Linux-AWK/C2/Built-in-Variables-in-awk/English-timed

Time	Narration
00:01	Hello and Welcome to this spoken tutorial on built-in functions in awk.
00:07	In this tutorial we will learn about different types of built-in functions like- Arithmetic functions
00:15	String functions
00:17	Input/Output functions and Time-stamp functions
00:23	We will do this through some examples.
00:26	To record this tutorial, I am using Ubuntu Linux 16.04 operating system and gedit text editor 3.20.1
00:38	You can use any text editor of your choice.
00:42	To practice this tutorial, you should have gone through the earlier awk tutorials on this website.
00:49	You should have some knowledge of any programming language like C or C++.
00:56	If not, then please go through the corresponding tutorials on our website.
01:02	The files used in this tutorial are available in the Code Files link on this tutorial page. Please download and extract them.
01:12	Built-in functions are always available for awk to call.
01:17	First we will learn about the arithmetic functions. square root function (sqrt (x)) returns positive square root of a number x
01:27	int function truncates x to an integer value
01:32	exponential function gives the exponential of x
01:37	log function returns natural logarithm value of x
01:43	sin and cos gives sine(x) and cosine(x) respectively
01:49	Please note that argument x should be mentioned in radians.
01:55	Let’s look at an example to understand these functions.
02:00	I have already written the code in a file arithmetic underscore function dot awk The same is available in the Code Files link.
02:10	Here, we are printing the square root of a positive and negative number respectively.
02:17	Next we are printing the integer value for a positive and negative number respectively.
02:24	Then we are printing exponential of a small number and a very large number.
02:31	After that, natural logarithm of positive and negative numbers are printed.
02:38	We are also printing sine and cosine values of 0.52 radian, that is actually 30 degree. Let us execute the file in the terminal.
02:50	Open the terminal by pressing Ctrl, Alt and T Keys.
02:55	Next go to the folder where you have downloaded and extracted the file using cd command.
03:03	Now type awk space -f space arithmetic_function.awk And press Enter to see the output.
03:14	Couple of things are clear from this output.
03:18	sqrt() function gives square root of a positive number.
03:23	It returns nan or not a number if the number is negative.
03:29	int() gives the truncated integer of any positive or negative number.
03:36	exp() gives exponential of a number. If the number is very large, the function will return inf.
03:47	Natural logarithm of positive number is given by log() function.
03:53	If the number if negative, the function returns nan.
03:58	Sine and cosine functions return corresponding values. You can verify the value using your calculator.
04:07	Now, let us look at random functions.
04:11	rand() returns any random number between 0 and 1. But never returns 0 or 1.
04:21	Generated numbers will be random within one awk execution.
04:27	But predictable across different executions of the awk program.
04:33	srand(x) is used to provide seed value x for random function.
04:39	In absence of x, date and time of day is used as the seed value. Let us understand these with an example.
04:49	I have written a code for the random function and saved it as random.awk
04:56	Here, inside the for loop, rand() function will generate a random number between 0 and 1.
05:04	Then the generated number will be multiplied by 50 and get printed.
05:10	So, this code will generate 5 random numbers within 50.
05:16	Switch to the terminal and execute the file. Let me clear the terminal.
05:23	Type: awk space hyphen f space random dot awk and press Enter.
05:31	See, it is giving 5 random numbers.
05:35	What happens if I execute the code again?
05:39	Press the Up arrow key to get the previously executed command and press Enter.
05:47	We are getting the same output. Which means, awk is generating the same set of random numbers for every execution of the script.
05:57	Then how can we get a new set of random numbers in every execution? Switch to the code once again.
06:06	Before the for loop, type srand() function
06:11	Press Crtl and S keys to the save the file.
06:16	Now switch to the terminal.
06:19	Press the Up arrow key to get the previously executed command and press Enter.
06:27	It is giving a different set of random numbers.
06:31	So, we can generate a new set of random numbers using srand function, when it’s used without an argument.
06:40	Next will see some string functions. length function gives the length of a particular string s
06:49	index function determines the position of string s2 within the larger string s1.
06:57	For example, index within parentheses within double quotes linux comma within double quotes n, returns 3. Let us see an example.
07:10	Open the file awkdemo.txt
07:14	We know that each student in the awkdemo.txt file has a 4 digit roll number.
07:21	Due to typing error, the roll numbers may have wrong number of digits. We can easily detect these using awk commands.
07:30	Switch to the terminal . Let me clear the terminal.
07:36	Now type the command as shown here. Here we are checking the length of the 1st field is equal to 4 or not.
07:46	If not, then that particular record will get printed. Press Enter.
07:53	See, there is one roll-number S02 that has been typed incorrectly.
08:00	It has three digits, whereas all others have four digits.
08:07	The substr(s,a,b) function extracts a substring from a larger string s.
08:14	Let me explain the parameters.
08:17	Here s is the string
08:20	a denotes the position in s from which the extraction would start
08:26	b denotes the number of characters that would be extracted. Let us see one example.
08:33	Switch to the awkdemo.txt file.
08:37	The first letter of the roll numbers represents the Hostel code where the particular student resides.
08:46	Say we want to find the list of students, who are staying in Hostel A.
08:52	To get that, let’s switch to the terminal.
08:56	Type the command as shown here.
09:00	Here we take the string denoted by $1.
09:05	As we know $1 represents the first field, that is roll number in our case.
09:12	Next, we extract a substring that starts at position one with the character length one.
09:19	Then, if it is equal to capital A, then that line from the file will get printed. Press Enter to see the output.
09:29	We got the list of students who are in Hostel A.
09:34	We have seen the function split earlier. So, I am not explaining the details here.
09:40	Please refer to the earlier awk tutorials if you have any doubt.
09:45	There are some other functions which are related to Input/Output. system() function - helps us to run any unix command within awk.
09:56	Now, we will run the unix command date through awk command.
10:01	In the terminal type the command as shown here. And press Enter.
10:09	Today’s date and time is displayed on the terminal as an output.
10:15	Now, why do we need this? We have kept only the BEGIN section of the awk command.
10:21	In real world scenarios, we may want to print the system date, before displaying the required output.
10:28	In that case, we would need to execute system commands from awk command.
10:34	There are some functions dealing with time stamps like systime(), strftime() , etc.
10:43	Browse through the Internet to know about these functions.
10:48	This brings us to the end of this tutorial. Let us summarize.
10:53	In this tutorial we learnt about different types of built-in functions like Arithmetic functions, String functions, Input/Output functions and Time stamps functions
11:06	As an assignment-Write an awk program to print the last field of every record
11:13	where name of the student has small u as the third letter using the awkdemo.txt file.
11:22	The video at the following link summarises the Spoken Tutorial project. Please download and watch it.
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11:59	The script has been contributed by Antara. And this is Praveen from IIT Bombay signing off. Thank you for joining

Contributors and Content Editors

PoojaMoolya, Sandhya.np14