What is <=> (the 'Spaceship' Operator) in PHP 7?

Question

PHP 7 introduced the Spaceship (<=>) operator. What is it and how does it work?

Already

wiki.php.net/rfc/combined-comparison-operator – Sleepy

Although feature release notes of PHP 7.0 say "eturns -1, 0 or 1 when $a is respectively less than, equal to, or greater than $b", the long term documentation of comparison operators is not so specific about that: "An int less than, equal to, or greater than zero when $a is less than, equal to, or greater than $b, respectively.", luckily that's how the sort functions expect it to be. – Dodecahedron

Answer 1

The <=> ("Spaceship") operator will offer combined comparison in that it will :

Return 0 if values on either side are equal
Return 1 if the value on the left is greater
Return -1 if the value on the right is greater

The rules used by the combined comparison operator are the same as the currently used comparison operators by PHP viz. <, <=, ==, >= and >. Those who are from Perl or Ruby programming background may already be familiar with this new operator proposed for PHP7.

   //Comparing Integers

    echo 1 <=> 1; //output  0
    echo 3 <=> 4; //output -1
    echo 4 <=> 3; //output  1
    
    //String Comparison
    
    echo "x" <=> "x"; //output  0
    echo "x" <=> "y"; //output -1
    echo "y" <=> "x"; //output  1

When you compare a string this way, it will go from left to right, and compare each character in the given string to see if they are different, until it finds a difference, then it will decide which string is bigger by comparing the ASCII value of this last character.

Kegan

But how the strings are compared? Is it looking into characters, count of characters in strings and so on? – Freshwater

@AltafHussain see this answer: (https://mcmap.net/q/36372/-why-does-a-less-than-or-more-than-comparison-in-php-of-two-strings-in-the-date-format-of-quot-yyyy-mm-dd-quot-work-even-though-they-are-strings) – Jehiah

It can be mostly used in sorting and combined comparison. – Verecund

Memo rule to remember it: -1 < 0 > 1 – Carce

Answer 2

According to the RFC that introduced the operator, $a <=> $b evaluates to:

• 0 if $a == $b
• -1 if $a < $b
• 1 if $a > $b

which seems to be the case in practice in every scenario I've tried, although strictly the official docs only offer the slightly weaker guarantee that $a <=> $b will return

an integer less than, equal to, or greater than zero when $a is respectively less than, equal to, or greater than $b

Regardless, why would you want such an operator? Again, the RFC addresses this - it's pretty much entirely to make it more convenient to write comparison functions for usort (and the similar uasort and uksort).

usort takes an array to sort as its first argument, and a user-defined comparison function as its second argument. It uses that comparison function to determine which of a pair of elements from the array is greater. The comparison function needs to return:

an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.

The spaceship operator makes this succinct and convenient:

$things = [
    [
        'foo' => 5.5,
        'bar' => 'abc'
    ],
    [
        'foo' => 7.7,
        'bar' => 'xyz'
    ],
    [
        'foo' => 2.2,
        'bar' => 'efg'
    ]
];

// Sort $things by 'foo' property, ascending
usort($things, function ($a, $b) {
    return $a['foo'] <=> $b['foo'];
});

// Sort $things by 'bar' property, descending
usort($things, function ($a, $b) {
    return $b['bar'] <=> $a['bar'];
});

More examples of comparison functions written using the spaceship operator can be found in the Usefulness section of the RFC.

Hilliard

To me, question "what is" is not just "what does it do", but also "what is it for, where can it be seen", therefore I consider this the right and complete answer :) – Dodecahedron

Answer 3

Its a new operator for combined comparison. Similar to strcmp() or version_compare() in behavior, but it can be used on all generic PHP values with the same semantics as <, <=, ==, >=, >. It returns 0 if both operands are equal, 1 if the left is greater, and -1 if the right is greater. It uses exactly the same comparison rules as used by our existing comparison operators: <, <=, ==, >= and >.

click here to know more

Nahama

Spaceship Operator is used to compare two expressions. It returns -1, 0 or 1 when first expression is respectively less than, equal to, or greater than second expression. nexladder.com/php7-tutorial/php7-spaceship-operator – Mayday