Classnote for COMS6100

Transcription

1 Classnote for COMS6100 Yiting Wang 3 November, 2016 Today we learn about subroutines, references, anonymous and file I/O in Perl. 1 Subroutines in Perl First of all, we review the subroutines that we had learned last class. 1.1 Syntax in subroutines The syntax in subroutines should be in this format: sub name{ # copy the arguments # check the arguments # perform computation # return results } Call a subroutines by it s name, generally followed by parentheses enclosing any arguments. And the parentheses are not needed if the subroutine is declared beforehand. 1.2 Arguments in subroutines The arguments in subroutines could be passed by following ways: 1. pass by value. 2. pass by reference. 3. We always to pass the array, the computer will fill in the blanks by itself. 1

2 1.3 Exercise in subroutines We do a small exercise which ask us to write a script for a subroutines: #! usr/bin/perl use strict; use warnings; my $a = 1; my $b = 2; add($a, $b); sub add{ my ($num1, $num2) my $c = $a + $b; return $c; } 2 References in Perl 2.1 References The references are the action of mentioning or alluding to something. So we can use the references instead of using the original value. Here are something we should pay attention to: References are scalars. So we should use $ before the references. Create a reference with the backslash operator: / before the variable.(scalar, array or hash) Perl References seem like a nickname or alias to another variable. For example, if we want to make a reference for an array, we can do like in this way: my $fruitsref = /@fruits; 2.2 Deferences Dereferencing is same as using the original variable. Variable Type Commands Scalar $ Hash % 2

3 For example, if we want to make a deference for the previous array, we can do like in this If we want to deal with the array or hash by using the references, we should use arrow operator >. If we change the value of the original variable, then we will also get the changed value when we use the deference. The following table are the commands for how to deference in different situations. Variable Type Scalar Array Hash Array s element Hash s element Commands $ {$arrayref} % {$hashref} $arrayref > [$index] $hashref > {key} Tips: we can use $#array to find the size of this array. 3 Anonymous Data in Perl We can create anonymous data, which is a reference. For example: my $fruitsanonymous = ["apple", "bananas", "cherries"]; $fruitsanonymous is a reference for this anonymous array. We use the different commands for the different type of the anonymous data: Anonymous Type Commands Array [ ] Hash { } TwodimensionArray [ [ ] ] 4 File I/O in Perl While we write some programs, we always have the data file to input the data for calculating or withdrawing. So how to read the data from a file is very important and necessary. The syntax of how to open a file is: 3

4 open( FILEHANDLE, FILENAME_WITH_ACCESS_TYPE); The commands open allows access to a file. The FILEHANDLE is always used instead of the file name. In this way, the scripts will be more easier to read and write. FILENAME WITH ACCESS TYPE include the file name and different type to access the file, specify access type with redirection characters (e.g., <, >, etc.): After we open a file, we should always test if the file is open correctly. Access Type Commands Read (default) < Write > Append >> Read & Write + < Etc. The special variable, $! shows that if it contains any IO error messages. The command will always be like this format: open( INPUT, "< datafile") or die( "Can t open input file, datafile: $!"); After we deal with the file, it is necessary to close the file. Otherwise, there will occur a big disaster that we are not expected. The command will always be like this format: close FILEHANDLE; 5 Standard Files in File I/O Here are three different types of standard files: STDIN: is standard input. STDOUT(default): is standard output. STDERR: is standard error output. We can read the file line by line in this way: my $pet = <STDIN>; chomp $pet; # Read a line from STDIN # Remove newline or in another way: my $pet = <>; chomp $pet; The syntax for using the STDOUT: 4

5 print FILEHANDLE STRING; The syntax for die and warn use STDERR be default: die STRING, warn STRING; And it is a good news to know that Perl allows you to re-open standard files. So we can do as many times as we want. 5