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

Title of script: Loops in Awk

Author: Antara Roy Choudhury

Keywords: for, while, do-while, next, nextfile

Visual Cue	Narration
Slide 1: Introduction	Welcome to this spoken tutorial on Loops in awk.
Slide 2: Learning Objective	In this tutorial we will learn about- while do-while for and more looping constructs in awk We will do this through some examples.
Slide 3: System requirement	To record this tutorial, I am using Ubuntu Linux 16.04 OS and gedit text editor 3.20.1 You can use any text editor of your choice.
Slide 4: Pre-requisite	* To practice this tutorial, you should have gone through the previous awk tutorials on our website. You should have familiarity with any programming language like C or C++ If not, then please go through the corresponding tutorials on our website.
Slide 5: Code Files	The files used in this tutorial are available in the Code Files link on this tutorial page. Please download and extract them.
Slide 6: Loops	A loop allows us to perform one or more actions repeatedly. while, do-while and for are the available loops in awk.
Slide 7: While loop	The syntax of while loop can be seen here. While loop first checks whether the specified condition is true. If yes, then it executes the code within the body. This loop will be repeated as long as the specified while condition is true.
Show awkdemo.txt file in gedit	We will use the same awkdemo.txt file, that we have used earlier.
show while_loop.awk in Gedit	I have already written a script named while_loop.awk The same is available in the Code Files link of this tutorial.
Point to the pipe symbol	Here we have set the field separator as Pipe symbol.
Highlight i = 1	Initially, we have to the set the value of the loop variable i as 1.
Highlight f = 1	Here, we have taken one more variable f and initialized it to 1. Variable f represents the field counter or the position of the fields for each record.
Highlight while (i<=3)	Now, in the while condition, we check if i is less than or equal to 3.
Highlight Print statement	If yes, then it will print the value in the fth field, for that record in awkdemo.txt file.
Highlight f++	Then we will increment the field counter f by 1.
Highlight i++	After that, we will also increment the value of loop variable i by 1.
Highlight printf("\n")	This printf is for printing a newline character at the end of each row.
	This loop will be executed for all the records in the awkdemo.txt file. Which means the first 3 fields will get printed for each record.
	Let’s execute this code now.
Open the terminal	Open the terminal by pressing Ctrl, Alt and T keys.
cd /<saved folder>	Go to the folder in which you downloaded and extracted the Code Files using cd command
Type in terminal: awk -f while_loop.awk awkdemo.txt [Enter]	Now type: awk space minus small f while_loop.awk space awkdemo.txt Press Enter
Show the output	Observe that we get the first three fields of all the rows in the output.
	Let us do the same with the do-while loop.
Slide 8: Do While Loop	The syntax of do-while loop can be seen here. The do-while loop always executes the code inside the body once. Then it checks the specified condition. And repeats the code inside the body, as long as the specified condition is true.
show do_loop.awk in Gedit	I have already written a script and named it as do_loop.awk The same file is available in the Code Files link.
Show the contents and highlight appropriately	In this code, these are the statements within the do loop, which will be executed first. This is the condition that will be checked. After that, the statements inside the loop will be executed repeatedly as long as the condition is true.
	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.
Type: awk -f do_loop.awk awkdemo.txt [Enter]	Let’s switch to the terminal. Let me clear the terminal. Now type: awk space hyphen small f do underscore loop dot awk awkdemo dot txt Press Enter
Scroll up and show outputs of while loop and do loop	We get the same output. Then why do we have both while and do-while loops? Let us understand the difference.
In while_loop.awk, assign i=4 [code uploaded as while_loop_mod.awk]	Switch to the file while underscore loop dot awk Now, change the value of loop counter i from 1 to 4.
Highlight (i<=3)	This will make the specified condition false from the beginning. So this means, we should not get any output.
Press Ctrl+S	Save the file and switch to the terminal.
awk -f while_loop.awk awkdemo.txt [Enter]	Clear the terminal. Now press Up arrow key until you get the command for executing the while loop. Now press Enter.
Show the output	See, we are not getting any output apart from blank lines. For each record in the awkdemo.txt file, blank lines are getting printed in the output.
	Now, let us make some changes in the do loop file.
In do_loop.awk, assign i=4 [code uploaded as do_loop_mod.awk]	Switch to the file do underscore loop dot awk Change the value of i from 1 to 4. Save the file and switch to the terminal.
awk -f do_loop.awk awkdemo.txt [Enter]	Clear the terminal. Now press the Up arrow key until you get the command for do loop. Press Enter.
Show the output	In the output, only the first field for each row is printed. What is the reason?
Highlight in code file	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. 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.
Highlight in code file	This loop will iterate for all the records in the awkdemo.txt file. Which means the first field for each record will get printed.
Highlight output	We are getting the output at least once for each record. Use the do-while loop, for a job to be executed at least once, irrespective of any other condition.
Retain same screen	We can do the same with the for loop also.
Slide 9: for loop	The syntax of for loop can be seen here. The for statement starts by executing initialization. Then, as long as thecondition is true, it repeatedly executes the statements within and then increments Assuming your familiarity with a language like C or C++, I am not explaining the syntax in detail.
show for_loop.awk in Gedit	This is how the for loop for this condition looks like. Here, initialization, condition checking and variable incrementation are done in the same line. Try this out by yourself.
Slide 10: looping constructs	There are some more looping constructs break continue exit We will see some relevant examples on these in further tutorials.
Show awkdemo_mod.txt	We may have a single and multiline comments in our file. Here notice that the single line comments are declared with single hash (#) symbol. The multiline comments are declared with the help of double hash (##) symbol.
Highlight	Now, there is no point of checking and printing these comments in the output. We have to skip the lines starting with hash (##) symbol. How can we do this?
	Recall the case of giving 50% increment in the stipend for those who are getting more than 8000. We will use the same example for skipping the comments.
show next.awk	I have created a file named next.awk as shown here for this execution.
Highlight $0~/^#/ {next;}	Now, what does this command mean?
Slide 11: next	awk will search for the pattern, caret sign hash symbol(^#) at the beginning of each line. If the pattern is found, the keyword next tells awk to skip the current line immediately. Then awk will start processing from the next line in the file. This will save the processing time.
Type awk -F "|" -f next.awk awkdemo_mod.txt [Enter]	Switch to the terminal and type the command as shown and press Enter.
Show the output	We get the output without any comments.
	Suppose, we have the students’ records in multiple files with the same format. Say in awkdemo_mod.txt and awkdemo2.txt
Show awkdemo2.txt	See, it is similar to our previous file. It also has comments preceeded with hash # sysmbol. And it has large text at the end with double hash ## symbol.
Slide 12: nextfile	So our data is in two different files. So, awk should process both the files to give an increment to all the students. Once we reach double hash(##) symbol of the first file, awk should stop processing that file entirely. Then it has to start the execution from the next file. This will save the processing time.
Type $0~/^#/ {next;} after the begin statement. Highlight $0~/^#/ {nextfile;}	Modify the next.awk as shown here. I have added dollar zero tilde slash caret symbol double hash slash within braces nextfile semicolon below the begin statement. This will search for double hash # symbol at the beginning of each line. If found, awk will skip the current file to process the next file.
Press Ctrl+s	Save this file.
Type awk -F "|" -f next.awk awkdemo_mod.txt awkdemo2.txt [Enter]	Switch to the the terminal and type the following command. Press Enter
Show the output	See, we are getting the output from both the files.
Slide 13: Summary	This brings us to the end of this tutorial. Let us summarize. In this tutorial we learnt about- while do… while for next nextfile in awk
Slide 14: Assignment	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.
Slide 13: About Spoken Tutorial project	The video at the following link summarises the Spoken Tutorial project. Please download and watch it.
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Slide 15: Forum for specific questions:	Do you have questions in THIS Spoken Tutorial? Please visit this site.
Slide 16: Acknowledgement	Spoken Tutorial Project is funded by NMEICT, MHRD, Government of India. More information on this mission is available at this link.
Slide 17: Thanks	The script has been contributed by Antara. And this is Praveen from IIT Bombay signing off. Thanks for joining.

Contributors and Content Editors

Antarade, Nancyvarkey