20.txt

一個計算機系教授的上課講義 主要是教導學生使用C語言編寫程序

CS 1355
Introduction to Programming in C
Thursday 2006.11.23
Lecture notes (at http://r638-2.cs.nthu.edu.tw/C/notes/20.txt)

Today: Morse Code homework example

1. main program
 - calls a function based on argv[1]
 - program name is in argv[0]

2. usage(): just substitute in the program name to a
   constant string

3. help(): print the code and symbol, five per column.
   if you do the way it is suggested, then it is a
   matter of printing all entries of a table.

4. encode(): 
  - converts each character to a code word.
  - lower case and upper case are not distinguished
  - accepted symbols: letters, digits, spaces, returns
  - any other symbol: interpreted as a space
  - return is printed as a return

  Tricky part: when do you actually print a blank space?
  - you need a blank space between each nonblank symbol
  - a blank symbol is encoded as three spaces
  - however, you should not always print an extra space to 
    separate code word and the three-blank code word!

   Hint: use a state variable to remember whether to print

5. decode(): 
  - convert back from Morse code to A-Z 0-9
  - three spaces get printed as one
  - all other chars printed as-is
  Tricky part: space handling
  - if you scan %s, it skips all blanks!
  - need to preserve blanks
    use the "character set" pattern,
    use repeat * to scan multiple
  - if blanks found, decide whether it is a codeword delimiter
    (single) or the codeword for the space symbol (three blanks)
  If not blank, check if it is one of the codewords.
  - ok use linear search or binary search to find codeword
    (what is linear search? see Lecture 14)
  - when found, use the index to retrieve the symbol
  - if not found, get the next char from standard input
    and print it to standard output

6. testing:
- write and test one routine thoroughly at a time
- test the help() usage() ones first
  test encode() before writing decode()
- run the reference executable to compare output format

Example: put input in the file named "in"
% cat in
Hello World lucky 7
Today is 2006 11 23
The Time is 14 56
Goodbye Mylove
% ./a.out -e < in      # try encoding, redirecting "in" to standard-input
.... . .-.. .-.. ---   .-- --- .-. .-.. -..   .-.. ..- -.-. -.- -.--   --...
- --- -.. .- -.--   .. ...   ..--- ----- ----- -....   .---- .----   ..--- ...--
- .... .   - .. -- .   .. ...   .---- ....-   ..... -....
--. --- --- -.. -... -.-- .   -- -.-- .-.. --- ...- .
% ./a.out -e < in | ./a.out -d  # pipe encoded output to decoder
HELLO WORLD LUCKY 7
TODAY IS 2006 11 23
THE TIME IS 14 56
GOODBYE MYLOVE
%

Extra Credit:
- If your code runs faster than the professor's code, 
  you qualify for extra credit, though you must explain the techniques
  you used to make the code run fast.
- Fastest encoder:
  if your program runs the fastest in the class, 
  you will get more extra credit.
- to measure how long a program takes to run, use the time command:
- time to be measured on the workstation
- input to be provided by TA