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These are the user uploaded subtitles that are being translated: 1 00:00:00,660 --> 00:00:05,070 Hello everyone and welcome to the lecture on Matrix selection and indexing. 2 00:00:05,070 --> 00:00:09,840 So just like with doctors we're going to be able to use the square bracket notation to select elements 3 00:00:10,020 --> 00:00:11,250 from a matrix. 4 00:00:11,250 --> 00:00:14,180 Let's go ahead and jump to our studio to see how this works. 5 00:00:14,670 --> 00:00:15,000 OK. 6 00:00:15,000 --> 00:00:20,430 So here are studio and let's go ahead and get a quick reminder of how we can use indexing for a vector 7 00:00:20,680 --> 00:00:21,040 . 8 00:00:21,080 --> 00:00:30,420 Imagine had a vector V of the elements 1 2 3 4 5 index and number two I would simply use square brackets 9 00:00:30,480 --> 00:00:34,040 and then put the index position where the element I want back. 10 00:00:34,080 --> 00:00:37,770 Or it could also put in the name if the vector was named. 11 00:00:37,800 --> 00:00:43,020 Now for matrices since we're dealing of two them mentions we'll need to use a comma to separate or indexing 12 00:00:43,020 --> 00:00:44,460 for each dimension. 13 00:00:44,460 --> 00:00:47,390 So let's go ahead and show you what this looks like. 14 00:00:47,400 --> 00:00:53,100 First off we'll start by making a matrix called Matt. 15 00:00:53,190 --> 00:00:59,530 The matrix function as a numeric sequential vector 1 through 50. 16 00:00:59,930 --> 00:01:03,110 We'll say by RHO is equal to false. 17 00:01:03,210 --> 00:01:11,250 Actually true excuse me and I'll specify the number of rows to be equal to 5. 18 00:01:11,580 --> 00:01:16,010 So it's going to take a look at that matrix real quick. 19 00:01:16,140 --> 00:01:21,690 So because my font's a little larger here it's going to be split into two parts but it is all just one 20 00:01:21,690 --> 00:01:27,900 matrix that's a 5 by 10 matrix so there's five rows with 10 columns here. 21 00:01:28,010 --> 00:01:33,990 You'll notice here that there's a square bracket index notation for the columns and the rows and that's 22 00:01:33,990 --> 00:01:38,560 the same kind of rotation we're going to be using when we select from this matrix. 23 00:01:38,580 --> 00:01:40,450 So let's go ahead and get started. 24 00:01:40,520 --> 00:01:43,780 The men really wanted to grab the first row. 25 00:01:43,920 --> 00:01:50,070 So for Matrix indexing notation what you're going to do for the square brackets is your first going 26 00:01:50,070 --> 00:01:56,970 to pass in the rows you want to if I just want the first row I'll put in one then you put a comma and 27 00:01:56,970 --> 00:02:01,580 then you pass in the columns you want if you want everything you just leave it blank. 28 00:02:01,590 --> 00:02:06,960 So if I want the first row that's going to return that first row for me. 29 00:02:06,960 --> 00:02:10,160 Let's go ahead and see what that looks like one more time. 30 00:02:10,170 --> 00:02:15,760 So I notice the first row was the numbers 1 2 3 4 5 6 7 8 9 and 10. 31 00:02:16,110 --> 00:02:24,940 So if I say first one comma nothing then I'm going to return all the columns for the first row. 32 00:02:24,960 --> 00:02:25,580 Great. 33 00:02:25,830 --> 00:02:30,720 Let's go ahead and see how I can grab the first column. 34 00:02:30,720 --> 00:02:34,580 So the first column is going to be nothing. 35 00:02:34,630 --> 00:02:36,460 Comma 1. 36 00:02:36,540 --> 00:02:43,740 Now what this means is grab everything all the rows from the first column so that in return will bring 37 00:02:43,740 --> 00:02:45,460 back the first column. 38 00:02:45,810 --> 00:02:47,850 So in general we can think of this. 39 00:02:47,910 --> 00:02:54,030 If I just go and take this in our script as something that looks like this we'll have some matrix and 40 00:02:54,030 --> 00:02:56,560 then we specify which rose we want. 41 00:02:56,750 --> 00:03:00,760 If a comma and then we specify specify which columns we want. 42 00:03:01,050 --> 00:03:04,320 So it goes Rose comma column within the square brackets. 43 00:03:04,320 --> 00:03:09,930 And if you want all the rows or all the columns you're just going to go ahead and leave that blank. 44 00:03:09,960 --> 00:03:13,980 So let's go ahead and see what else we can do here. 45 00:03:13,980 --> 00:03:18,140 Imagine you wanted to grab the first three rows of a matrix. 46 00:03:18,260 --> 00:03:23,160 Well we would just say Matrix name but the square brackets in there. 47 00:03:23,160 --> 00:03:26,280 And here we can use this slicing notation. 48 00:03:26,280 --> 00:03:33,120 So one colon three that's going to return 1 2 and 3 for the rows and then I'm going to leave a comma 49 00:03:33,150 --> 00:03:39,750 there because I want all the values for those rows every column value for those three rows and then 50 00:03:39,750 --> 00:03:44,700 we can see that it gets returned those three rows for all 10 columns. 51 00:03:44,730 --> 00:03:46,060 All right great. 52 00:03:46,080 --> 00:03:51,650 So let's go ahead and take a look at our matrix one more time. 53 00:03:51,840 --> 00:03:53,560 So we have this matrix Matt. 54 00:03:54,030 --> 00:03:57,110 It's 10 columns by five rows. 55 00:03:57,120 --> 00:04:02,930 Imagine I want it to grab let's say just the top left right rectangle So that's going to be let's say 56 00:04:02,950 --> 00:04:07,140 just want to grab one two three and 11 12 13. 57 00:04:07,190 --> 00:04:08,780 I would actually do that. 58 00:04:08,880 --> 00:04:12,510 So I'm going to say Matt square brackets. 59 00:04:12,660 --> 00:04:15,540 I want to go ahead and grab that left top left rectangle. 60 00:04:15,600 --> 00:04:18,570 So it's going to be looks like the first two rows. 61 00:04:18,590 --> 00:04:21,730 So it's I want one two three and 11 12 13. 62 00:04:21,900 --> 00:04:23,560 So gonna go ahead and put in the first two rows. 63 00:04:23,550 --> 00:04:27,450 So that's one goal and two for that slice of the first two rows. 64 00:04:27,780 --> 00:04:30,520 And I also want the first three columns. 65 00:04:30,540 --> 00:04:31,560 One two three. 66 00:04:31,560 --> 00:04:34,420 So that's going to be one comma three. 67 00:04:35,700 --> 00:04:36,420 And there you have it. 68 00:04:36,420 --> 00:04:39,780 We have that rectangle back that top left rectangle. 69 00:04:40,320 --> 00:04:44,640 OK let's go ahead and just show two more quick examples. 70 00:04:44,640 --> 00:04:47,410 Imagine I wanted to grab the last two columns. 71 00:04:47,670 --> 00:04:51,270 I could do Matt since I want just columns. 72 00:04:51,270 --> 00:04:53,070 That means I want all the rows for it. 73 00:04:53,080 --> 00:04:57,600 So going to put a comma and I'm going to go to specify columns 9 through 10. 74 00:04:57,600 --> 00:04:59,380 So those are the last two columns. 75 00:04:59,760 --> 00:05:00,600 And there you have it. 76 00:05:00,600 --> 00:05:02,870 We have the last two columns there. 77 00:05:02,970 --> 00:05:05,710 Let's show one more example of this. 78 00:05:05,820 --> 00:05:08,390 Go ahead and see what the map looks like. 79 00:05:08,400 --> 00:05:11,310 Imagine you want it to grab let's say just a center square. 80 00:05:11,310 --> 00:05:15,200 So I want 15 16 and 25 26. 81 00:05:15,240 --> 00:05:23,100 I want this square in the matrix when I go ahead and say mats and let's see we're going to need the 82 00:05:23,100 --> 00:05:31,560 rows for 15 16 25 26 will need two and three are going to go and slice rows two for three comma. 83 00:05:31,620 --> 00:05:36,240 Then I'm going to do the columns in this case looks like it's five through six and you can notice. 84 00:05:36,240 --> 00:05:42,230 Now hopefully this notation starts to make sense as to why our studios are reporting it as blank. 85 00:05:42,240 --> 00:05:46,460 Comma 5 because this stands for all the rows in column 5. 86 00:05:46,500 --> 00:05:48,560 Likewise for the rows. 87 00:05:48,570 --> 00:05:52,780 It's going to be the row comma blank because it's all the columns for the first row. 88 00:05:52,800 --> 00:05:59,400 So if I go ahead and run this it returns back that square from the Matrix. 89 00:05:59,400 --> 00:06:00,060 OK. 90 00:06:00,240 --> 00:06:05,020 So that's it as far as the general syntax for grabbing chunks from the Matrix. 91 00:06:05,140 --> 00:06:10,950 But I encourage you to do is just go ahead and pick out with your eyes some gen's rectangles or rows 92 00:06:10,950 --> 00:06:15,540 or columns from our general matrix here and see if you can index them correctly. 93 00:06:15,630 --> 00:06:22,290 So maybe try grabbing the rectangle here 33 through 35 and 43 for 45. 94 00:06:22,290 --> 00:06:23,780 See if you can grab that rectangle. 95 00:06:23,790 --> 00:06:24,930 How would you do it. 96 00:06:24,950 --> 00:06:26,230 Just go ahead and practice. 97 00:06:26,280 --> 00:06:30,240 Choose some rectangles or squares in the matrix and see if you can index them out. 98 00:06:30,600 --> 00:06:31,500 OK. 99 00:06:31,500 --> 00:06:33,860 Thanks everyone and I'll see you at the next lecture. 9877