Linux-AWK/C2/User-Defined-Functions-in-awk/English

Title of script: User Defined Function in Awk

Author: Antara Roy Choudhury

Keywords: Function definition syntax, Function call, Return statement, User defined function in awk

Visual Cue	Narration
Slide 1: Introduction	Welcome to this spoken tutorial on user-defined function in awk.
Slide 2: Learning Objective	In this tutorial we will learn about- The syntax of function definition Function call and Return statement 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: Prerequisite	To practice this tutorial, you should have gone through the earlier awk tutorials on this website. You should have some knowledge of 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: User defined functions	Now let us learn about user defined functions. The function syntax is as follows. And the syntax is self-explanatory.
In Slide 6, highlight function keyword	Here the keyword function is mandatory.
Slide 7: Function Call	To call a function, write the name of the function, followed by the arguments in parentheses. Note: space is not allowed between the function name and the open parentheses of the argument.
Slide 8: Example	We will see one example now. Now, in our awkdemo.txt file, the sixth field represents stipend. Assume that stipend is either zero or consists of four digits. Suppose, stipend is 8900. I would like to print it as 8 thousand 9 hundred. If stipend is 0, print as zero
Show user_function.awk in gedit	I have already written the code in a file named user_function.awk
Highlight function changeit(argval)	Here I have written a function named changeit with a single argument argval. Here value is basically our sixth field which is stipend.
Highlight if(argval==0)	Inside the function, first the code will check if argval is zero or not.
Highlight printf("zero")	If yes, it will print “Zero”
Highlight else part	If not, then the else part of the code will be executed.
Highlight a[1]=substr(value,1,1) a[2]=substr(value,2,1) a[3]=substr(value,3,1) a[4]=substr(value,4,1)	In the else part, first we will extract each digit one by one using the substr function. And will store the values in an array a at different indices.
Highlight a[1]=substr(value,1,1)	For example, a[1] will give the first digit from left hand side or the thousand’s place digit. Since we have only four digits, I used four iterations.
Highlight all the if loops inside else block	Next, we will check whether the elements are not equal to zero. And print them in a proper order.
Highlight printf("\n")	At the end, we print a backslash n character to provide new line break in the output.
Highlight $2 in the last print statement	Then inside the awk script, we have printed dollar 2, which is the second field which is name.
Highlight changeit($6)	Then we call the function changeit with the parameter dollar 6, which is stipend. Let’s execute the file.
Type: awk -F "|" -f user_function.awk awkdemo.txt [Enter]	In the terminal type the following command and press Enter.
Show the output	We get the output as expected.
Slide 9: Return statement	A user-defined function can also include a return statement. This statement returns control to the calling part of the awk program. It can also be used to return a value for use in the rest of the awk program.
Slide 9: Return statement	It looks like this:return space expression Here the expression part is optional.
	Let’s write a function to return average of an array.
Show in gedit average.awk	I have written the code in the file average.awk Let’s view the contents.
Highlight appropriately in function avg(arr, i, sum,n,ret)	We have defined a function named avg for this purpose. It has five parameters. arr is the array for which the average is to be calculated. i is array loop variable. sum is the summation of all array elements. n indicates the number of elements in array. ret represents the variable to be returned from the function avg. ret will store the calculated average.
Highlight extra space function avg(arr, i, sum,n,ret)	The extra space before i indicates that the variables i, n, sum and ret are local variables. Actually, the local variables are not intended to be arguments. You should follow this convention when defining functions.
Highlight inside for (i in arr) { n++ sum += arr[i] }	Inside the for loop, we have calculated the total number and summation of array elements.
Highlight ret=sum/n return ret	We have calculated the average by dividing summation with the total number of elements. And stored the value in variable ret. This function avg() returns the value of the variable ret.
Highlight appropriately in the file	Inside the BEGIN section we have defined array nums with 5 different numbers.
Highlight print avg(nums)	In the print statement, we called the function avg() with one argument, which is the array name. So, you do not have to pass local variables as arguments.
Type at command prompt awk -f average.awk [Enter]	To execute the file, at command prompt, type awk -f average.awk And press Enter.
Show the output	We get the output as 3.6. You can validate it by using a calculator.
	Let us look at one more example.
Show reverse.awk in Gedit	I have written a code to reverse a string and named it as reverse.awk recursive function is used to reverse a string. Pause the video here and look at the code to understand how the control flows. Then execute it to see the output.
Display Slide	As an assignment, use the function rev to reverse the Roll number field in awkdemo.txt file. For example, if the roll number is A001, output should be 100A. The code for the same is provided as reverse_roll.awk in the Code Files link.
Display Slide 9 Summary	This brings us to the end of this tutorial. Let us summarize. In this tutorial we learnt about- The syntax of function definition Function call and Return statement
Display Slide 10 Assignment	As an assignment- 1. Write a function to create a transpose of a 2D matrix. 2. Write a function to return the minimum value element from an array.
Display Slide 11 About Spoken Tutorial project	The video at the following link summarises the Spoken Tutorial project. Please download and watch it.
Display Slide 12 Spoken Tutorial workshops	The Spoken Tutorial Project team conducts workshops using spoken tutorials. And gives certificates on passing online tests. For more details, please write to us.
Display Slide 13 Forum for specific questions:	Please post your timed queries in this forum.
Display Slide 14 Acknowledgement	Spoken Tutorial Project is funded by NMEICT, MHRD, Government of India. More information on this mission is available at this link.
	The script has been contributed by Antara. And this is Praveen from IIT Bombay signing off. Thank you for joining

Contributors and Content Editors

Antarade, Nancyvarkey