Laboratory: Files

Summary: In this laboratory, we explore file creation, input, and output in Scheme.

Contents:

• Exercises
  • Exercise 0: Preparation
  • Exercise 1: Summing Values
  • Exercise 2: File Length
  • Exercise 3: Missing Files
  • Exercise 4: Reading All Lines
  • Exercise 5: Displaying Files
  • Exercise 6: Finding Divisors
  • Exercise 7: Reusing Output Files
  • Exercise 8: Counting Characters

Exercises

Exercise 0: Preparation

a. Scan through this lab to determine what kinds of tasks you'll need to complete.

b. You should also scan through the reading on files in Scheme.

c. Start DrScheme.

Exercise 1: Summing Values

The file /home/rebelsky/Web/Courses/CS151/2003S/Examples/numbers contains five hundred and twenty-eight natural numbers.

a. Use sum-of-file from the reading to determine their sum.

b. How would you quickly determine if your attempt to sum those numbers was correct?

Note: That file was copied from a similar file produced by Mr. Stone.

Exercise 2: File Length

Using sum-of-file as a pattern, write a Scheme procedure file-size that takes as argument a string that names a file and returns the number of characters in that file (that is, the number of times that read-char can be called to read a character from the file without returning the end-of-file object).

Exercise 3: Missing Files

Find out what happens if sum-of-file or file-size is given a string that does not name any existing file.

Exercise 4: Reading All Lines

Write a procedure, read-lines, that takes the name of a file as a parameter and returns a list of all the lines of the file (with each line represented as a string). You can certainly use the read-line procedure from the reading as a helper for read-lines. You might use sum-of-file as a pattern for read-lines.

Exercise 5: Displaying Files

Write a procedure, display-file, that takes the name of a file as a parameter and displays the contents of the file with each line preceded by its line number. You may find it useful to call upon the previous procedure to do the reading.

Exercise 6: Finding Divisors

Use the store-divisors procedure to draw up a list of the divisors of 120, storing them in a file named divisors-of-120. Examine the file afterwards and confirm that the answer is correct. (Don't give this procedure an extremely large number as argument -- it's too slow. There are more efficient ways to find divisors!)

Exercise 7: Reusing Output Files

The Scheme standard says that if you try to open an output port to a file that already exists, the effect is unspecified, i.e., anything might happen. Hence, designers of a particular implementation of Scheme are free to do what they choose.

a. Find out through experimentation what DrScheme does in this situation.

b. Find out using DrScheme's Help Desk what DrScheme allows programmers to do in this situation. (The help desk document is entitled Opening file ports.)

Exercise 8: Counting Characters

Write a Scheme procedure that takes as arguments two file names (an input file and an output file), counts the number of occurences of each vowel in the input file, and writes the result to the output file.

If you have time or inclination, extend your procedure to deal with all 127 ASCII characters.