Difference between revisions of "Linux-AWK/C2/Built-in-Functions-in-awk/English-timed"
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Revision as of 12:34, 4 July 2019
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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 is 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. |
11:30 | The Spoken Tutorial Project team conducts workshops using spoken tutorials.
And gives certificates on passing online tests. For more details, please write to us. |
11:43 | Please post your timed queries in this forum. |
11:47 | Spoken Tutorial Project is funded by NMEICT, MHRD, Government of India.
More information on this mission is available at this link. |
11:59 | The script has been contributed by Antara. And this is Praveen from IIT Bombay signing off.
Thank you for joining |