1 1 00:00:00,380 --> 00:00:02,090 Hi guys, and welcome back. 2 2 00:00:02,090 --> 00:00:05,030 In this video we are going to talk about dictionaries. 3 3 00:00:05,030 --> 00:00:08,140 Dictionaries are another structure in Python, 4 4 00:00:08,140 --> 00:00:10,060 like lists, sets, and doubles, 5 5 00:00:10,060 --> 00:00:13,600 that allow us to interact with data in a certain way. 6 6 00:00:13,600 --> 00:00:15,310 And what dictionaries are for 7 7 00:00:15,310 --> 00:00:18,870 is to associate keys and values together 8 8 00:00:18,870 --> 00:00:20,510 so that if you know the key 9 9 00:00:20,510 --> 00:00:23,430 you can access the value very easily. 10 10 00:00:23,430 --> 00:00:25,190 For example, you could have a dictionary 11 11 00:00:25,190 --> 00:00:27,730 of your friend names being the keys 12 12 00:00:27,730 --> 00:00:29,880 and their ages being the values. 13 13 00:00:29,880 --> 00:00:31,730 That way if you knew the name of your friend, 14 14 00:00:31,730 --> 00:00:34,840 you can very easily get the value out, their age. 15 15 00:00:34,840 --> 00:00:35,740 Let's get started. 16 16 00:00:36,890 --> 00:00:39,530 Let's say we have a dictionary friend ages 17 17 00:00:39,530 --> 00:00:41,900 and it's equal to the curly braces, 18 18 00:00:41,900 --> 00:00:44,690 and inside it you've got your friend name, Rolf, 19 19 00:00:44,690 --> 00:00:48,300 and then a colon and then something like 24. 20 20 00:00:48,300 --> 00:00:51,010 This is a dictionary of a single element 21 21 00:00:51,010 --> 00:00:54,440 where the element is this key value pair. 22 22 00:00:54,440 --> 00:00:56,630 The pairing is done with the colon 23 23 00:00:56,630 --> 00:00:59,000 and that means that 24 is a value 24 24 00:00:59,000 --> 00:01:02,513 associated with they key, Rolf, which is a string. 25 25 00:01:03,500 --> 00:01:06,270 Keys must be strings, they can also be integers 26 26 00:01:06,270 --> 00:01:09,010 and any other hashable types. 27 27 00:01:09,010 --> 00:01:11,240 Now I know we haven't talked about hashable types yet, 28 28 00:01:11,240 --> 00:01:15,200 but for now just think of them as integers and strings. 29 29 00:01:15,200 --> 00:01:18,140 If we want multiple values in a single dictionary, 30 30 00:01:18,140 --> 00:01:21,080 you can separate the key value pairs with a comma. 31 31 00:01:21,080 --> 00:01:26,080 So we can have Adam, 30, and Anne, 27, for example. 32 32 00:01:26,360 --> 00:01:30,170 And here we have a dictionary with three values, three keys. 33 33 00:01:30,170 --> 00:01:32,370 Rolf, Adam, and Anne. 34 34 00:01:32,370 --> 00:01:35,400 And each of those keys has a value associated with them. 35 35 00:01:35,400 --> 00:01:38,914 Rolf has 24, Adam has 30, and Anne has 27. 36 36 00:01:38,914 --> 00:01:42,583 If you wanted to access one particular element, 37 37 00:01:42,583 --> 00:01:45,710 you can do friend underscore ages 38 38 00:01:45,710 --> 00:01:47,490 and then using subscript notation, 39 39 00:01:47,490 --> 00:01:49,250 access the appropriate key. 40 40 00:01:49,250 --> 00:01:51,917 So in here we would put, for example, Adam 41 41 00:01:51,917 --> 00:01:54,680 and then we would access the Adam key 42 42 00:01:54,680 --> 00:01:56,972 inside the friend ages dictionary. 43 43 00:01:56,972 --> 00:01:59,070 Remember that in lists and doubles 44 44 00:01:59,070 --> 00:02:00,450 you could use subscript notation 45 45 00:02:00,450 --> 00:02:03,960 to access a particular index such as zero, one, and two, 46 46 00:02:03,960 --> 00:02:05,680 but that won't work in dictionaries. 47 47 00:02:05,680 --> 00:02:08,278 You must access the keys themselves. 48 48 00:02:08,278 --> 00:02:10,527 Of course, if we printed this out, 49 49 00:02:10,527 --> 00:02:15,527 line:15% you'll see that we get 30 out, just there. 50 50 00:02:17,370 --> 00:02:21,010 If you wanted to change a particular element or add one, 51 51 00:02:21,010 --> 00:02:23,310 you can do that by doing friend ages, 52 52 00:02:23,310 --> 00:02:26,700 and then for example Bob equal 20. 53 53 00:02:26,700 --> 00:02:28,350 Then if you print friend ages, 54 54 00:02:28,350 --> 00:02:31,110 line:15% you'll see that Bob will have been added to the dictionary. 55 55 00:02:31,110 --> 00:02:34,620 line:15% So when we print it out, we've got Rolf, Adam, Anne, and Bob 56 56 00:02:34,620 --> 00:02:37,380 And if you wanna change one, just do the same thing, 57 57 00:02:37,380 --> 00:02:38,763 but with an existing key. 58 58 00:02:40,210 --> 00:02:42,890 line:15% And now Rolf is 20. 59 59 00:02:42,890 --> 00:02:44,650 So that is how you can add a key, 60 60 00:02:44,650 --> 00:02:46,680 just access it and set a value to it, 61 61 00:02:46,680 --> 00:02:48,303 or change an existing key. 62 62 00:02:50,000 --> 00:02:51,960 Usually when we are working with dictionaries that are used 63 63 00:02:51,960 --> 00:02:54,730 to represent multiple things in the real world, 64 64 00:02:54,730 --> 00:02:57,420 such as multiple friends or things like that, 65 65 00:02:57,420 --> 00:03:00,559 it's quite convenient to have a list of dictionaries. 66 66 00:03:00,559 --> 00:03:03,163 For example, let's make a list of our friends. 67 67 00:03:04,480 --> 00:03:06,840 Now in the past, we've had a list of friends, 68 68 00:03:06,840 --> 00:03:11,250 such as Rolf, Adam, and Anne like this, 69 69 00:03:11,250 --> 00:03:13,570 but you can actually have a list of dictionaries, 70 70 00:03:13,570 --> 00:03:16,270 each dictionary describing a friend. 71 71 00:03:16,270 --> 00:03:17,103 By the way, 72 72 00:03:17,103 --> 00:03:19,350 I'm not sure if we've looked at this split syntax for lists, 73 73 00:03:19,350 --> 00:03:21,340 but lists don't care about white space, 74 74 00:03:21,340 --> 00:03:23,130 so you can put multiple values in one line, 75 75 00:03:23,130 --> 00:03:24,880 or you can split them across several lines, 76 76 00:03:24,880 --> 00:03:25,763 however you like. 77 77 00:03:26,770 --> 00:03:29,880 Going back to our dictionary, you can, instead of Rolf, 78 78 00:03:29,880 --> 00:03:33,290 create a dictionary that contains name 79 79 00:03:33,290 --> 00:03:35,270 and Rolf associated with it, 80 80 00:03:35,270 --> 00:03:39,550 and age and 24 associated with it. 81 81 00:03:39,550 --> 00:03:41,644 Now if we do this for each of our friends, 82 82 00:03:41,644 --> 00:03:46,050 and I will do that with Adam and Anne, 83 83 00:03:46,050 --> 00:03:47,400 and their appropriate ages, 84 84 00:03:50,040 --> 00:03:52,040 That means that now what we've got 85 85 00:03:52,040 --> 00:03:53,680 is a list of dictionaries 86 86 00:03:53,680 --> 00:03:56,370 and each dictionary represent a friend. 87 87 00:03:56,370 --> 00:03:58,850 But that means that we can store more 88 88 00:03:58,850 --> 00:04:02,010 than just their name in each element. 89 89 00:04:02,010 --> 00:04:03,230 Before we were constrained 90 90 00:04:03,230 --> 00:04:05,290 to just storing their name in our list, 91 91 00:04:05,290 --> 00:04:07,703 but now we can store more data if we want. 92 92 00:04:09,750 --> 00:04:12,530 If you print friends out, you'll see that you get back 93 93 00:04:12,530 --> 00:04:15,500 line:15% your list of friends there. 94 94 00:04:15,500 --> 00:04:17,490 And of course if you wanted to print a particular friend, 95 95 00:04:17,490 --> 00:04:21,290 you would need to use the index, because this is a list. 96 96 00:04:21,290 --> 00:04:24,330 The friends list needs to be accessed by index 97 97 00:04:24,330 --> 00:04:26,620 so if you wanted to access index zero, 98 98 00:04:26,620 --> 00:04:28,580 that would give you back this dictionary. 99 99 00:04:28,580 --> 00:04:30,670 Index one would give you this dictionary. 100 100 00:04:30,670 --> 00:04:33,040 And index two would give you this dictionary. 101 101 00:04:33,040 --> 00:04:35,070 So let's say we want to access index one, 102 102 00:04:35,070 --> 00:04:36,820 which is this dictionary, 103 103 00:04:36,820 --> 00:04:38,080 if you print this out 104 104 00:04:38,080 --> 00:04:39,550 line:15% you'll see that what you get back is, 105 105 00:04:39,550 --> 00:04:41,990 line:15% naturally, the entire dictionary. 106 106 00:04:41,990 --> 00:04:46,360 That means that now, after friends one, 107 107 00:04:46,360 --> 00:04:50,150 you can put another subscript notation to access the name, 108 108 00:04:50,150 --> 00:04:51,570 for example. 109 109 00:04:51,570 --> 00:04:54,070 line:15% And doing this would give you Adam. 110 110 00:04:54,070 --> 00:04:57,300 So this is how you can access a list of dictionaries, 111 111 00:04:57,300 --> 00:05:00,540 or a particular value inside a list of dictionaries. 112 112 00:05:00,540 --> 00:05:02,220 If you had a list of lists, 113 113 00:05:02,220 --> 00:05:05,780 you would access multiple indices in succession like this. 114 114 00:05:05,780 --> 00:05:07,970 And if you had a dictionary of dictionaries, 115 115 00:05:07,970 --> 00:05:10,483 then you would access multiple keys in succession. 116 116 00:05:11,350 --> 00:05:14,030 Let me say of course that you can nest these lists 117 117 00:05:14,030 --> 00:05:17,030 and doubles and dictionaries however way you want in Python. 118 118 00:05:19,500 --> 00:05:21,220 Let's take a look at another example 119 119 00:05:21,220 --> 00:05:24,492 where we have a dictionary that maps the student names 120 120 00:05:24,492 --> 00:05:28,410 to the percentage of attendance they have to their lessons. 121 121 00:05:28,410 --> 00:05:31,140 So Rolf has attended 96% of his lessons. 122 122 00:05:31,140 --> 00:05:34,700 Bob 80% and Anne 100%. 123 123 00:05:34,700 --> 00:05:39,550 You can iterate over a dictionary, using a for loop. 124 124 00:05:39,550 --> 00:05:43,630 And what this does is it gives you the keys. 125 125 00:05:43,630 --> 00:05:45,210 So if you print this student variable 126 126 00:05:45,210 --> 00:05:46,870 that we're using here for iteration 127 127 00:05:46,870 --> 00:05:49,480 line:15% what you get back are the student names. 128 128 00:05:49,480 --> 00:05:51,463 The keys in your dictionary. 129 129 00:05:52,350 --> 00:05:55,883 Using that key, you can print something like this. 130 130 00:05:58,960 --> 00:06:01,780 Which will print out the student variable 131 131 00:06:01,780 --> 00:06:03,610 which is the students name 132 132 00:06:03,610 --> 00:06:06,570 and then you can access the student key 133 133 00:06:06,570 --> 00:06:08,100 in the student attendance dictionary 134 134 00:06:08,100 --> 00:06:11,400 using subscript notation to get their attendance. 135 135 00:06:11,400 --> 00:06:13,200 So if we save this and run it 136 136 00:06:13,200 --> 00:06:17,160 line:15% you now see that we get Rolf 96, Bob 80, Anne 100. 137 137 00:06:17,160 --> 00:06:20,291 You can of course put a percentage sign there if you like 138 138 00:06:20,291 --> 00:06:21,124 to make it a little bit nicer. 139 139 00:06:21,124 --> 00:06:23,820 However accessing a dictionary like this in a for loop 140 140 00:06:23,820 --> 00:06:27,130 doesn't look the best because there are other, better ways. 141 141 00:06:27,130 --> 00:06:32,040 The better way is if you do student attendance dot items 142 142 00:06:32,040 --> 00:06:36,467 and then here you can do for student comma attendance 143 143 00:06:36,467 --> 00:06:39,100 in student attendance dot items. 144 144 00:06:39,100 --> 00:06:41,960 And then here you no longer need to access dictionary 145 145 00:06:41,960 --> 00:06:44,940 because you've got it in the attendance variable. 146 146 00:06:44,940 --> 00:06:48,654 So what we've got here is that student attendance dot items 147 147 00:06:48,654 --> 00:06:51,610 gives you two values for each student 148 148 00:06:51,610 --> 00:06:55,590 that you can get as two separate variables here in Python. 149 149 00:06:55,590 --> 00:06:57,321 Saving and running this 150 150 00:06:57,321 --> 00:06:59,073 line:15% will give you exactly the same as before. 151 151 00:07:00,080 --> 00:07:02,520 You can use the in keyword in a dictionary 152 152 00:07:02,520 --> 00:07:05,530 to check whether a value is one of the keys 153 153 00:07:05,530 --> 00:07:06,363 of the dictionary. 154 154 00:07:06,363 --> 00:07:10,880 So for example, if Bob in student attendance, 155 155 00:07:10,880 --> 00:07:11,910 you can print something like 156 156 00:07:11,910 --> 00:07:14,733 Bob attended this many lectures. 157 157 00:07:16,450 --> 00:07:17,760 Otherwise, you can print that 158 158 00:07:17,760 --> 00:07:21,363 Bob is not a student in this class. 159 159 00:07:23,140 --> 00:07:25,450 So, the in keyword allows you to check 160 160 00:07:25,450 --> 00:07:27,820 whether a value is one of the keys. 161 161 00:07:27,820 --> 00:07:30,640 Notice that you can't check whether one of the values 162 162 00:07:30,640 --> 00:07:33,903 is in the dictionary, you can only check the keys. 163 163 00:07:35,110 --> 00:07:38,850 Finally, you can also get just the values back if you like. 164 164 00:07:38,850 --> 00:07:41,210 If you wanted 96, 80, and 100 165 165 00:07:41,210 --> 00:07:43,050 without worrying about the keys, 166 166 00:07:43,050 --> 00:07:46,030 for example if wanted to calculate an average, 167 167 00:07:46,030 --> 00:07:49,010 you could have a variable called attendance values 168 168 00:07:49,010 --> 00:07:53,080 and make it equal to student attendance dot values. 169 169 00:07:53,080 --> 00:07:56,042 And then you can print the sum of attendance values 170 170 00:07:56,042 --> 00:08:01,042 divided by the length of attendance values. 171 171 00:08:01,210 --> 00:08:02,700 And what this is going to give you is 172 172 00:08:02,700 --> 00:08:07,200 add 96, 80, and 100 together and divide by three 173 173 00:08:07,200 --> 00:08:10,860 which is the length of what you get back which is a list. 174 174 00:08:10,860 --> 00:08:13,493 line:15% So if you run that you get back 92.0. 175 175 00:08:15,050 --> 00:08:17,350 Going back to what we had earlier, 176 176 00:08:17,350 --> 00:08:20,940 which is this syntax where you have two variables 177 177 00:08:20,940 --> 00:08:23,860 being defined, separated by a comma inside a for loop, 178 178 00:08:23,860 --> 00:08:26,150 we're going to learn more about that in the next video. 179 179 00:08:26,150 --> 00:08:28,370 So if that's unclear, don't worry about it, 180 180 00:08:28,370 --> 00:08:29,570 we'll look at it just now. 181 181 00:08:29,570 --> 00:08:31,153 I'll see you in the next video.