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

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Time
Narration
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 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 fth 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 conditionis 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.

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 preceded with hash symbol.
09:45 And, it has large text at the end with double hash ## 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.

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12:01 This script has been contributed by Antara.

And this is Praveen from IIT Bombay, signing off.

Thanks for joining.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14