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17.2.1 Satisfying a Two-Argument Test

When an object O is being considered iteratively against each element Ei

of a sequence S by an operator F listed in the next figure, it is sometimes useful to control the way in which the presence of O is tested in S is tested by F. This control is offered on the basis of a function designated with either a :test or :test-not argument.

adjoin nset-exclusive-or search
assoc nsublis set-difference
count nsubst set-exclusive-or
delete nsubstitute sublis
find nunion subsetp
intersection position subst
member pushnew substitute
mismatch rassoc tree-equal
nintersection remove union
nset-difference remove-duplicates

Figure 17.2: Operators that have Two-Argument Tests to be Satisfied

The object O might not be compared directly to Ei. If a :key argument is provided, it is a designator for a function of one argument to be called with each Ei as an argument, and yielding an object Zi to be used for comparison. (If there is no :key argument, Zi is Ei.)

The function designated by the :key argument is never called on O itself. However, if the function operates on multiple sequences (e.g., as happens in set-difference), O will be the result of calling the :key function on an element of the other sequence.

A :test argument, if supplied to F, is a designator for a function of two arguments, O and Zi. An Ei is said (or, sometimes, an O and an Ei are said) to satisfy the test if this :test function returns a generalized boolean representing true.

A :test-not argument, if supplied to F, is designator for a function of two arguments, O and Zi. An Ei is said (or, sometimes, an O and an Ei are said) to satisfy the test if this :test-not function returns a generalized boolean representing false.

If neither a :test nor a :test-not argument is supplied, it is as if a :test argument of #'eql was supplied.

The consequences are unspecified if both a :test and a :test-not argument are supplied in the same call to F.

17.2.1.1 Examples of Satisfying a Two-Argument Test
 (remove "FOO" '(foo bar "FOO" "BAR" "foo" "bar") :test #'equal)
→ (foo bar "BAR" "foo" "bar")
 (remove "FOO" '(foo bar "FOO" "BAR" "foo" "bar") :test #'equalp)
→ (foo bar "BAR" "bar")
 (remove "FOO" '(foo bar "FOO" "BAR" "foo" "bar") :test #'string-equal)
→ (bar "BAR" "bar")
 (remove "FOO" '(foo bar "FOO" "BAR" "foo" "bar") :test #'string=)
→ (BAR "BAR" "foo" "bar")

 (remove 1 '(1 1.0 #C(1.0 0.0) 2 2.0 #C(2.0 0.0)) :test-not #'eql)
→ (1)
 (remove 1 '(1 1.0 #C(1.0 0.0) 2 2.0 #C(2.0 0.0)) :test-not #'=)
→ (1 1.0 #C(1.0 0.0))
 (remove 1 '(1 1.0 #C(1.0 0.0) 2 2.0 #C(2.0 0.0)) :test (complement #'=))
→ (1 1.0 #C(1.0 0.0))

 (count 1 '((one 1) (uno 1) (two 2) (dos 2)) :key #'cadr) → 2

 (count 2.0 '(1 2 3) :test #'eql :key #'float) → 1

 (count "FOO" (list (make-pathname :name "FOO" :type "X")
                    (make-pathname :name "FOO" :type "Y"))
        :key #'pathname-name
        :test #'equal)
→ 2