Transcript perl - Tower of Hanoi

Programming Perls*
• Objective: To introduce students to the perl
language.
– Perl is a language for getting your job done.
– Making Easy Things Easy & Hard Things Possible
– Perl is a language for easily manipulating text, files,
and processes
– Combines concepts from unix, sed, awk, shell scripts
– Language of system administrators, web developers and
more…
– Practical Extraction and Report Language
– Pathologically Eclectic Rubbish Lister
*Many of the examples in this lecture
come from “Learning Perl”, 3rd Ed,
R. Schwartz & T. Phoenix, O’Reilly, 2001
Topics
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Getting Started
scalars
lists and arrays
hashes
I/O
File handles
regular expressions
Hello, World!
#!/usr/bin/perl
print "Hello, World!\n";
#!/usr/bin/perl
@lines = `perldoc -u -f atan2`;
foreach (@lines) {
s/(\w)<([^>]+)>/$1<\U$2>/g;
print;
}
A More Complicated Example
#!/usr/bin/perl -w
open(FIND,"find . -print |") || die "Couldn't run find: $!\n";
FILE:
while ($filename = <FIND>) {
chomp($filename);
next FILE unless -T $filename;
if (!open(TEXTFILE, $filename)) {
print STDERR "Can't open $filename--continuing...\n";
next FILE;
}
while (<TEXTFILE>) {
foreach $word (@ARGV) {
if (index($_,$word) >= 0) {
print "$filename: $word\n";
next;
}
}
}
}
Getting Help
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man perl
perldoc
Learning Perl
Programming Perl
www.cpan.org, www.pm.org,
www.perlmonks.org
scalars
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numbers: 3, 3.14159, 7.24e15
strings: ‘fred’, “barney”, “hello\n”
variables: $name, $count
assignment: $name = ‘fred’; $count = 1;
$count += 1; $name = $fred . ‘flinstone’;
• special variables: $_
operators
• numbers
– 2+3, 5.1-2.4, 3 * 12, 14/2, 10/3
– ==, !=, <, >, <=, >=
• strings
– concatenation: str1 . str2
– replication: str x num
– eq, ne, lt, gt, le, ge
print
• single vs. double quotes
– ‘$firstname flinstone’
– “$firstname flinstone”
• print “My name is $name\n”
• print $firstname, $lastname, “\n”
• STDOUT, STDERR
Conditionals
• Boolean value (any scalar value)
– false: undef, 0, ‘’, ‘0’
– true: everything else
$count = 10;
if ($count > 0) {
print $count, “\n”;
$count -= 1;
} else {
print “blast off\n”;
}
Loops
$count =10;
while ($count > 0) {
print $count, “\n”;
$count -= 1;
}
print “blast off\n”;
Getting User Input
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line input operator: <STDIN>
$line = <STDIN> # includes \n
chomp removes \n
chomp($line)
Sum Odd Numbers
#!/usr/bin/perl
#Add up some odd numbers
$n = 1;
print "How many odd numbers do you want to add? ";
$howmany = <STDIN>;
chomp($howmany);
while ($n <= $howmany) {
$sum += 2*$n - 1;
$n += 1;
}
print "The sum of the first $howmany odd numbers = $sum\n";
Exercise 2.1
• Write a program that computes the
circumference of a circle with radius 12.5.
Use $pi = 3.141592654
Exercise 2.2
• Modify the previous program to prompt and
read the radius
Exercise 2.3
• Modify the previous program so that if the
radius is less than zero, the circumference is
set to zero.
Exercise 2.4
• Write a program that prompts for and reads
two numbers, on separate lines, and prints
their product.
Exercise 2.5
• Write a program that prompts for and reads
a string and a number (on separate lines)
and prints the string the number of times
indicated by the number (on separate lines).
Arrays and Lists
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Used interchangeably
List variables @name
List literals (“fred”,2,3)
@primes = (2,3,5,7,11,13,17)
@range = 1..10
Accessing elements: $primes[3]
Length of a list: $#primes
List assignment: ($p1, $p2, $p3) = (2,3,5)
List Operators
• @array = 1..5;
• The pop operator removes the last element of a list
• $last = pop(@array);
– @array = (1,2,3,4); $last=5
• The push operator appends an element to the end
of a list
• push(@array,5);
– @array = (1,2,3,4,5)
List Operators
• @array = 1..5;
• The shift operator removes the first element of a
list
– $first = shift(@array);
– $first = 1; @array = (2,3,4,5)
• The unshift operator prepends an element to the
beginning of a list
– unshift(@array,1);
– @array = (1,2,3,4,5)
List Operators
• @array = 1..5;
• The reverse operator reverses the elements
of a list
– @rarray = reverse(@array);
• The sort operator sorts the elements of a list
– @sarray = sort(@rarray);
– @students = (“Sam”, “Fred”, “Anna”, “Sue”);
– print sort(@students);
foreach Control Structure
foreach $i (1..10) {
print “$i\n”;
}
foreach (1..10) {
print “$_\n”;
}
Reading Lines
#!/usr/bin/perl
chomp(@lines = <STDIN>); # read lines, not newlines
foreach $line (@lines) {
print "$line\n";
}
Exercise 3.1
• Write a program that reads a list of strings
on separate lines until end-of-input and
prints the list in reverse order.
Exercise 3.2
• Write a program that reads a list of numbers
on separate lines until end-of-input and then
prints for each number the corresponding
person’s name from the list
– fred betty barney dino wilma pebbles bammbamm
Exercise 3.3
• Write a program that reads a list of strings
on separate lines until the end-of-input.
Then it should print the strings in
alphabetical order.
Hashes
• An array that can be indexed by arbitrary
strings
• $family_name{“fred”} = “flintstone;
• $family_name{“barney”} = “rubble”;
foreach $person in keys( %family_name ) {
print “Full name = $family_name{$person}\n”;
}
Hashes
• The hash as a whole is referred to by a
variable whose name starts with %
– %hash = (“barney”, “rubble”, “fred”,
“flinstone”);
– %hash =(“barney” => “rubble”,
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“fred” => “flinstone”);
– @key-value-list = %hash
Hashes
• To obtain the keys in a hash
– @first_names = keys(%hash);
• To obtain the values in a hash
– @last_names = values(%hash);
The each Function
• You can loop over the key-value pairs in a hash
while ( ($key, $value) = each %hash ) {
print “$key => $value\n”;
}
• The order is not specified – use sort if you care.
foreach $key (sort keys %hash) {
$value = $hash{$key};
print “$key => $value\n”;
}
The exists Function
• You can query to see if an entry with a
given key has been inserted into a hash
if (exists $last_name{$person}) {
print “$person has a last name\n”;
}
Deleting Entries from a Hash
• delete($family_name{fred});
Exercise 5.1
• Write a program that will ask the user for a
given name and report the corresponding
family name.
Exercise 5.2
• Write a program that reads a series of words
(with one word per line) until end-of-input,
then prints a summary of how many times
each word was seen.