Fundamentals of Computer Science I (CS151.02 2007S)

Laboratory: Input and Output

This lab is also available in PDF.

Summary: In this laboratory, you will experiment with the use and application of some of Scheme's basic input and output procedures.

Procedures Covered: read, write, display, and newline.



Exercise 0: Preparation

a. Make sure that you understand what read, write, and display are supposed to do. You may find the reading on input and the reading on output helpful.

b. Ensure that you understand the sample code from the reading on input.

c. Start DrScheme.

Exercise 1: Practice with read

For each of part of this exercise, you should use something like

(define val (read))

to read in a value.

For example,

> (define val (read))
> val

a. Use read to obtain a number. That is, call read as above and enter a number.

b. Use read to obtain a string. (You'll need to type in a string.)

c. Use read to obtain a character. (You'll need to type in a character in traditional form.)

d. Use read to obtain a list of numbers. (You should not write the word list.)

e. Use read to obtain a symbol. Verify that the value you obtained is a symbol using the symbol? predicate.

f. Use read to obtain a list that contains a number, a string, and a symbol.

g. What happens when you use read to obtain a list and you hit <Enter> in the middle of the list?

Exercise 2: Simple Input and Output

Consider the following sequence of Scheme commands:

(display "Please enter a value and I will square it: ")
(define val (read))
(define val-squared (* val val))
(display (string-append "The value of "
                        (number->string val)
                        " squared is "
                        (number->string val-squared)))

a. What do you expect the code to do?

b. Verify your answer via experimentation.

Exercise 3: Local Input Values

Rewrite the code from the previous exercise to use let or let* (or both) rather than define.

Exercise 4: Running from the Command Line

a. Save the above code in a file (e.g., square.scm in your home directory).

b. Open a terminal window.

c. In that terminal window, type

/usr/bin/mzscheme -r file.scm

Where file.scm is the name you chose in part a.

d. Reflect on what happened in step c. Did you need to type any Scheme? Could someone else use step c without understanding the underlying Scheme?

e. Create a file called square that contains the following lines

/usr/bin/mzscheme -r file.scm

where file.scm is the name you chose in part a.

Although the preceding is not Scheme code, you can still enter it in DrScheme and save it into a file.

f. In the terminal window, type

chmod 755 square

g. In the terminal window, type


h. Reflect on what just happened.

Exercise 5: Error Checking

a. What happens in your square program if someone enters something other than a number?

b. Update your program so that it prints a friendly error message (using display) and then asks again if someone enters something other than a number.

Exercise 6: Computing Roots of a Quadratic Equation

a. Write a Scheme program that reads in the three coefficients of a quadratic equation (the a, b, and c in ax2 + bx + c) and prints out the roots of the equation. You should model this program on the previous exercises. In case you've forgotten, the roots of the quadratic equation are

(-b +/- sqrt(b2 - 4ac)) / 2a

b. Save the program in a file and execute it from the command line.

c. Reflect on what happened. Did you need to type any Scheme? Could someone else use step b without understanding the underlying Scheme?

Exercise 7: Simple Stories

Here is a simple story procedure from the reading on program output.

(define story1
  (lambda (lst)
     (if (null? lst)
	   (display "Daniel likes other things, too.")
	   (display "Daniel likes ")
	   (display (car lst))
	   (display ".")
	   (story1 (cdr lst))))))

a. Make sure that it can tell a story.

b. Write your own variant of this procedure.

Exercise 8: Repetition

The reading on input contains a program that repeatedly asks for a value and computes its square root.

a. Verify that it works correctly.

b. Create a shell script (as above) that lets us compute square roots within a terminal window.

Exercise 9: An Interactive Storyteller

Rewrite the story telling procedure from exercise 7 so that it prompts for the words in the story (that is, the things that Daniel likes). Try to ensure that the story does not get told until after all of the words are entered.




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