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String Basics

In Scheme, literal strings are enclosed in quotes. According to R5RS, "A string constant may continue from one line to the next, but the exact contents of such a string are unspecified.", probably to account for platform differences in handling newlines. The PLT and mzScheme interpreters allow you to write string literals with linebreaks inline, i.e.
>(define a-string "A literal string with a linebreak (\"\\n\")
in it")
>a-string
"A literal string with a linebreak (\"\\n\")\nin it"
>(display a-string)
A literal string with a linebreak ("\n")
in it
R5RS provides the string->list and list->string functions to convert between strings and lists:
> (string->list "Hello")
(#\H #\e #\l #\l #\o)
> (list->string '(#\S #\c #\h #\e #\m #\e))
"Scheme"
If you want to find out what character is at a specific position in a string, use (string-ref string *i*), where i is a 0-based index in the string:
>(string-ref a-string 0)
#\A
string-length will tell you how long a string is.
> (string-length a-string)
46
You may have heard that Scheme provides mutable strings. If you want to modify a string in place, the string-set! function will do that for you. However, if you gave the string as a literal to the REPL, string-set! will give you a surprise:
> (string-set! a-string 0 #\1)
string-set!: expects type <mutable-string> as 1st argument, given: "A literal string with a linebreak (\"\\n\")\r\nin it"; other arguments were: 0 #\1
You can make a copy of a string using string-copy. This will yield a mutable string:
> (define a-copy (string-copy a-string))
> (string-set! a-copy 0 #\1)
> a-copy
"1 literal string with a linebreak (\"\\n\")\r\nin it"
You can allocate a new string with make-string. make-string comes in 2 forms, the second allows you to specify an initial character:
> (make-string 5)
"\0\0\0\0\0"
> (make-string 5 #\*)
"*****"
You can also use string-append to join strings. This
> (string-append "Hello" ", " "Scheme" "!")
"Hello, Scheme!"
R5RS provides a number of [useful string functions, but you should also be aware of SRFI 13 (String Library) and SRFI-14 (Character Set Library), which add a number of very useful functions.

-- GordonWeakliem - 05 Apr 2004

 
 
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