mismatch

mismatch Function

Syntax:

mismatch sequence-1 sequence-2 &key from-end test test-not key start1 start2 end1 end2 → position

Arguments and Values:

Sequence-1—a sequence.

Sequence-2—a sequence.

from-end—a generalized boolean. The default is false.

test—a designator for a function of two arguments that returns a generalized boolean. test-not—a designator for a function of two arguments that returns a generalized boolean.

start1, end1—bounding index designators of sequence-1. The defaults for start1 and end1 are 0 and nil, respectively.

start2, end2—bounding index designators of sequence-2. The defaults for start2 and end2 are 0 and nil, respectively.

key—a designator for a function of one argument, or nil.

position—a bounding index of sequence-1, or nil.

Description:

The specified subsequences of sequence-1 and sequence-2 are compared element-wise. The key argument is used for both the sequence-1 and the sequence-2.

If sequence-1 and sequence-2 are of equal length and match in every element, the result is false. Otherwise, the result is a non-negative integer , the index within sequence-1 of the leftmost or rightmost position, depending on from-end, at which the two subsequences fail to match. If one subsequence is shorter than and a matching prefix of the other, the result is the index relative to sequence-1 beyond the last position tested.

If from-end is true, then one plus the index of the rightmost position in which the sequences differ is returned. In effect, the subsequences are aligned at their right-hand ends; then, the last elements are compared, the penultimate elements, and so on. The index returned is an index relative to sequence-1.

Examples:

(mismatch "abcd" "ABCDE" :test #’char-equal) → 4 
(mismatch ’(3 2 1 1 2 3) ’(1 2 3) :from-end t) → 3 
(mismatch ’(1 2 3) ’(2 3 4) :test-not #’eq :key #’oddp) → NIL 
(mismatch ’(1 2 3 4 5 6) ’(3 4 5 6 7) :start1 2 :end2 4) → NIL 

See Also:

Section 3.6 (Traversal Rules and Side Effects)

Notes:

The :test-not argument is deprecated.

Expanded Reference: mismatch

Finding where two sequences differ

mismatch compares two sequences element-by-element and returns the index in sequence-1 where they first differ. Returns NIL if the sequences match completely.

(mismatch '(a b c d) '(a b x d))
=> 2
(mismatch "hello" "hello")
=> NIL
(mismatch "hello" "help")
=> 3

Sequences of different lengths

If one sequence is a prefix of the other, mismatch returns the index just past the shorter sequence.

(mismatch "abc" "abcdef")
=> 3
(mismatch '(1 2 3 4 5) '(1 2 3))
=> 3

Case-insensitive comparison with :test

(mismatch "abcd" "ABCDE" :test #'char-equal)
=> 4
(mismatch "HELLO" "hello" :test #'char-equal)
=> NIL

Comparing from the end

With :from-end t, mismatch returns one plus the index of the rightmost position where the sequences differ.

(mismatch '(3 2 1 1 2 3) '(1 2 3) :from-end t)
=> 3

Using :key for derived comparisons

(mismatch '(1 2 3) '(2 3 4) :key #'oddp)
=> 0

Restricting comparison ranges

Use :start1, :end1, :start2, and :end2 to compare only portions of each sequence.

(mismatch '(1 2 3 4 5 6) '(3 4 5 6 7) :start1 2 :end2 4)
=> NIL