CSCI A348/548
Lab Notes Four

Spring 2001 (Second semester 2000-2001)

One line summary of lab four.

Please use this lab to bring yourself up to date with the second assignment.

Lecture Notes Eight contain all the details, step by step. Assignment 3, coming soon, will be more straightforward, and will take less time. So we can extend the second assignment one more week, so you can take your time finishing it.

In this lab, for those of you who are already done, a few substitutions, from Tuesday.

Perl Substitutions (p. 12, 88 Learning Perl - Chapter 7)

We're using the =~ operator, together with the letter s on its right hand side, followed by a slash delimited pattern to be matched, and a string. When the pattern matches the string that follows the second slash will replace it. There are several rules and exceptions and we will summarize those that we care for here, through a couple of examples.

The dot (.) matches any one character except newline.

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
$a = "1234567890"; 
$a =~ s/./a/; 
print $a; 
frilled.cs.indiana.edu%./alpha
a234567890frilled.cs.indiana.edu%

To have the substitution happen everywhere it can happen, use g (global) aftre the third slash.

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
$a = "1234567890"; 
$a =~ s/./a/g; 
print $a; 
frilled.cs.indiana.edu%./alpha
aaaaaaaaaafrilled.cs.indiana.edu%

The pattern can be bigger (or longer):

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
$a = "1234567890"; 
$a =~ s/../a/g; 
print $a; 
frilled.cs.indiana.edu%./alpha
aaaaafrilled.cs.indiana.edu%

Parentheses can be used as memory elements:

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
$a = "1234567890"; 
$a =~ s/(.)(.)/$2$1/g; 
print $a; 
frilled.cs.indiana.edu%./alpha
2143658709frilled.cs.indiana.edu%

And they can include larger patterns:

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
$a = "1234567890"; 
$a =~ s/(..)/$1+1/g; 
print $a; 
frilled.cs.indiana.edu%./alpha
12+134+156+178+190+1frilled.cs.indiana.edu%

To have the part between the last two slashes act as Perl code use e (evaluate) after the third slash.

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
$a = "1234567890"; 
$a =~ s/(..)/$1+1/ge; 
print $a; 
frilled.cs.indiana.edu%./alpha
1335577991frilled.cs.indiana.edu%

Miscellaneous A few other things needed in ReadParse are listed below.

Characters have (decimal) ASCII codes that can be obtain with ord.

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
@values = ('A', 'B', 'C', 'D', 'E'); 
foreach $value (@values) {
  print $value, " has ASCII code: ", ord($value), "\n"; 
} 
frilled.cs.indiana.edu%./alpha
A has ASCII code: 65
B has ASCII code: 66
C has ASCII code: 67
D has ASCII code: 68
E has ASCII code: 69
frilled.cs.indiana.edu%

ASCII codes can be turned into characters with chr.

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
@values = (65, 66, 67, 68, 69); 
foreach $value (@values) {
  print "ASCII code $value stands for: ", chr($value), "\n"; 
} 
frilled.cs.indiana.edu%./alpha
ASCII code 65 stands for: A
ASCII code 66 stands for: B
ASCII code 67 stands for: C
ASCII code 68 stands for: D
ASCII code 69 stands for: E
frilled.cs.indiana.edu%

The hex function turns a hexadecimal value in a decimal one.

frilled.cs.indiana.edu%cat alpha
#!/usr/bin/perl
@values = (1, 10, 20, 100, 110, 111); 
foreach $value (@values) {
  print "$value in base 16 is equal to ", hex($value), " in base 10.\n"; 
} 
frilled.cs.indiana.edu%./alpha
1 in base 16 is equal to 1 in base 10.
10 in base 16 is equal to 16 in base 10.
20 in base 16 is equal to 32 in base 10.
100 in base 16 is equal to 256 in base 10.
110 in base 16 is equal to 272 in base 10.
111 in base 16 is equal to 273 in base 10.
frilled.cs.indiana.edu%

You're now ready for Tuesday.

Last updated on Feb 1, 2001, by Adrian German for A348/A548

CSCI A348/548 Lab Notes Four

CSCI A348/548
Lab Notes Four