Linux-AWK/C2/MultiDimensional-Array-in-awk/English-timed

Time	Narration
00:01	Hello and welcome to the spoken tutorial on creating multidimensional arrays in awk.
00:07	In this tutorial, we will learn to- create multidimensional array in awk and scan the multidimensional array.
00:18	We will do this through some examples.
00:21	To record this tutorial, I am using: Ubuntu Linux 16.04 Operating System and gedit text editor 3.20.1
00:33	You can use any text editor of your choice.
00:37	To practice this tutorial, you should have gone through previous awk tutorials on array, on this website.
00:45	You should have some basic knowledge of any programming language like C or C++.
00:52	If not, then please go through the corresponding tutorials on our website.
00:58	The files used in this tutorial are available in the Code Files link on this tutorial page. Please download and extract them.
01:08	What is a multidimensional array in awk?
01:12	We know that in single dimensional arrays, an array element is identified by a single index.
01:19	For example, array week is identified by a single index, day.
01:26	However, in multidimensional array, an element is identified by a sequence of multiple indices.
01:34	For example, a two dimensional array element is identified by a sequence of 2 indices.
01:42	Here, multiple indices are concatenated into a single string, with a separator between them.
01:50	The separator is the value of the built-in variable SUBSEP.
01:55	The combined string is used as a single index for a simple one dimensional array.
02:01	For example, suppose we write multi within square brackets 4 comma 6 equal to value in double quotes.
02:11	Here, multi is the name of multi-dimensional array. Then, the numbers 4 and 6 are converted to a string.
02:21	Suppose, the value of SUBSEP is hash symbol (#).
02:26	Then, those numbers are concatenated with a hash (#) symbol between them.
02:32	So, the array element multi within square brackets within double quotes 4 hash 6 is set to value within double quotes.
02:43	The default value of SUBSEP is the string within double quotes backslash 034.
02:50	It is actually a nonprinting character. It will not appear usually in most input data.
02:58	Let us try to declare a two dimensional array as shown in the slide.
03:03	Row 1 contains two elements A and B.
03:08	Row 2 has two elements C and D.
03:12	Open the terminal by pressing Ctrl, Alt and T keys.
03:17	Go to the folder in which you have downloaded and extracted the Code Files using 'cd' command.
03:24	Now, define the array as follows. Type the command carefully as shown here. Then press Enter.
03:35	We get a command prompt back without any error. So, the array is defined.
03:41	We do not get any output because we have not given anything to print in the code.
03:47	Let us add the print statement.
03:50	Press the up arrow key to get the previously executed command in the terminal.
03:56	Just before the closing curly bracket, type: semicolon space print space a within square brackets 2 comma 2. Press Enter to execute the command.
04:13	Notice, we get the output as capital D.
04:18	How to test if a particular index sequence exists in a given multidimensional array?
04:25	We can use in operator.
04:28	We have already seen it in single-dimensional array earlier in this series.
04:34	We have to write the entire sequence of indices within parentheses and separated by commas.
04:42	Let us see this in an example.
04:45	I have already written a script named test_multi.awk.
04:51	The same is available in the Code Files link of this tutorial page.
04:56	I have defined a 2 by 2 array as seen in our previous discussion.
05:02	Then I have written two 'if' conditions.
05:06	The first if condition checks whether the element at the index one comma one, is present or not.
05:13	We have to write the index for multidimensional array within parentheses.
05:18	If the condition is true, it will print one comma one is present.
05:23	Else, it will print one comma one is absent.
05:28	Similarly, we will check for the presence of the element at index three comma one. Let us execute the file.
05:36	Switch to the terminal and type: awk space hyphen small f space test underscore multi dot awk and press Enter.
05:49	The output says: one comma one is present and three comma one is absent.
05:55	Let us take one more example. Say, we want to create the transpose of a matrix.
06:02	The transpose of a given matrix is formed by interchanging the rows and columns of a matrix. How can we do this?
06:11	I have created a two-dimensional array matrix in the file 2D-array.txt.
06:19	I have written a code named transpose.awk.
06:24	First, look at the action section of this awk script.
06:29	Here, we are calculating the maximum number of fields in a row. And, stored the calculated value in the variable max_nf.
06:40	As we know, NR is the number of current records processed by awk. Value of NR is stored in max_nr variable.
06:50	Awk will process the input file from the first record to the last record.
06:56	When awk is processing the first record, max_nr will be equal to 1.
07:03	While processing second record, max_nr will be 2 and it continues this way.
07:11	When awk is processing the last record, max_nr will store the total number of records.
07:19	Now, we should read the data from input file and store the data into a two dimensional array.
07:26	Inside the 'for' loop, we have iterator variable x.
07:31	x will traverse from one to NF and x will be incremented by 1 after each iteration.
07:39	For every value of x, $x(dollar x) represents the value at field x.
07:46	That value will be stored in array matrix at index NR comma x.
07:53	For example, matrix of 1 comma 1 stores the value which is present at index 1 comma 1 from the input file.
08:02	So, after awk has processed the entire input file with this code, matrix array will be completely formed.
08:10	It will store entire data of input file into a two dimensional array format.
08:16	Now, let’s us look inside the END section.
08:20	We have written a nested for loop to print the transpose of the matrix.
08:25	I assume your familiarity with basic C programming. So, I am not explaining this portion of code in detail.
08:34	Pause the video here to look at the code in detail and understand on your own.
08:40	Now, we will learn how to scan a multidimensional array.
08:45	Awk does not have a multi-dimensional array in the truest sense.
08:50	So, there cannot be any special 'for' statement to scan the multidimensional array.
08:56	You can have multidimensional way to scan an array.
09:00	You can combine the 'for' statement with the split function for this.
09:05	Let us see what the split function is. split function is used to chop up or split a string into pieces
09:14	and place the various pieces into an array.
09:18	The syntax is as follows. First argument contains the string to be chopped.
09:25	Second argument specifies the name of the array where split() will put the chopped pieces into.
09:33	The third argument mentions the separator that will be used to chop the string up.
09:39	The first piece is stored in arr 1,
09:43	the second piece in arr 2 and so forth.
09:48	Suppose, we want to recover the original sequence of indices from an already created array. How can we do this?
09:56	I have written a code named multi_scan.awk.
10:02	Entire code is written inside the BEGIN section.
10:06	First we have created an array named a and assigned these values to it.
10:12	Then we have the for loop with an iterator.
10:16	The iterator will be set to each of the indices values for each iteration- say, 1 comma 1, then 1 comma 2 and so on.
10:27	The split() function breaks the iterator into pieces separated by SUBSEP.
10:34	The pieces will be stored in the array arr.
10:38	So, arr[1] and arr[2] will contain the first index and second index respectively. Let us execute this file.
10:48	Switch to the terminal and type- awk space hyphen small f space multi underscore scan dot awk and press Enter.
11:01	See the output; the original sequence of indices are recovered.
11:07	Let us summarize. In this tutorial, we learnt to- create a multidimensional array in awk and scan a multidimensional array.
11:18	As an assignment, write an awk script to rotate a two dimensional array by 90 degree and print the rotated matrix.
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12:05	The script has been contributed by Antara. And this is Praveen from IIT Bombay, signing off. Thank you for joining.

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PoojaMoolya, Sandhya.np14