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.
Contents
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.
For each of part of this exercise, you should use something like
to read in a value.
For example,
> (define val (read))
45
> val
45
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?
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.
Rewrite the code from the previous exercise to use let or
let* (or both) rather than define.
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
#!/bin/bash
/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
g. In the terminal window, type
h. Reflect on what just happened.
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.
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?
Here is a simple story procedure from the
reading on program output.
(define story1
(lambda (lst)
(if (null? lst)
(begin
(display "Daniel likes other things, too.")
(newline))
(begin
(display "Daniel likes ")
(display (car lst))
(display ".")
(newline)
(story1 (cdr lst))))))
a. Make sure that it can tell a story.
b. Write your own variant of this procedure.
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.
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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