Lisp - Strings

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Strings in Common Lisp are vectors, i.e., one-dimensional array of characters.

String literals are enclosed in double quotes. Any character supported by the character set can be enclosed within double quotes to make a string, except the double quote character (") and the escape character (\). However, you can include these by escaping them with a backslash (\).

Example

Create a new source code file named main.lisp and type the following code in it.

main.lisp

; print statements
(write-line "Hello World")
(write-line "Welcome to Tutorials Point")

; escaping the double quote character
(write-line "Welcome to \"Tutorials Point\"")

Output

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

Hello World
Welcome to Tutorials Point
Welcome to "Tutorials Point"

String Comparison Functions

Numeric comparison functions and operators, like, < and > do not work on strings. Common LISP provides other two sets of functions for comparing strings in your code. One set is case-sensitive and the other case-insensitive.

The following table provides the functions −

Case Sensitive Functions Case-insensitive Functions Description
string= string-equal Checks if the values of the operands are all equal or not, if yes then condition becomes true.
string/= string-not-equal Checks if the values of the operands are all different or not, if values are not equal then condition becomes true.
string< string-lessp Checks if the values of the operands are monotonically decreasing.
string> string-greaterp Checks if the values of the operands are monotonically increasing.
string<= string-not-greaterp Checks if the value of any left operand is greater than or equal to the value of next right operand, if yes then condition becomes true.
string>= string-not-lessp Checks if the value of any left operand is less than or equal to the value of its right operand, if yes then condition becomes true.

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; case-sensitive comparison for equality
(write (string= "this is test" "This is test"))
; terminate printing
(terpri)
; case-sensitive comparison for greater than
(write (string> "this is test" "This is test"))
; terminate printing
(terpri)
; case-sensitive comparison for less than
(write (string< "this is test" "This is test"))
; terminate printing
(terpri)

;case-insensitive comparision for equality
(write (string-equal "this is test" "This is test"))
; terminate printing
(terpri)
;case-insensitive comparision for greater than
(write (string-greaterp "this is test" "This is test"))
; terminate printing
(terpri)
;case-insensitive comparision for less than
(write (string-lessp "this is test" "This is test"))
; terminate printing
(terpri)

; checking non-equality
(write (string/= "this is test" "this is Test"))
; terminate printing
(terpri)
(write (string-not-equal "this is test" "This is test"))
; terminate printing
(terpri)
(write (string/= "lisp" "lisping"))
; terminate printing
(terpri)
(write (string/= "decent" "decency"))

Output

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

NIL
0
NIL
T
NIL
NIL
8
NIL
4
5

Case Controlling Functions

The following table describes the case controlling functions −

Sr.No. Function & Description
1

string-upcase

Converts the string to upper case
2

string-downcase

Converts the string to lower case
3

string-capitalize

Capitalizes each word in the string

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; convert to upper case and print statement
(write-line (string-upcase "a big hello from tutorials point"))
; convert to Capital case and print statement
(write-line (string-capitalize "a big hello from tutorials point"))

Output

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

A BIG HELLO FROM TUTORIALS POINT
A Big Hello From Tutorials Point

Trimming Strings

The following table describes the string trimming functions −

Sr.No. Function & Description
1

string-trim

It takes a string of character(s) as first argument and a string as the second argument and returns a substring where all characters that are in the first argument are removed off the argument string.
2

String-left-trim

It takes a string of character(s) as first argument and a string as the second argument and returns a substring where all characters that are in the first argument are removed off the beginning of the argument string.
3

String-right-trim

It takes a string character(s) as first argument and a string as the second argument and returns a substring where all characters that are in the first argument are removed off the end of the argument string.

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; trim and print the statement
(write-line (string-trim " " "   a big hello from tutorials point   "))
; trim spaces on left and print the statement
(write-line (string-left-trim " " "   a big hello from tutorials point   "))
; trim spaces on right and print the statement
(write-line (string-right-trim " " "   a big hello from tutorials point   "))
; trim spaces on both left and right and print the statement
(write-line (string-trim " a" "   a big hello from tutorials point   "))

Output

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

a big hello from tutorials point
a big hello from tutorials point   
   a big hello from tutorials point
big hello from tutorials point

Other String Functions

Strings in LISP are arrays and thus also sequences. We will cover these data types in coming tutorials. All functions that are applicable to arrays and sequences also apply to strings. However, we will demonstrate some commonly used functions using various examples.

Calculating Length

The length function calculates the length of a string.

Extracting Sub-string

The subseq function returns a sub-string (as a string is also a sequence) starting at a particular index and continuing to a particular ending index or the end of the string.

Accessing a Character in a String

The char function allows accessing individual characters of a string.

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; print length of the statement
(write (length "Hello World"))
; terminate printing
(terpri)
; print substring of statement starting from index 6
(write-line (subseq "Hello World" 6))
; print char from string at index 6
(write (char "Hello World" 6))

Output

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

11
World
#\W

Sorting and Merging of Strings

The sort function allows sorting a string. It takes a sequence (vector or string) and a two-argument predicate and returns a sorted version of the sequence.

The merge function takes two sequences and a predicate and returns a sequence produced by merging the two sequences, according to the predicate.

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

;sorting the strings
(write (sort (vector "Amal" "Akbar" "Anthony") #'string<))
; terminate printing
(terpri)

;merging the strings
; merge sequences and print
(write (merge 'vector (vector "Rishi" "Zara" "Priyanka") 
   (vector "Anju" "Anuj" "Avni") #'string<))

Output

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

#("Akbar" "Amal" "Anthony")
#("Anju" "Anuj" "Avni" "Rishi" "Zara" "Priyanka")

Reversing a String

The reverse function reverses a string.

For example, update the source code file named main.lisp and type the following code in it.

main.lisp

; print reverse of the statement
(write-line (reverse "Are we not drawn onward, we few, drawn onward to new era"))

Output

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

are wen ot drawno nward ,wef ew ,drawno nward ton ew erA

Concatenating Strings

The concatenate function concatenates two strings. This is generic sequence function and you must provide the result type as the first argument.

For example, update the source code file named main.lisp and type the following code in it.

main.lisp

; concatenate and print statements
(write-line (concatenate 'string "Are we not drawn onward, " "we few, drawn onward to new era"))

Output

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

Are we not drawn onward, we few, drawn onward to new era
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