Perl Scripting=Practical Extraction and Report Language.
make sure /usr/local/bin/perl is in your path
how to run. -> $perl -wc hello_world.pl
To check errors use -c option.
To turn on warning use -w option.
die statement shows you a way to get out of a program when you are in trouble.
chomp is a function to remove the newline that's attached to our answer after hitting the carriage return.
Simple variables in Perl can have two types of values: integers and strings
➢There are also object variables (maybe see this later)
Integers: 1, 2, -10
Strings: sequences of characters, quoted either as ' ' or “ .... “
➢a string in between ' ' has value exactly the sequence of characters in between quotes
➢“ “ some substitutions occurs
$i=10;
$s1=' winter for the last $i months ';
$s2=” winter for the last $i months “;
print $i;
print $s1;
print $s2;
Result:
10
winter for the last $i months
winter for the last 10 months
$s3=”winter for the last \n $i months”
winter for the last \n stands for “new line”
10 months
Important to notice:
➢Unlike shell scripting, you use $var on the left side of an assignment
$i=10
➢Like in shell scripting, you do not need to make explicit the type of the variable
$i=10 # understood as an integer
$s=”10” # treated as a string
➢Everything in a Perl script is a statement, and statements must end in semicolon
$i=10;
$s1=' winter for the last $i months ';
$s2=” winter for the last $i months “;
print $i;
print $s1;
print $s2;
To echo values on the terminal display, use a print statement: print expr, ...., expr;
print 'winter ', “ for the last $i months, \n”, “unfortunately”
winter for the last 10 months,
unfortunately
Perl automatically converts a string to an integer or the other way around,
depending on the context:
$a=”10”
print “ a is $a \n”
$a1=$a + 20 + only makes sense as an integer operand
print “a1 is $a1 \n”
$a2=$a.” months” . (concatenation) only makes sense for strings
print “a2 is $a2 \n”
$a3=$a.$a1
print “ a3 is $a3 \n”
$a4=$a3-1
print “ a4 is $a4”
a is 10 integer
a1 is 30 integer
a2 is 10 months string
a3 is 1030 string
a4 is 1029 integer
Perl Operators
Arithmetic operators : + , -, *, /, %, ** (exponent) integers
unary +, -
Assignment operators: =, +=, -=, *=, /=,%=, **= integers
.= strings
Standard comparisons for integers: <, >, <=, >= , ==, !=
String comparison: eq, ne, lt, le, gt, ge (alphabetical order)
✔ “10”==10 # automatic conversion of string “10” to integer 10
✔ “ 10 “ == 10 # automatic conversion of string “ 10 “ to int 10
✗ “ 10 “ eq “10” # fails: first string has extra spaces
✔ “ 10 “ eq “ “.”10”.” “ # pass
Logical operators: && (and), || (or), ! (not)
✔ (“abc” lt “cde” ) && (“abc” lt “Abc”)
Conditionals
if (comparison) {
statement;
statement;
...
}
$i=1; # prints in order numbers from 1 to 10, on separate lines
if ($i <= 10) {
print “$i\n”; $i+=1;
}
$i=”1”;
until ( $s eq “10000” ) {
print “$s\n”; $s=$s.”0”
}
Loops
while (comparison) { for var (val, ..., val) { for (setup; cond; inc) {
statement; statement; statement;
statement; statement; statement;
.... ... ...
} } }
$i=1; for $i (2,4,6) { for ($i=1; $i<=10; $i+=1) {
while ($i<=10) { print “$i\n”; print “$i\n”;
print “$i\n”; } }
$i+=1;
}
File handling
Open a file myin.txt for reading open (inh, “
➢ inh is a file handler (think of it as a number the system assigns to the opened
file)
open (inh;”
while ($line=) { #reads the input file myin.txt line by line
print “$line”; # displays each line on standard output
}
close (inh);
Open a file myout.txt for writing open (outh, “>myout.txt”);
➢ if the file does not exist, it creates it
➢ if the file exists, it overwrites it
➢ open a file and append information to it open (outh, “>>myout.txt”);
open (inh;”
open (outh,”>>myout.txt”);
while ($line=) { #reads the input file myin.txt line by line
print outh “$line”; # appends each line to the output file
}
close (inh);
close (outh);
Dealing with errors in opening files:
if ( ! open (inh,”
print “Error opening myin.txt!\n”;
exit (1);
}
else { if (! open (outh,”>>myout.txt”)) {
print “Error opening myout.txt!\n”;
exit (1);
}
else {
while ($line=) {
print outh “$line”;
}
close (outh);
}
close (inh);
}
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