A string is a sequence of zero or more characters. Most strings can be named by enclosing the characters they contain between double quotation marks: for instance, "hyperbola" is the nine-character string consisting of the characters #\h, #\y, #\p, #\e, #\r, #\b, #\o, #\l, and #\a, in that order, and "" is the zero-character string (the ``null string'').

Strings may contain spaces and newline characters; when such characters are between double quotation marks, they are treated like any other characters. There is a slight problem when one wants to put a double quotation mark into a string: To indicate that the double quotation mark is part of the string (rather than marking the end of the string), one must place a backslash character immediately in front of it. For instance, "Say \"hi\"" is the eight-character string consisting of the characters #\S, #\a, #\y, #\space, #\", #\h, #\i, and #\", in that order. The backslashes are ``escape'' characters, present in the name only to indicate that the characters immediately after them form part of the string.

This use of the backslash character causes yet another slight problem: What if one wants to put a backslash into a string? The solution is to place another backslash character immediately in front of it. For instance, "a\\b" is the three-character string consisting of the characters #\a, #\\, and #\b, in that order.

Scheme provides the following basic procedures for working with strings:

• The string? predicate determines whether its argument is or is not a string.

1. Is the symbol hyperbola a string?

2. Is the character #\A a string?

3. Does the null string count as a string?

• The make-string procedure constructs and returns a string that consists of repetitions of a single character; its first argument indicates how long the string should be, and the second argument specifies which character it should be made of. For instance, (make-string 5 #\a) constructs and returns the string "aaaaa".

• The string procedure takes any number of characters as arguments and constructs and returns a string consisting of exactly those characters. For instance, (string #\H #\i #\!) constructs and returns the string "Hi!".

4. Suggest two ways of constructing the string ??? -- one using a call to make-string, the other a call to string.

• The string-length procedure takes any string as argument and returns the number of characters in that string. For instance, the value of (string-length "parabola") is 8 and the value of (string-length "a\\b") is 3.

• The string-ref procedure is used to select the character at a specified position within a string. Like list-ref, string-ref presupposes zero-based indexing; the position is specified by the number of characters that precede it in the string. (So the first character in the string is at position 0, the second at position 1, and so on.) For instance, the value of (string-ref "ellipse" 4) is #\p -- the character that follows four other characters and so is at position 4 in zero-based indexing.

5. Here are two opposing views about the relationship between string-length and string-ref:

   • ``No matter what string str is, provided that it's not the null string, (string-ref str (string-length str)) will return the last character in the string.''

   • ``No matter what string str is, (string-ref str (string-length str)) is an error.''

   Which, if either, of these views is correct? Why?

• Strings can be compared for ``lexicographic order,'' the extension of alphabetical order that is derived from the collating sequence of the local character set. Once more, Scheme provides both case-sensitive and case-insensitive versions of these predicates: string<?, string<=?, string=?, string>=?, and string>? are the case-sensitive versions, and string-ci<?, string-ci<=?, string-ci=?, string-ci>=?, and string-ci>? the case-insensitive ones.

• The substring procedure takes three arguments. The first is a string and the second and third are non-negative integers not exceeding the length of that string. Substring returns the part of its first argument that starts after the number of characters specified by the second argument and ends after the number of characters specified by the third argument. For instance: (substring "hypocycloid" 3 8) returns the substring "ocycl" -- the substring that starts after the initial "hyp" and ends after the eighth character, the l.

• The string-append procedure takes any number of strings as arguments and returns a string formed by concatenating those arguments. For instance, the value of (string-append "al" "fal" "fa") is "alfalfa".

6. Write a Scheme procedure tally-vowels that takes one argument, a string, and determines how many characters in that string are vowels. (For this purpose, you should count only the ten characters #\a, #\e, #\i, #\o, #\u, #\A, #\E, #\I, #\O, and #\U as vowels.)

7. Write a Scheme procedure consonant-part that takes one argument, a string, and returns a string similar to its argument except that all of the vowels have been removed. (For instance, the value of (consonant-part "asparagus") should be "sprgs".)

This document is available on the World Wide Web as

http://www.math.grin.edu/~stone/courses/scheme/strings.html

created March 5, 1997
last revised October 9, 1997

Henry Walker (walker@math.grin.edu) and John David Stone (stone@math.grin.edu)