Laboratory: Input and Output

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, and display.

Contents

• Exercises
  • Exercise 0: Preparation
  • Exercise 1: Practice with read
  • Exercise 2: Simple Input and Output
  • Exercise 3: Running from the Command Line
  • Exercise 4: Local Input Values
  • Exercise 5: Error Checking
  • Exercise 6: Computing Roots of a Quadratic Equation
  • Exercise 7: Repetition
• For Those with Extra Time

Exercises

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 helpful.

b. Start DrScheme.

Exercise 1: Practice with read

For each of the parts of this exercise, you should use something like

(define val (read))

to read in a value.

a. Use read to obtain a number.

b. Use read to obtain a string.

c. Use read to obtain a character.

d. Use read to obtain a list of numbers.

e. Use read to obtain a symbol.

f. Use read to obtain a list that contains a number, string, and 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)))
(newline)

a. What do you expect the code to do?

b. Verify your answer via experimentation.

Exercise 3: Running from the Command Line

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

b. Open a terminal window.

c. In that terminal window, type

mzscheme -r file.scm

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

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

Exercise 4: Local Input Values

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

Exercise 5: Error Checking

a. What happens 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: Repetition

Write a program repeatedly asks for a value and computes its square root. You will probably need to

a. Put the stuff (request for value, computation of a value, display of that value) in a procedure.

b. Have that procedure recurse.

c. Decide upon a base case to stop recursion. (I'd suggest that you stop when someone enters something other than a number.)

For Those with Extra Time

Write a command-line version of the dice game from a recent lab.