Characters and Strings in Scheme

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?

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.

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?

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.

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

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

Consider the definition

(define like
  (string-append 
     "I like " 
     person 
     " because " 
     person 
     " is "
     adjective
     "."))

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.

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.

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

(define cr (string #\newline))
(define letter
  (string-append
    "Dear " recipient ", " cr
    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
    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.

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

b. Check your answer experimentally.

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?

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

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