Equality Predicates
===================
Here we describe two functions that test for equality between any two
objects. Other functions test equality between objects of specific
types, e.g., strings. For these predicates, see the appropriate chapter
describing the data type.
- Function: eq object1 object2
This function returns `t' if OBJECT1 and OBJECT2 are the same
object, `nil' otherwise. The "same object" means that a change in
one will be reflected by the same change in the other.
`eq' returns `t' if OBJECT1 and OBJECT2 are integers with the same
value. Also, since symbol names are normally unique, if the
arguments are symbols with the same name, they are `eq'. For
other types (e.g., lists, vectors, strings), two arguments with
the same contents or elements are not necessarily `eq' to each
other: they are `eq' only if they are the same object.
(eq 'foo 'foo)
=> t
(eq 456 456)
=> t
(eq "asdf" "asdf")
=> nil
(eq '(1 (2 (3))) '(1 (2 (3))))
=> nil
(setq foo '(1 (2 (3))))
=> (1 (2 (3)))
(eq foo foo)
=> t
(eq foo '(1 (2 (3))))
=> nil
(eq [(1 2) 3] [(1 2) 3])
=> nil
(eq (point-marker) (point-marker))
=> nil
The `make-symbol' function returns an uninterned symbol, distinct
from the symbol that is used if you write the name in a Lisp
expression. Distinct symbols with the same name are not `eq'.
Note:Creating Symbols.
(eq (make-symbol "foo") 'foo)
=> nil
- Function: equal object1 object2
This function returns `t' if OBJECT1 and OBJECT2 have equal
components, `nil' otherwise. Whereas `eq' tests if its arguments
are the same object, `equal' looks inside nonidentical arguments
to see if their elements or contents are the same. So, if two
objects are `eq', they are `equal', but the converse is not always
true.
(equal 'foo 'foo)
=> t
(equal 456 456)
=> t
(equal "asdf" "asdf")
=> t
(eq "asdf" "asdf")
=> nil
(equal '(1 (2 (3))) '(1 (2 (3))))
=> t
(eq '(1 (2 (3))) '(1 (2 (3))))
=> nil
(equal [(1 2) 3] [(1 2) 3])
=> t
(eq [(1 2) 3] [(1 2) 3])
=> nil
(equal (point-marker) (point-marker))
=> t
(eq (point-marker) (point-marker))
=> nil
Comparison of strings is case-sensitive, but does not take account
of text properties--it compares only the characters in the strings.
A unibyte string never equals a multibyte string unless the
contents are entirely ASCII (Note:Text Representations).
(equal "asdf" "ASDF")
=> nil
However, two distinct buffers are never considered `equal', even if
their textual contents are the same.
The test for equality is implemented recursively; for example, given
two cons cells X and Y, `(equal X Y)' returns `t' if and only if both
the expressions below return `t':
(equal (car X) (car Y))
(equal (cdr X) (cdr Y))
Because of this recursive method, circular lists may therefore cause
infinite recursion (leading to an error).