Fundamentals of Computer Science I: Media Computing (CS151.02 2007F)

Characters and Strings in Scheme

Summary: In these exercises, you will explore a number of the standard Scheme procedures for handling characters and strings. You will also explore an application of these procedures for marking up text.

Useful Procedures and Notation

  • Constant notation: #\ch (character constants) "string" (string constants).

  • Character constants: #\a (lowercase a) ... #\z (lowercase z); #\A (uppercase A) ... #\Z (uppercase Z); #\0 (zero) ... #\9 (nine); #\space (space); #\newline (newline); and #\? (question mark).

  • Character conversion: char->integer, integer->char, char-downcase, and char-upcase

  • Character predicates: char?, char-alphabetic?, char-numeric?, char-lower-case?, char-upper-case?, char-whitespace?, char<?, char<=?, char=?, char>=?, char>?, char-ci<?, char-ci<=?, char-ci=?, char-ci>=?, and char-ci>?.

  • String predicates: string?

  • String constructors: make-string, string, string-append

  • String extractors: string-ref, substring

  • String conversion: list->string, number->string, string->list

  • String analysis: string-length,

  • String comparison: string<?, string<=?, string=?, string>=?, string>?, string-ci<?, string-ci<=?, string-ci=?, string-ci>=?, string-ci>?


a. If you have not done so already, you may also want to open separate tabs in your web browser with the reading on characters and strings.

b. If you have not done so already, you may want to skim Section 6.3.5 of the Scheme Report.

c. No images needed in today's lab!


Exercise 1: Collating Sequences

As you may recall, Scheme uses a collating sequence for the letters, assigning a sequence number to each letter. DrFu uses the ASCII collating sequence.

a. Determine the ASCII collating-sequence numbers for the capital letter A and for the lower-case letter a.

b. Find out what ASCII character is in position 38 in the collating sequence.

c. Do the digit characters precede or follow the capital letters in the ASCII collating sequence?

d. If you were designing a character set, where in the collating sequence would you place the space character? Why?

e. What position does the space character occupy in ASCII?

Exercise 2: Character Predicates

a. Determine whether our implementation of Scheme considers #\newline a whitespace character.

b. Determine whether our implementation of Scheme indicates that capital B precedes lower-case a.

c. Determine whether our implementation of Scheme indicates that lower-case a precedes capital B.

d. Verify that the case-insensitive comparison operation (char-ci<?) gives the expected result for the previous two tests.

e. Determine whether our implementation of Scheme indicates that #\a and #\A are the same letter. (It should not.)

f. Find an equality predicate that returns #t when given #\a and #\A as parameters.

Exercise 3: String Basics

a. Write a Scheme expression to determine whether the symbol 'plaid is a string.

b. Write a Scheme expression to determine whether the character #\A is a string.

c. Does the empty string (represented as "") count as a string?

Exercise 4: Creating Questions

Develop three ways of constructing the string "???" -- one using a call to make-string, one a call to string, and one a call to list->string.

Exercise 5: Referencing Lengths

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 empty 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?

Exercise 6: Building Simple Sentences

Consider the definition

(define like
     "I like " 
     " because " 
     " is "

a. What other values must be defined in order for this definition to work?

b. What type must those values have?

c. Suppose you had previously defined person as "Ms. Davis" and adjective as "cheerful". What do you expect the value of like to be?

d. Check your previous answer experimentally.

Exercise 7: Building Sentences, Revisited

One criticism of the like definition in the previous exercise is that it takes a lot of lines. We could define a similar sentence as follows:

(define tunes
  (string-append "I listen to " band " because their music is " adjective "."))

a. What are the comparative advantages and disadvantages of the single-line sentence-building definition?

b. Define band and adjective in such a way that tunes can be successfully defined.

Exercise 8: A Simple Form Letter

We can, of course, use a similar technique to build longer form letters. Consider the following definitions

(define cr (string #\newline))
(define letter
    "Dear " recipient ", " cr
    "Thank you for your submission to " magazine ".  Unfortunately, we " cr
    "consider the subject of your article, " article ", inappropriate for our" cr
    "readership.  In fact, it is probably inappropriate for any readership." cr
    "Please do not contact us again, and do not bother other magazines with" cr
    "this inappropriate material or we will be forced to contact the " cr
    "appropriate authorities." cr
    "Regards," cr
    "Ed I. Tor" cr))

a. What must be defined for the definition of letter to succeed?

b. Check that the definition of letter works by using the following associated definitions.

(define recipient "Professor Schneider")
(define magazine "College Trustee News")
(define article "Using Grinnell's Endowment to Eliminate Tuition")

c. You may note that the output is fairly ugly when you simply ask for letter. You can get nicer output by using the display procedure, as in (display letter). Try doing so.

Exercise 9: Adding Quotation Marks

a. What changes are necessary to letter so that name of the article appears in quotation marks?

b. Check your answer experimentally.

Exercise 10: More Form Letters

a. Create a file, sam.scm, with the following lines:

(define recipient "Mr. Rebelsky")
(define magazine "Liberal Arts Letters")
(define article "Why Every Faculty Member Should Take Introductory Computer Science")

b. Create a separate file, letter.scm that contains the definition of cr and the updated definition of letter from the previous exercises.

c. In the definitions window, type the following

(load "sam.scm")
(load "letter.scm")
(display letter)

d. What do you expect to happen when you click Run?

e. Check your answer experimentally.

f. What ideas does this exercise suggest to you?

For Those with Extra Time

Extra 1: Generating Brush Names

You may recall that the names of the ten basic circle brushes follow a regular pattern: the word “Circle” followed by a space, an open paren, an odd number, and a close paren. For example, the first circular brush is "Circle (01)" and the sixth circular brush is "Circle (11)".

Write a procedure, (circular-brush n), that computes the name of the nth circular brush. This procedure should throw an error if n is invalid. Please use the string procedures, such as number->string and string-append, in defining your procedure.

> (circular-brush 1)
"Circle (01)"
> (circular-brush 5)
"Circle (09)"
> (circular-brush 8)
"Circle (15)"
> (circular-brush "One")
Error: circular-brush requires an integer between 1 and 10
> (circular-brush -5)
Error: circular-brush requires an integer between 1 and 10
> (circular brush 1.5)
Error: circular-brush requires an integer between 1 and 10
> (circular brush 1925)
Error: circular-brush requires an integer between 1 and 10

Extra 2: More Form Letters

Write another procedure to generate a form letter of your choice. Possibilities include acceptance letters to Grinnell, rejection letters, and recommendation letters.

Creative Commons License

Samuel A. Rebelsky,

Copyright 2007 Janet Davis, Matthew Kluber, and Samuel A. Rebelsky. (Selected materials copyright by John David Stone and Henry Walker and used by permission.)

This material is based upon work partially supported by the National Science Foundation under Grant No. CCLI-0633090. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. To view a copy of this license, visit or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.