Difference between revisions of "Linux-AWK/C2/Built-in-Functions-in-awk/English-timed"
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Latest revision as of 17:51, 30 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 give 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() function gives the truncated integer of any positive or negative number. |
| 03:36 | exp() function 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 values using your calculator. |
| 04:07 | Now, let us look at random functions. |
| 04:11 | rand() function 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) function 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 Ctrl and S keys to 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 we 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 | Here, 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. |
