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

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 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 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 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 ## 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.
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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