to use not even...

$number = 27;

if ($number % 2 == 1) {
  print "It's odd";
}

Answer 14

<?php
// Recursive function to check whether
// the number is Even or Odd 
function check($number){
    if($number == 0)
        return 1;
    else if($number == 1)
        return 0;
    else if($number<0)
        return check(-$number);
    else
        return check($number-2);        
}
  
// Check the number odd or even
$number = 35;
if(check($number))
    echo "Even";
else
    echo "Odd";
?>

So, the output will be Odd

Answer 15

//checking even and odd
$num =14;

$even = ($num % 2 == 0);
$odd = ($num % 2 != 0);

if($even){
    echo "Number is even.";
} else {
    echo "Number is odd.";
}

Answer 16

Try this one with #Input field

<?php
    //checking even and odd
    echo '<form action="" method="post">';
    echo "<input type='text' name='num'>\n";
    echo "<button type='submit' name='submit'>Check</button>\n";
    echo "</form>";

    $num = 0;
    if ($_SERVER["REQUEST_METHOD"] == "POST") {
      if (empty($_POST["num"])) {
        $numErr = "<span style ='color: red;'>Number is required.</span>";
        echo $numErr;
        die();
      } else {
          $num = $_POST["num"];
      }


    $even = ($num % 2 == 0);
    $odd = ($num % 2 != 0);
    if ($num > 0){
        if($even){
            echo "Number is even.";
        } else {
            echo "Number is odd.";
        }
    } else {
        echo "Not a number.";
    }
    }
?>

Answer 17

Two simple bitwise functions, returning a 0 for False and 1 for True.

# is_odd: 1 for odd , 0 for even
odd = number & 1

# is_even: 1 for even , 0 for odd
even = number & 1 ^ 1