Nancyvarkey at 06:33, 25 July 2015

2015-07-25T06:33:04Z

Nirmala Venkat: Created page with "'''>>Title of script''':''' File Handling''' '''Author: Nirmala Venkat''' '''Keywords: Open File, Close File, Read Mode, Write Mode, Append Mode, FILEHANDLE, video tutorial'..."

2015-07-24T05:36:07Z

Created page with "'''>>Title of script''':''' File Handling''' '''Author: Nirmala Venkat''' '''Keywords: Open File, Close File, Read Mode, Write Mode, Append Mode, FILEHANDLE, video tutorial'..."

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'''>>Title of script''':''' File Handling'''

'''Author: Nirmala Venkat'''

'''Keywords: Open File, Close File, Read Mode, Write Mode, Append Mode, FILEHANDLE, video tutorial'''

{| style="border-spacing:0;"
| style="background-color:#c0c0c0;border:1pt solid #000000;padding:0.097cm;"| '''Visual Cue'''
| style="background-color:#c0c0c0;border:1pt solid #000000;padding:0.097cm;"| '''Narration'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 1:

| style="border:1pt solid #000000;padding:0.097cm;"| Welcome to the''' Spoken Tutorial''' on''' File Handling in PERL.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 2:

| style="border:1pt solid #000000;padding:0.097cm;"| In this tutorial we will learn how to

* Open a file in '''read mode'''
* Write to a file
* Open a file in '''append mode'''
* Close the file handle

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 3:

| style="border:1pt solid #000000;padding:0.097cm;"| For this tutorial, I am using

* '''Ubuntu Linux 12.04''' operating system
* '''Perl 5.14.2'''
* and the''' gedit''' Text Editor

You can use any text editor of your choice.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 4:

| style="border:1pt solid #000000;padding:0.097cm;"| To follow this tutorial, you should have working knowledge of '''Perl''' programming.

If not, then go through the relevant''' Perl''' spoken tutorials on the''' spoken tutorial''' website.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 5:

| style="border:1pt solid #000000;padding:0.097cm;"|
The basic operations we can do with files in '''Perl '''are:

* open a file
* read from a file
* write to a file and
* close a file
|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Slide 6:
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| The default file handles are

* '''STDIN'''
* '''STDOUT'''
* '''STDERR'''

i.e '''standard input, standard output '''and '''standard error.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 6:

'''Opening a File:'''

'''Syntax:'''
open (FILEHANDLE, MODE, EXPR);

'''Example:'''
open (DATA<nowiki>, “<”, “</nowiki>First.txt”);

open (DATA<nowiki>, “<</nowiki>First.txt”);
| style="border:1pt solid #000000;padding:0.097cm;"| This is the syntax for '''open '''function.

In the syntax''', FILEHANDLE''' is the''' file handle''' returned by the '''open '''function.

'''MODE''' represents the mode of opening the file.
For eg: read, write, etc.

'''EXPR''' is the physical filename used to read or write. In this case, '''“First.txt” '''is the filename.

There is another way to write the '''open '''function, as shown here.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us understand how to open an existing file and read the data from it.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the '''Terminal and type'''

'''gedit first.txt'''
| style="border:1pt solid #000000;padding:0.097cm;"| First we will create a text file and store some data in it.

Go to the '''terminal''' and type''' gedit first.txt''' and press '''Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Point to the filename first.txt in the Titlebar of gedit.'''
| style="border:1pt solid #000000;padding:0.097cm;"| In the '''first dot txt''' file, type the following text:
'''Welcome to File Handling'''

'''Chapter 1'''

'''Chapter 2'''

'''Chapter 3'''

Save the file and close''' gedit'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now we will look at a '''Perl '''program that opens the file''' first.txt''' and reads the content.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| On the '''terminal >> '''type '''gedit openfile dot pl ampersand''' >> press''' Enter'''.
| style="border:1pt solid #000000;padding:0.097cm;"| Let me open the sample program''' openfile.pl''' which I have already saved.

Type''' gedit openfile dot pl ampersand''' and press''' Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| In the '''openfile dot pl''' file, type the following code as displayed on the screen.

Let us understand the code now.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''<nowiki>#!/</nowiki>usr/bin/perl'''

'''open(DATA<nowiki>, "<</nowiki>” ,”first.txt") or die "Couldn't open file first.txt, $!";'''

| style="border:1pt solid #000000;padding:0.097cm;"| The''' open''' '''function''' opens a file for reading.

The first parameter''' DATA''' is the''' filehandle''' which allows''' Perl''' to refer to the file in future.

The second parameter '''<nowiki>“<”</nowiki>''' less than symbol denotes the '''READ''' mode.

If you fail to specify the “'''Mode'''”, by default the file will be opened in “'''READ'''” mode.

The third parameter''' first.txt''' is the''' filename''' from where the data has to be read.

What will happen if the file''' first.txt''' does not exist?

The script will die with the appropriate error message stored in the''' dollar exclamation ($!)'''variable.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''while<nowiki>(<</nowiki>DATA>){'''

'''print "$_";'''

'''}'''
| style="border:1pt solid #000000;padding:0.097cm;"| The '''while '''loop will:
* read line by line
* and loop through the'''<nowiki> <</nowiki>DATA>''' file
* until all the lines have been read.

Print''' dollar underscore''' ('''$_)''' variable will print the contents of the current line.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''close (DATA);'''
| style="border:1pt solid #000000;padding:0.097cm;"| Lastly, close the file with the''' FILEHANDLE''' name, which we had given in the '''open''' statement.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Closing a file prevents any accidental file changes or overwriting of the content.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl+S'''
| style="border:1pt solid #000000;padding:0.097cm;"| Now, press''' Ctrl+S''' to save the file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| On the''' terminal''' type
'''perl openfile dot pl'''
>> press''' Enter'''.
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the''' terminal''' and type

'''perl openfile dot pl'''
and press''' Enter'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight

'''Welcome to File Handling'''

'''Chapter 1'''

'''Chapter 2'''

'''Chapter 3'''
| style="border:1pt solid #000000;padding:0.097cm;"| The output is displayed as shown.

This is the same content that we saw earlier in '''first dot txt '''file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Next we will see how to write data into a file.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 7:

'''Opening a file for writing:'''

'''Syntax:'''
open (FILEHANDLE, “> filename”)

'''Example:'''
open (DATA, “ >write.txt”);
| style="border:1pt solid #000000;padding:0.097cm;"| The '''open '''statement with greater than''' (>)''' symbol defines the '''WRITE''' mode.

'''Filename''' represents the name of the file where the data has to be written.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the '''terminal.'''
| style="border:1pt solid #000000;padding:0.097cm;"| Let me open the sample program''' writefile.pl''' which I have already saved.

Switch to the '''terminal.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Type''' gedit writefile dot pl ampersand''' >>press '''Enter.'''
| style="border:1pt solid #000000;padding:0.097cm;"| Now, type''' gedit writefile dot pl ampersand''' and press '''Enter.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| In the '''writefile dot pl''' file, type the following code as displayed on the screen.

Let me explain the code now.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''<nowiki>#!/</nowiki>usr/bin/perl'''

'''open FILE1, ">second.txt" or die "Couldn't open file second.txt, $!";'''

'''print FILE1 "Working with files makes data storage and retrieval a simple task! \n";'''
| style="border:1pt solid #000000;padding:0.097cm;"| The '''open function '''opens a file '''second.txt''' in '''write mode.'''

“>” Greater than symbol before the filename denotes the '''write mode.'''

The first parameter '''FILE1''' is the''' FILEHANDLE'''.

The '''print function''' prints the given text to''' FILEHANDLE'''. i.e''' FILE1'''
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl+S'''
| style="border:1pt solid #000000;padding:0.097cm;"| Now, press''' Ctrl+S''' to save the file

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to terminal

'''perl writefile dot pl >> '''press''' Enter'''.
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the '''terminal''' and type

'''perl writefile dot pl'''
and press''' Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now let us check whether the text has been written in '''second.txt''' file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| In terminal >> type''' gedit second.txt''' and press '''Enter'''.
| style="border:1pt solid #000000;padding:0.097cm;"| Type''' gedit second.txt''' and press '''Enter'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight in second.txt file
| style="border:1pt solid #000000;padding:0.097cm;"| We can see the text''' Working with files makes data storage and retrieval a simple task! '''In our second.txt file.

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"|
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Let us close the '''second.txt '''file.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| What will happen if we open the same file again in '''write mode'''?

Let us see that.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to '''writefile dot pl.'''
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to '''writefile dot pl.'''
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''<nowiki># </nowiki>print FILE1 "Working with files makes data storage and retrieval a simple task! \n";'''

'''print FILE1 " > overwrites the content of the file! \n";'''
| style="border:1pt solid #000000;padding:0.097cm;"| Comment the previous '''print''' '''Statement'''.

Add the below '''print''' command.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl+S'''
| style="border:1pt solid #000000;padding:0.097cm;"| Now, press''' Ctrl+S''' to save the file.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| On '''terminal''' >> type

'''perl writefile dot pl'''
>> press''' Enter'''.
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the '''terminal''' and type
'''perl writefile dot pl'''
and press''' Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now let us check the''' second.txt''' file once again.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| In terminal, type''' gedit second.txt''' >> press '''Enter'''.
| style="border:1pt solid #000000;padding:0.097cm;"| Type''' gedit second.txt''' and press '''Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight in second.txt file
| style="border:1pt solid #000000;padding:0.097cm;"| We can see “'''Greater than symbol (>) will overwrite the content of the file!" '''

The previous content of the '''second.txt''' file has been overwritten.

This is because we had opened the file in the '''write mode''' again.
|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"|
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Let us close the '''second.txt''' file.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Next, we will see how to append data to an existing file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 8:

'''Opening a file for Appending:'''

'''Syntax:'''
open (FILEHANDLE, “>> filename”)

'''Example:'''
open (DATA, “ >>write.txt”);
| style="border:1pt solid #000000;padding:0.097cm;"| The '''open '''statement with two '''greater than''' (>> ) symbols denotes the '''APPEND''' mode.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Point to writefile.pl
| style="border:1pt solid #000000;padding:0.097cm;"| Now I will open the '''writefile dot pl''' again in '''gedit'''.
|-
| style="border-top:1pt solid #000000;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1.5pt solid #000000;padding:0.097cm;"| Type two '''greater (>>) than''' symbols.
| style="border:1.5pt solid #000000;padding:0.097cm;"| In the '''open''' statement, type two '''greater (>>) than''' symbols.

This will denote that the file is in '''append mode.'''
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Comment
| style="border-top:1.5pt solid #000000;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Comment the previous '''print''' '''statement''' as it is already executed.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Add the line as per narration
| style="border:1pt solid #000000;padding:0.097cm;"| Add the line

'''print FILE1 "Two greater than symbols >> opens the file in append mode"'''
to append to the existing data.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl+S'''
| style="border:1pt solid #000000;padding:0.097cm;"| Now, press''' Ctrl+S''' to save the file.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| On the terminal >> type

'''perl writefile.pl& >> '''Press '''Enter'''
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the '''terminal''' and type
'''perl writefile dot pl'''

and press''' Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now let us check whether the text has been appended to the''' second.txt''' file.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Type''' gedit second.txt''' >> press '''Enter'''.
| style="border:1pt solid #000000;padding:0.097cm;"| Type''' gedit second.txt''' and press '''Enter'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight the appended text
| style="border:1pt solid #000000;padding:0.097cm;"| We can see the text has been appended in our''' second.txt''' file.
|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"|
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Let us close the '''second.txt''' file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Similarly, there are other modes also.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 9:

'''File Attributes:'''
| style="border:1pt solid #000000;padding:0.097cm;"|
* <nowiki>+< </nowiki>file – READ, WRITE
* +> file – READ, WRITE, CREATE, TRUNCATE,
* +>> file - READ, WRITE, CREATE, APPEND
Try out these options on your own and understand what happens.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| This brings us to the end of this tutorial. Let us summarise.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 10:
Summary
| style="border:1pt solid #000000;padding:0.097cm;"| In this tutorial we learnt to
* Open a file in read mode
* Write to a file
* Open a file in append mode and
* Close the file handle
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 11:

'''Assignment'''

| style="border:1pt solid #000000;padding:0.097cm;"| Here is an assignment for you.

# Change the file attribute to''' +>''' in the''' writefile.pl''' program.
# Save and execute the program.
# Open the''' second.txt''' file to see the output.
# Analyse the usage of file attribute''' +>'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 12:

About Spoken Tutorial project

| style="border:1pt solid #000000;padding:0.097cm;"| The video at the following link summarises the Spoken Tutorial project.

Please download and watch it

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 13:

Spoken Tutorial workshops

The Spoken Tutorial Project Team

| style="border:1pt solid #000000;padding:0.097cm;"| The''' Spoken Tutorial Project''' Team
* conducts workshops using spoken tutorials and
* gives certificates on passing online tests.

For more details, please write to us.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 14:

Acknowledgement

| style="border:1pt solid #000000;padding:0.097cm;"| Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.

More information on this mission is available at
this link.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

|}

@@ Line 61: / Line 61: @@
 * '''STDOUT'''
 * '''STDERR'''
 |-
@@ Line 304: / Line 302: @@
 |-
 | style="border:1pt solid #000000;padding:0.097cm;"| Highlight in second.txt file
-| style="border:1pt solid #000000;padding:0.097cm;"| We can see “'''Greater than symbol (>) will overwrite the content of the file!" '''
+| style="border:1pt solid #000000;padding:0.097cm;"| We can see the output  “'''Greater than symbol (>) will overwrite the content of the file!" '''
 The previous content of the '''second.txt''' file has been overwritten.
@@ Line 343: / Line 341: @@
 '''print FILE1 "Two greater than symbols >> opens the file in append mode"'''
 to append to the existing data.
 |-

PERL/C3/File-Handling/English - Revision history

Nancyvarkey at 06:33, 25 July 2015

Nirmala Venkat: Created page with "'''>>Title of script''':''' File Handling''' '''Author: Nirmala Venkat''' '''Keywords: Open File, Close File, Read Mode, Write Mode, Append Mode, FILEHANDLE, video tutorial'..."