INF3110/4110
		   Problems week 37 (6.-10.9.2004)

Problem 1

Write a Perl program which reads one or more text files and prints the
number of non-blank lines. For example, assume that the file
`short.file´ contains the following four lines:

	First line
	^T
	
	Fourth line

(Line two contains just a TAB character, while the third line is
empty.)

The program `nonblank.perl´ will then work like this:

    % nonblank.perl short.file
    2


Problem 2

The match pattern used in the previous problem is

         ^\s*$

What will match if the pattern is

a)   \s*$
b)   ^\s+
c)   \s*
d)   \s+
e)   \s


Problem 3

The standard Unix program `tail´ prints the last n lines of a text
file (by default n=10). Write this program in Perl.

The program should accept one file name and maybe an option that gives
the number of lines to print; if this option is not present, the last
10 will be printed.

Hint. The simplest way of finding the last n lines is to read all the
lines into an array.

Example:

	 % tail.perl short.file
	 First line
	 ^T
	 
	 Fourth line
	 % tail.perl -3 short.file
	 ^T
	 
	 Fourth line
	 % tail.perl -1 short.file
	 Fourth line
	 %


Problem 4

Write a program `extract.perl´ which will print the specified lines
from a file. It will accept two parameters: a line number range and a
file name. The line numbers may be given as

     4:17   lines 4, 5, ..., 17
     6	    line 6
     2:	    line 2, 3, ...
     :5	    line 1, 2, 3, 4, 5

Example

	 % extract.perl 4 short.file
	 Fourth line
	 % extract.perl 3: short.file
	 
	 Fourth line
	 %


Problem 5

The standard Unix command `wc´ (for "word count") counts the number of
lines, words and characters in a file. Write this program in Perl.

Example

	% wc short.file
	      4      4     26 short.file

Hint. To count the number of words in a line, try to substitute the
word with nothing using the construct

         s/...//

(where you must find a suitable pattern ...) as many times as you
can. When the substitution returns 0, you know that there are no more
words. 

Hint. The pattern `\s´ matches any space character, while `\S´ matches
any non-space character.


Problem 6

The real Unix `wc´ can handle several files. It will produce counts
for each file and the a sum of all these. Modify your program from the
previous problem to handle this.

Example

Assume that file `empty.text´ is empty.

	 % wc empty.text
               0       0       0 empty.text
	 % wc short.file empty.text short.file
	       4       4      26 short.file
	       0       0       0 empty.text
	       4       4      26 short.file
	       8       8      52 total


Problem 7

The Unix program `last´ tells of all users who have logged on to the
computer for some time:

chyunc   pts/3        tournai.uio.no   Wed Sep  1 17:54 - 17:58  (00:04)    
cristobd pts/17                        Wed Sep  1 17:10 - 17:25  (00:15)    
cristobd pts/12       benabarre.ifi.ui Wed Sep  1 17:10 - 17:25  (00:15)    
christkk pts/3                         Wed Sep  1 17:09 - 17:14  (00:04)    
christkk pts/9                         Wed Sep  1 17:09 - 17:14  (00:04)    
christkk X4744        tauste.ifi.uio.n Wed Sep  1 17:09 - 17:14  (00:04)    
cjhaga   pts/31       montecarlo.uio.n Wed Sep  1 15:43 - 18:04  (02:20)    
christkk pts/37                        Wed Sep  1 15:16 - 15:17  (00:00)    
christkk pts/27                        Wed Sep  1 15:11 - 15:37  (00:26)    
christkk pts/28                        Wed Sep  1 15:11 - 15:37  (00:26)    
christkk X4744        tauste.ifi.uio.n Wed Sep  1 15:11 - 15:37  (00:26)    
chrisvid pts/57                        Wed Sep  1 12:46 - 13:24  (00:38)    
chrisvid X46ba        ainodake.ifi.uio Wed Sep  1 12:45 - 13:24  (00:38)    
chrisjo  pts/19       madero.ifi.uio.n Wed Sep  1 10:54 - 14:03  (03:09)    
chrisoh  pts/9        169.80-202-71.ne Wed Sep  1 08:15 - 15:50 (1+07:34)    

The first column gives the user's account name and the last one how
long the user was logged on. The notation "1+07:34" means "1 day
(i.e., 24 hours) plus 7 hours and 34 minutes.

Write a Perl program that reads the log produces by `last´ and print
an alphabetically sorted list of who has logged in on the computer,
how many times, and their average connection time.

Example

If the log lines given above constitute the complete log, the result
should be

chrisjo         once; average   189 min
chrisoh         once; average   454 min
christkk     7 times; average    13 min
chrisvid     2 times; average    38 min
chyunc          once; average     4 min
cjhaga          once; average   140 min
cristobd     2 times; average    15 min