Comments on Quiz 3a/3b

(This was the first quiz, in Week 3.)

Grading:

The two versions were similar.

Most people had difficulties. In some cases it was hard to tell if this was because of not understanding the problem. Most people would benefit from retaking the test.

Therefore we'll do the following:

1. Instead of having people retake the test, the same items will be included on the midterm, in more or less the same form.
2. I made a note of people who did the test essentially correctly. Their papers are marked with + .
3. The papers of other people are just checked off as received, without a grade being recorded.

Comments on answers:

• It's important to distinguish between what UNIX sees and what is inside a Perl script. For UNIX:
  • UNIX will run programs that are Perl scripts, compiled C++ programs, etc.
  • Each program has a standard input and a standard output. The program doesn't ``know'' where the standard input will come from or where the standard output will go.
  • Programs can also open named files for reading or writing. This is different from reading standard input or writing standard output.
  • On a UNIX command line, you had these three sources of standard input:
    1. a named file: myprog < filename
    2. a pipe: ... | myprog
    3. the keyboard: myprog (with no ``redirection'')
  • On a UNIX command line, you had these three places that standard outut can go:
    1. a named file: myprog > filename
    2. a pipe: myprog | ...
    3. the screen: myprog (with no ``redirection'')

    So in a pipe prg1 | prg2 the standard output of prg1 becomes the standard input to prg2 .

    Some people mentioned HTML. It's true that a CGI program produces output in HTML, but it's not run from a command line.

  • On a UNIX command line, a program can also be given arguments: myprog arg1 arg2 arg3 These are simply strings that are passed to the program. They may be file names or not.

• For Perl:
  • A Perl script gets standard input from <STDIN>
  • A Perl script writes standard output using an ordinary print statement.
  • A Perl script gets arguments through @ARGV
  • The expression <> is special: If there are no command-line arguments then <> reads from standard input. If there are command-line arguments, then <> treats them as file names and opens and reads the files one by one. This is not by standard input, though.
• Do not confuse standard input with arguments.

  In sort filename or wc filename, the file name is an argument, so sort is not reading the file using standard input.