Expressions And Operators: Equality

Note: The semantics of operators == and != were inherited from PHP, and are sometimes not what is expected. As such, use === and !== instead. See the discussion below.

The binary equality operators are, as follows:

• ==, which represents value-equality
• !=, which represents value-inequality
• ===, which represents same-type-and-value-equality
• !==, which represents not-same-type-and-value-equality.
• <=>, which indicates less-than, equal-to, or greater-than (see later below)

However, when comparing two objects, operator === represents identity and operator !== represents non-identity. Specifically, in this context, these operators check to see if the two operands are the exact same object, not two different objects of the same type and value.

The type of the result of ==, !=, ===, and !==, is bool.

null == 0   // result has value true
null === 0  // result has value false
true != 100  // result has value false
true !== 100  // result has value true
// -----------------------------------------
"10" != 10  // result has value false
"10" !== 10 // result has value true
// -----------------------------------------
vec[10,20] == vec[10,20.0]  // result has value true
vec[10,20] === vec[10,20.0] // result has value false
dict["red"=>0,"green"=>0] === dict["red"=>0,"green"=>0] // result has value true
dict["red"=>0,"green"=>0] === dict["green"=>0,"red"=>0] // result has value false

When using == and != to compare strings that start with numeric digits, such strings are converted to int or float and then compared numerically. As such,

'0e789' == '0e123'   // True
'0e789' != '0e123'   // False

because both strings actually have the same numeric value, zero! (Zero to the power 789 is the same as zero to the power 123.) However,

'0e789' === '0e123'   // False
'0e789' !== '0e123'   // True

because there is no numeric conversion; the strings are compared character by character.

Often referred to as the spaceship operator, the binary operator <=> compares the values of its operands and returns an int result. If the left-hand value is less than the right-hand value, the result is some unspecified negative value; else, if the left-hand value is greater than the right-hand value, the result is some unspecified positive value; otherwise, the values are equal and the result is zero. For example:

1 <=> 1         // 0; equal
1 <=> 2         // negative; 1 < 2
2 <=> 1         // positive; 2 > 1

"a" <=> "a"     // 0; same length and content
"a" <=> "b"     // negative; a is lower than b in the collating sequence
"b" <=> "a"     // positive; b is higher than a in the collating sequence
"a" <=> "A"     // positive; lowercase a is higher than uppercase A

"a" <=> "aa"    // negative; same leading part, but a is shorter than aa
"aa" <=> "a"    // positive; same leading part, but aa is longer than a
"aa" <=> "aa"   // 0; same length and content