Sandhya.np14 at 16:55, 25 July 2019

2019-07-25T16:55:16Z

Sandhya.np14 at 09:16, 10 July 2019

2019-07-10T09:16:36Z

PoojaMoolya: Created page with "{|border=1 | '''Time''' | '''Narration''' |- |00:01 | Hello and Welcome to this spoken tutorial on '''Loops''' in '''awk'''. |- | 00:06 | I..."

2018-12-14T06:22:27Z

Created page with "{|border=1 | <center>'''Time'''</center> | <center>'''Narration'''</center> |- |00:01 | Hello and Welcome to this spoken tutorial on '''Loops''' in '''awk'''. |- | 00:06 | I..."

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{|border=1
| <center>'''Time'''</center>
| <center>'''Narration'''</center>

|-
|00:01
| Hello and Welcome to this spoken tutorial on '''Loops''' in '''awk'''.

|-
| 00:06
| In this tutorial we will learn about- '''while'''

'''do-while''', '''for ''' and more '''looping constructs '''in''' awk'''

|-
| 00:16
|We will do this through some examples.

|-
| 00:20
| To record this tutorial, I am using

'''Ubuntu Linux 16.04 Operating System '''and''' ''' '''gedit text editor 3.20.1'''

|-
| 00:32
|You can use any text editor of your choice.

|-
| 00:36
| To practice this tutorial, you should have gone through the previous '''awk''' tutorials on our website.

|-
| 00:43
| You should have familiarity with any '''programming language''' like C or C++

|-
| 00:50
| If not, then please go through the corresponding tutorials on our website.

|-
| 00:56
| The files used in this tutorial are available in the '''Code Files''' link on this tutorial page.

Please download and extract them.

|-
| 01:06
| A '''loop''' allows us to perform one or more actions repeatedly.

|-
| 01:12
|'''while, do-while '''and''' for '''are the available '''loops''' in '''awk'''.

|-
|01:18
| The syntax of '''while''' '''loop''' can be seen here.

|-
| 01:22
|'''While loop '''first checks whether the specified '''condition''' is true.

|-
| 01:27
|If yes, then it executes the code within the '''body.'''

This '''loop '''will be repeated as long as the specified '''while condition '''is '''true'''.

|-
| 01:37
| We will use the same '''awkdemo.txt '''file, that we have used earlier.

|-
| 01:43
| I have already written a script named '''while_loop.awk'''

|-
| 01:48
|The same file is available in the '''Code Files''' link of this tutorial.

|-
| 01:53
| Here we have set the '''field separator''' as '''Pipe symbol.'''

|-
| 01:58
| Initially, we have to the set the value of the '''loop variable i '''as 1.

|-
| 02:04
| Here, we have taken one more variable '''f''' and initialized it to 1.

|-
| 02:10
|'''Variable f''' represents the '''field counter '''or the position of the '''fields''' for each '''record'''.

|-
| 02:17
| Now, in the '''while condition''', we check if '''i''' is less than or equal to 3.

|-
| 02:23
| If yes, then it will print the value in the '''f'''<sup>th</sup>''' field, '''for that record in '''awkdemo.txt '''file.

|-
| 02:31
| Then we will increment the '''field counter f''' by 1.

|-
| 02:36
| After that, we will also increment the value of '''loop variable i''' by 1.

|-
| 02:43
| This '''printf''' is for printing a '''newline character''' at the end of each row.

|-
| 02:49
| This '''loop''' will be executed for all the records in the '''awkdemo.txt''' file.

|-
| 02:55
|Which means the first 3 fields will get printed for each record.

|-
| 03:00
| Let’s execute this code now.

|-
| 03:03
| Open the '''terminal''' by pressing '''Ctrl, Alt''' and '''T''' keys.

|-
| 03:09
| Go to the folder in which you downloaded and extracted the '''Code Files''' using '''cd command'''

|-
|03:16
| Now type: '''awk space hyphen small f space while_loop.awk space awkdemo.txt'''

Press '''Enter'''

|-
| 03:29
| Observe that we get the first three '''fields''' of all the rows in the output.

|-
| 03:35
| Let us do the same with the '''do-while loop'''.

|-
| 03:38
| The syntax of '''do-while loop''' can be seen here.

|-
| 03:42
|The '''do-while loop''' always executes the code inside the '''body''' once.

|-
| 03:47
|Then it checks the specified '''condition.''' And repeats the code inside the '''body''', as long as the specified '''condition'''is '''true'''.

|-
| 03:56
| I have already written a script and named it as '''do_loop.awk'''

The same file is available in the '''Code Files '''link.

|-
| 04:06
| In this code, these are the '''statements '''within the '''do loop, '''which will be executed first.

This is the '''condition '''that will be checked.

|-
| 04:15
|After that, the '''statements '''inside the '''loop '''will be executed repeatedly as long as the '''condition '''is '''true'''.

|-
| 04:23
| This '''loop''' will iterate for all the records in the '''awkdemo.txt''' file

Which means the first 3 fields will get printed for all the records.

|-
| 04:33
| Let’s switch to the '''terminal. '''Let me clear the''' terminal.'''

|-
| 04:38
|Now type: '''awk space hyphen small f space do underscore loop dot awk space awkdemo dot txt'''

Press '''Enter'''

|-
| 04:52
| We get the same output. Then why do we have both '''while '''and '''do-while loops'''?

|-
| 04:58
|Let us understand the difference.

|-
| 05:00
| Switch to the file '''while underscore loop dot awk'''

|-
| 05:05
|Now, change the value of '''loop counter i '''from 1 to 4.

|-
| 05:11
| This will make the specified '''condition false''' from the beginning.

So this means, we should not get any output.

|-
| 05:19
| Save the file and switch to the '''terminal'''.

|-
| 05:22
| Clear the '''terminal'''.

Now press the Up arrow key until you get the command for executing the '''while loop.'''

|-
| 05:30
|Now press '''Enter.'''

|-
| 05:32
| See, we are not getting any output apart from blank lines.

|-
| 05:37
|For each record in the '''awkdemo.txt''' file, blank lines are getting printed in the output.

|-
| 05:44
| Now, let us make some changes in the '''do loop '''file.

|-
| 05:48
| Switch to the file '''do underscore loop dot awk'''

|-
| 05:53
|Change the value of '''i '''from 1 to 4.

|-
| 05:57
|Save the file and switch to the '''terminal.'''

|-
| 06:01
| Clear the '''terminal'''.

Now press the Up arrow key until you get the command for '''do loop.'''

Press '''Enter.'''

|-
|06:10
| In the output, only the first field for each row is printed.

What is the reason?

|-
| 06:16
| For each row, '''awk''' first prints the value at the first''' field, ''' because value of '''variable f '''is initialized to '''1.'''

Then the '''condition '''is checked.

|-
| 06:28
|Since the value of the '''loop counter i '''is 4, then the '''condition '''is '''false'''.

Hence, the '''loop '''is terminated there only, for that record.

|-
| 06:39
| This '''loop''' will iterate for all the records in the '''awkdemo.txt''' file.

|-
| 06:44
|Which means the first '''field''' for each record will get printed.

|-
| 06:49
| We are getting the output at least once for each record.

|-
| 06:53
|Use the '''do-while loop, '''for a job to be executed at least once, irrespective of any other '''condition'''.

|-
| 07:01
| We can do the same with the '''for loop''' also.

|-
| 07:05
| The syntax of '''for loop '''can be seen here.

|-
| 07:09
|The '''for statement''' starts by executing '''''initialization'''''.

|-
| 07:14
|Then, as long as the '''''condition''''' is true, it repeatedly executes the '''''statements''' ''within and then '''''increments'''''

|-
| 07:23
|Assuming your familiarity with a language like '''C''' or '''C++''', I am not explaining the syntax in detail.

|-
| 07:30
| This is how '''for loop '''for this '''condition''' looks like.

|-
| 07:35
|Here, '''initialization, condition''' checking and variable incrementation are done in the same line.

|-
| 07:43
|Try this out by yourself.

|-
| 07:46
| There are some more''' looping constructs '''

'''break''', '''continue''', '''exit'''

|-
| 07:53
|We will see some relevant examples on these in further tutorials.

|-
| 07:58
| We may have a single and multiline '''comments''' in our file.

|-
| 08:03
|Here notice that the single line '''comments''' are declared with single '''hash '''(#) symbol'''.'''

|-
| 08:10
|The multiline '''comments''' are declared with the help of double '''hash '''(##) symbol.

|-
| 08:16
| Now, there is no point of checking and printing these '''comments''' in the output.

|-
| 08:22
|We have to skip the lines starting with '''hash '''(##) symbol.

How can we do this?

|-
| 08:28
| Recall the case of giving 50% increment in the stipend for those who are getting more than 8000.

|-
| 08:36
|We will use the same example for skipping the '''comments'''.

|-
| 08:40
| I have created a file named''' next.awk '''as shown here for this execution.

|-
|08:47
| Now, what does this command mean?

|-
| 08:50
| '''awk''' will search for the pattern, '''caret sign hash symbol(^#) '''at the beginning of each line.

|-
| 08:57
|If the pattern is found, the keyword '''next '''tells '''awk''' to skip the current line immediately.

|-
| 09:04
|Then '''awk''' will start processing from the next line in the file.

This will save the processing time.

|-
| 09:12
| Switch to the '''terminal''' and type the command as shown here and

Press '''Enter.'''

|-
| 09:20
| We get the output without any '''comments.'''

|-
| 09:24
| Suppose, we have the students’ records in multiple files with the same format.

Say in '''awkdemo_mod.txt''' and '''awkdemo2.txt'''

|-
| 09:37
| See, it is similar to our previous file.

|-
| 09:41
|It also has '''comments''' preceeded with '''hash ''' sysmbol.

|-
| 09:45
|And it has large text at the end with double '''hash #<nowiki>#</nowiki>''' symbol.

|-
| 09:50
| So our data is in two different files.

'''awk''' should process both the files to give an increment to all the students.

|-
| 09:59
|Once we reach double '''hash'''(##) symbol of the first file, '''awk''' should stop processing that file entirely.

|-
| 10:06
|Then it has to start the execution from the next file.

This will save the processing time.

|-
| 10:13
| Modify the '''next.awk '''as shown here.

|-
| 10:17
|I have added '''dollar zero tilde slash caret symbol double hash slash within braces nextfile semicolon '''below the '''begin statement'''.

|-
| 10:29
|This will search for double '''hash #''' symbol at the beginning of each line.

|-
| 10:34
|If found, '''awk''' will skip the current file to process the next file.

|-
| 10:39
| Save this file.

|-
|10:41
| Switch to the the '''terminal '''and type the following command.

Press''' Enter'''

|-
| 10:48
| See, we are getting the output from both the files.

|-
| 10:53
| This brings us to the end of this tutorial.

Let us summarize.

|-
| 10:58
|In this tutorial we learnt about- '''while''', '''do… while''', '''for''', '''next''', '''nextfile '''in''' awk'''

|-
| 11:06
| As an assignment for the student records of '''awkdemo2.txt''', print only even '''fields'''(i.e. '''field''' 2, '''field''' 4 etc, irrespective of how many '''fields''' are there in the '''input file'''.

|-
| 11:22
| The video at the following link summarises the Spoken Tutorial project.

Please download and watch it.

|-
| 11:30
| The Spoken Tutorial Project team conducts workshops using spoken tutorials and gives certificates on passing online tests.

For more details, please write to us.

|-
| 11:43
| Do you have questions in THIS Spoken Tutorial?

Please visit this site.

|-
| 11:49
| Spoken Tutorial Project is funded by NMEICT, MHRD, Government of India.

More information on this mission is available at this link.

|-
| 12:01
| This script has been contributed by Antara.

And this is Praveen from IIT Bombay signing off.

Thanks for joining.

|}

@@ Line 411: / Line 411: @@
 |-
 | 09:41
-|It also has '''comments''' preceded with '''hash ''' sysmbol.
+|It also has '''comments''' preceded with '''hash ''' symbol.
 |-

Linux-AWK/C2/Loops-in-awk/English-timed - Revision history

Sandhya.np14 at 16:55, 25 July 2019

Sandhya.np14 at 09:16, 10 July 2019

PoojaMoolya: Created page with "{|border=1 | '''Time''' | '''Narration''' |- |00:01 | Hello and Welcome to this spoken tutorial on '''Loops''' in '''awk'''. |- | 00:06 | I..."