Lisp - Sequence SubSequence

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Lisp provides multiple functions to get subsequnce of a various type of sequence like list, vectors and strings. Here are the commonly used functions explained with examples.

subseq function

subseq function extracts a subsequence from a sequence.

Syntax

(subseq sequence startIndex [endIndex])

Arguments

• sequence− A sequnce whose subsequnce is to be retrieved.

• startIndex− start index from which SubSequence is to be retrieved.

• endIndex− an optional end index upto which subsequence is to be retrieved. endIndex is excluded.

Returns

This function returns a new sequence containing the resulted subsequnce.

Example

; prints subsequence (B C D)
(print(subseq '(a b c d e) 1 4))  
(terpri)
; print substring "point"
(write-line(subseq "tutorialspoint" 9))

Output

When you execute the code, it returns the following result −

(B C D)
point

copy-seq function

copy-seq function creates copy of the sequence. It is useful when we want to modify the sequence without altering the original sequnce.

Syntax

(copy-seq sequence)

Arguments

• sequence− A sequnce whose copy is to be retrieved.

Returns

This function returns a new sequence as a copy of sequence.

Example

; prints list
(print(copy-seq '(a b c d e)))  

Output

When you execute the code, it returns the following result −

(A B C D E)

elt function

elt function accesses an element at particular index in a sequence. Index starts from zero.

Syntax

(elt sequence index)

Arguments

• sequence− A sequnce whose element is to be retrieved.

• index− A non-negative index. Index starts from zero and should be less than total number of elements in a sequence.

Returns

This function returns element at particular index of the sequence.

Example

; prints C
(print(elt '(a b c d e) 2))   
(terpri)

; prints #\h
(print(elt "hello" 0))       

Output

When you execute the code, it returns the following result −

C 
#\h