Linux-AWK/C2/Basics-of-Single-Dimensional-Array-in-awk/English-timed

Time	Narration
00:01	Welcome to this spoken tutorial on Basics of single dimensional array in awk.
00:07	In this tutorial, we will learn about- Arrays in awk,
00:12	assigning array elements,
00:15	how it is different from arrays in other programming languages and refer the elements of an array .
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 previous awk tutorials on our website.
00:49	You should have some basic 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.
01:11	What is an array in awk? awk supports arrays for storing related elements.
01:18	Elements can be a number or a string.
01:21	Arrays in awk are associative.
01:24	This means that each array element is an index-value pair.
01:29	It looks very similar to arrays in any other programming language.
01:33	But there are some important differences.
01:36	First, we do not need to declare an array before using it.
01:41	Also, there is no need to specify how many elements the array will contain.
01:47	In programming languages, array index is generally a positive integer.
01:52	Usually the index starts from 0, then 1, then 2 and so on.
01:58	But in awk, the index can be anything – any number or a string.
02:03	This is the syntax of assigning an array element in awk. Array name can be any valid variable name.
02:11	Here the index can be an integer or a string.
02:16	Strings have to be written inside double quotes, whether it is index name or a value.
02:23	Let’s understand this with an example.
02:27	I have already written the code and saved it as array_intro.awk.
02:34	This file is available in the Code Files link below the player. Please download and use it.
02:41	Here, I have taken weekdays as an example and written it inside the BEGIN section.
02:48	Here, the name of the array is day.
02:52	I have set the index as 1 and the value as "Sunday".
02:57	In this array element, I have used a string as the index. So, for index "first", the value is "Sunday".
03:06	The entire array is constructed likewise.
03:10	Notice here, the array elements are not in a sequence. I have declared day four before day three.
03:18	In awk arrays, index need not to be in a sequential manner.
03:23	Advantage of associative array is that new pairs can be added at any time.
03:29	Let me add day 6 in the array.
03:33	Place the cursor at the end of the last line and press Enter. Then type the following.
03:42	Save the file.
03:44	We have declared the array. But how should we refer to the array element?
03:49	Write the arrayname and the index within square brackets to refer an element, at a particular index. Let us try this.
03:58	Switch to the code once again.
04:01	Place the cursor in front of the closing curly brace.
04:05	Press Enter and type: print space day within square brackets 6.
04:13	Save the code.
04:15	Open the terminal by pressing Ctrl, Alt and T keys.
04:20	Go to the folder in which you downloaded and extracted the Code Files using 'cd' command.
04:27	Now, type: awk space hyphen small f space array_intro.awk . Press Enter.
04:38	See, we get Friday as the output.
04:42	Next we will check whether any element exists in an array at a certain index.
04:48	For this, we have to use the in operator. Let me explain this with an example.
04:55	Switch to the code in the editor window.
04:59	Place the cursor at the end of the print statement and press Enter. Then type as shown.
05:09	Save the code.
05:11	Now I have added two 'if' conditions.
05:15	The first if condition checks whether the index two is present in day.
05:21	If yes, then the corresponding print statement will get executed.
05:26	Then the second condition checks whether the index seven is present in day. It will execute the print statement if it is true.
05:35	As we can see, index two is in the array and seven is not. Let’s execute this file to verify the output.
05:44	Switch to the terminal. Press the up arrow key to get back the previously executed command.
05:51	Press Enter to execute.
05:54	We get the output as expected.
05:57	We will now make some more changes to the code. Update the code as shown here.
06:04	Below the 7 in day condition, I have added one more condition.
06:09	This will check whether the value of index seven is null or not.
06:14	If true, it will print Index 7 is not null.
06:18	We already know that we don’t have any index with 7. So, it will not print anything.
06:24	Next, we have changed the print statement of the condition 7 in day.
06:30	Save the code. Let’s see what happens when we execute the code.
06:35	Switch to terminal. Press the up arrow key to get the previously executed command.
06:43	Press Enter to execute.
06:46	We got an unexpected output.
06:49	The statement "Index 7 is present after null comparison." is printed. How is it possible?
06:57	When we write, day[7] not equal to null, we are trying to access the element at index 7.
07:04	This access itself will first create an element at index 7 and initialize it with the value null.
07:12	Next, we are trying to check if any element is actually present at index 7.
07:18	As null element is already created, the output shows that Index 7 is present after null comparison.
07:26	So, remember this: day at index 7 not equal to null is a wrong way to check the presence of an element.
07:34	It will create a null element at index 7.
07:38	Instead, we have to use the in operator.
07:41	It will not create any extra element in the array. This brings us to the end of this tutorial.
07:50	In this tutorial, we learnt about- Arrays in awk,
07:54	Assigning array elements,
07:56	How it is different from arrays in other programming languages,
08:00	Refer the elements of an array .
08:03	As an assignment- Define an array flowerColor.
08:07	Index will be the names of the flowers.
08:10	Value will be the corresponding color of the flowers.
08:14	Insert entries for any five flowers of your choice.
08:18	Print the color of the fourth flower. Check if the flower “Lotus” is present in the array.
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09:01	The script has been contributed by Antara. And, this is Praveen from IIT Bombay, signing off. Thanks for joining.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14