# 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

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))
```

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.

c. Use `read` to obtain a character.

d. Use `read` to obtain a list of numbers.

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-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?

### 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`).

b. Open a terminal window.

c. In that terminal window, type

```/usr/local/plt/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/local/plt/bin/mzscheme -r file.scm
```

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

f. In the terminal window, type

```chmod 755 square
```

g. In the terminal window, type

```square
```

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: 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

### Extra 1: A Simple Game

Write a command-line version of some simple game that uses `random`. For example, you might ask someone to guess whether the next random number will be even or odd and give them a point if they guess correctly.

## History

Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

This document was generated by Siteweaver on Tue Dec 9 13:59:05 2003.
The source to the document was last modified on Tue Oct 28 14:53:09 2003.
This document may be found at `http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2003F/Labs/io.html`.

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Samuel A. Rebelsky, rebelsky@grinnell.edu