WEBVTT 1 00:00:00.380 --> 00:00:02.090 Hi guys, and welcome back. 2 00:00:02.090 --> 00:00:05.030 In this video we are going to talk about dictionaries. 3 00:00:05.030 --> 00:00:08.140 Dictionaries are another structure in Python, 4 00:00:08.140 --> 00:00:10.060 like lists, sets, and doubles, 5 00:00:10.060 --> 00:00:13.600 that allow us to interact with data in a certain way. 6 00:00:13.600 --> 00:00:15.310 And what dictionaries are for 7 00:00:15.310 --> 00:00:18.870 is to associate keys and values together 8 00:00:18.870 --> 00:00:20.510 so that if you know the key 9 00:00:20.510 --> 00:00:23.430 you can access the value very easily. 10 00:00:23.430 --> 00:00:25.190 For example, you could have a dictionary 11 00:00:25.190 --> 00:00:27.730 of your friend names being the keys 12 00:00:27.730 --> 00:00:29.880 and their ages being the values. 13 00:00:29.880 --> 00:00:31.730 That way if you knew the name of your friend, 14 00:00:31.730 --> 00:00:34.840 you can very easily get the value out, their age. 15 00:00:34.840 --> 00:00:35.740 Let's get started. 16 00:00:36.890 --> 00:00:39.530 Let's say we have a dictionary friend ages 17 00:00:39.530 --> 00:00:41.900 and it's equal to the curly braces, 18 00:00:41.900 --> 00:00:44.690 and inside it you've got your friend name, Rolf, 19 00:00:44.690 --> 00:00:48.300 and then a colon and then something like 24. 20 00:00:48.300 --> 00:00:51.010 This is a dictionary of a single element 21 00:00:51.010 --> 00:00:54.440 where the element is this key value pair. 22 00:00:54.440 --> 00:00:56.630 The pairing is done with the colon 23 00:00:56.630 --> 00:00:59.000 and that means that 24 is a value 24 00:00:59.000 --> 00:01:02.513 associated with they key, Rolf, which is a string. 25 00:01:03.500 --> 00:01:06.270 Keys must be strings, they can also be integers 26 00:01:06.270 --> 00:01:09.010 and any other hashable types. 27 00:01:09.010 --> 00:01:11.240 Now I know we haven't talked about hashable types yet, 28 00:01:11.240 --> 00:01:15.200 but for now just think of them as integers and strings. 29 00:01:15.200 --> 00:01:18.140 If we want multiple values in a single dictionary, 30 00:01:18.140 --> 00:01:21.080 you can separate the key value pairs with a comma. 31 00:01:21.080 --> 00:01:26.080 So we can have Adam, 30, and Anne, 27, for example. 32 00:01:26.360 --> 00:01:30.170 And here we have a dictionary with three values, three keys. 33 00:01:30.170 --> 00:01:32.370 Rolf, Adam, and Anne. 34 00:01:32.370 --> 00:01:35.400 And each of those keys has a value associated with them. 35 00:01:35.400 --> 00:01:38.914 Rolf has 24, Adam has 30, and Anne has 27. 36 00:01:38.914 --> 00:01:42.583 If you wanted to access one particular element, 37 00:01:42.583 --> 00:01:45.710 you can do friend underscore ages 38 00:01:45.710 --> 00:01:47.490 and then using subscript notation, 39 00:01:47.490 --> 00:01:49.250 access the appropriate key. 40 00:01:49.250 --> 00:01:51.917 So in here we would put, for example, Adam 41 00:01:51.917 --> 00:01:54.680 and then we would access the Adam key 42 00:01:54.680 --> 00:01:56.972 inside the friend ages dictionary. 43 00:01:56.972 --> 00:01:59.070 Remember that in lists and doubles 44 00:01:59.070 --> 00:02:00.450 you could use subscript notation 45 00:02:00.450 --> 00:02:03.960 to access a particular index such as zero, one, and two, 46 00:02:03.960 --> 00:02:05.680 but that won't work in dictionaries. 47 00:02:05.680 --> 00:02:08.278 You must access the keys themselves. 48 00:02:08.278 --> 00:02:10.527 Of course, if we printed this out, 49 00:02:10.527 --> 00:02:15.527 line:15% you'll see that we get 30 out, just there. 50 00:02:17.370 --> 00:02:21.010 If you wanted to change a particular element or add one, 51 00:02:21.010 --> 00:02:23.310 you can do that by doing friend ages, 52 00:02:23.310 --> 00:02:26.700 and then for example Bob equal 20. 53 00:02:26.700 --> 00:02:28.350 Then if you print friend ages, 54 00:02:28.350 --> 00:02:31.110 line:15% you'll see that Bob will have been added to the dictionary. 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 00:02:34.620 --> 00:02:37.380 And if you wanna change one, just do the same thing, 57 00:02:37.380 --> 00:02:38.763 but with an existing key. 58 00:02:40.210 --> 00:02:42.890 line:15% And now Rolf is 20. 59 00:02:42.890 --> 00:02:44.650 So that is how you can add a key, 60 00:02:44.650 --> 00:02:46.680 just access it and set a value to it, 61 00:02:46.680 --> 00:02:48.303 or change an existing key. 62 00:02:50.000 --> 00:02:51.960 Usually when we are working with dictionaries that are used 63 00:02:51.960 --> 00:02:54.730 to represent multiple things in the real world, 64 00:02:54.730 --> 00:02:57.420 such as multiple friends or things like that, 65 00:02:57.420 --> 00:03:00.559 it's quite convenient to have a list of dictionaries. 66 00:03:00.559 --> 00:03:03.163 For example, let's make a list of our friends. 67 00:03:04.480 --> 00:03:06.840 Now in the past, we've had a list of friends, 68 00:03:06.840 --> 00:03:11.250 such as Rolf, Adam, and Anne like this, 69 00:03:11.250 --> 00:03:13.570 but you can actually have a list of dictionaries, 70 00:03:13.570 --> 00:03:16.270 each dictionary describing a friend. 71 00:03:16.270 --> 00:03:17.103 By the way, 72 00:03:17.103 --> 00:03:19.350 I'm not sure if we've looked at this split syntax for lists, 73 00:03:19.350 --> 00:03:21.340 but lists don't care about white space, 74 00:03:21.340 --> 00:03:23.130 so you can put multiple values in one line, 75 00:03:23.130 --> 00:03:24.880 or you can split them across several lines, 76 00:03:24.880 --> 00:03:25.763 however you like. 77 00:03:26.770 --> 00:03:29.880 Going back to our dictionary, you can, instead of Rolf, 78 00:03:29.880 --> 00:03:33.290 create a dictionary that contains name 79 00:03:33.290 --> 00:03:35.270 and Rolf associated with it, 80 00:03:35.270 --> 00:03:39.550 and age and 24 associated with it. 81 00:03:39.550 --> 00:03:41.644 Now if we do this for each of our friends, 82 00:03:41.644 --> 00:03:46.050 and I will do that with Adam and Anne, 83 00:03:46.050 --> 00:03:47.400 and their appropriate ages, 84 00:03:50.040 --> 00:03:52.040 That means that now what we've got 85 00:03:52.040 --> 00:03:53.680 is a list of dictionaries 86 00:03:53.680 --> 00:03:56.370 and each dictionary represent a friend. 87 00:03:56.370 --> 00:03:58.850 But that means that we can store more 88 00:03:58.850 --> 00:04:02.010 than just their name in each element. 89 00:04:02.010 --> 00:04:03.230 Before we were constrained 90 00:04:03.230 --> 00:04:05.290 to just storing their name in our list, 91 00:04:05.290 --> 00:04:07.703 but now we can store more data if we want. 92 00:04:09.750 --> 00:04:12.530 If you print friends out, you'll see that you get back 93 00:04:12.530 --> 00:04:15.500 line:15% your list of friends there. 94 00:04:15.500 --> 00:04:17.490 And of course if you wanted to print a particular friend, 95 00:04:17.490 --> 00:04:21.290 you would need to use the index, because this is a list. 96 00:04:21.290 --> 00:04:24.330 The friends list needs to be accessed by index 97 00:04:24.330 --> 00:04:26.620 so if you wanted to access index zero, 98 00:04:26.620 --> 00:04:28.580 that would give you back this dictionary. 99 00:04:28.580 --> 00:04:30.670 Index one would give you this dictionary. 100 00:04:30.670 --> 00:04:33.040 And index two would give you this dictionary. 101 00:04:33.040 --> 00:04:35.070 So let's say we want to access index one, 102 00:04:35.070 --> 00:04:36.820 which is this dictionary, 103 00:04:36.820 --> 00:04:38.080 if you print this out 104 00:04:38.080 --> 00:04:39.550 line:15% you'll see that what you get back is, 105 00:04:39.550 --> 00:04:41.990 line:15% naturally, the entire dictionary. 106 00:04:41.990 --> 00:04:46.360 That means that now, after friends one, 107 00:04:46.360 --> 00:04:50.150 you can put another subscript notation to access the name, 108 00:04:50.150 --> 00:04:51.570 for example. 109 00:04:51.570 --> 00:04:54.070 line:15% And doing this would give you Adam. 110 00:04:54.070 --> 00:04:57.300 So this is how you can access a list of dictionaries, 111 00:04:57.300 --> 00:05:00.540 or a particular value inside a list of dictionaries. 112 00:05:00.540 --> 00:05:02.220 If you had a list of lists, 113 00:05:02.220 --> 00:05:05.780 you would access multiple indices in succession like this. 114 00:05:05.780 --> 00:05:07.970 And if you had a dictionary of dictionaries, 115 00:05:07.970 --> 00:05:10.483 then you would access multiple keys in succession. 116 00:05:11.350 --> 00:05:14.030 Let me say of course that you can nest these lists 117 00:05:14.030 --> 00:05:17.030 and doubles and dictionaries however way you want in Python. 118 00:05:19.500 --> 00:05:21.220 Let's take a look at another example 119 00:05:21.220 --> 00:05:24.492 where we have a dictionary that maps the student names 120 00:05:24.492 --> 00:05:28.410 to the percentage of attendance they have to their lessons. 121 00:05:28.410 --> 00:05:31.140 So Rolf has attended 96% of his lessons. 122 00:05:31.140 --> 00:05:34.700 Bob 80% and Anne 100%. 123 00:05:34.700 --> 00:05:39.550 You can iterate over a dictionary, using a for loop. 124 00:05:39.550 --> 00:05:43.630 And what this does is it gives you the keys. 125 00:05:43.630 --> 00:05:45.210 So if you print this student variable 126 00:05:45.210 --> 00:05:46.870 that we're using here for iteration 127 00:05:46.870 --> 00:05:49.480 line:15% what you get back are the student names. 128 00:05:49.480 --> 00:05:51.463 The keys in your dictionary. 129 00:05:52.350 --> 00:05:55.883 Using that key, you can print something like this. 130 00:05:58.960 --> 00:06:01.780 Which will print out the student variable 131 00:06:01.780 --> 00:06:03.610 which is the students name 132 00:06:03.610 --> 00:06:06.570 and then you can access the student key 133 00:06:06.570 --> 00:06:08.100 in the student attendance dictionary 134 00:06:08.100 --> 00:06:11.400 using subscript notation to get their attendance. 135 00:06:11.400 --> 00:06:13.200 So if we save this and run it 136 00:06:13.200 --> 00:06:17.160 line:15% you now see that we get Rolf 96, Bob 80, Anne 100. 137 00:06:17.160 --> 00:06:20.291 You can of course put a percentage sign there if you like 138 00:06:20.291 --> 00:06:21.124 to make it a little bit nicer. 139 00:06:21.124 --> 00:06:23.820 However accessing a dictionary like this in a for loop 140 00:06:23.820 --> 00:06:27.130 doesn't look the best because there are other, better ways. 141 00:06:27.130 --> 00:06:32.040 The better way is if you do student attendance dot items 142 00:06:32.040 --> 00:06:36.467 and then here you can do for student comma attendance 143 00:06:36.467 --> 00:06:39.100 in student attendance dot items. 144 00:06:39.100 --> 00:06:41.960 And then here you no longer need to access dictionary 145 00:06:41.960 --> 00:06:44.940 because you've got it in the attendance variable. 146 00:06:44.940 --> 00:06:48.654 So what we've got here is that student attendance dot items 147 00:06:48.654 --> 00:06:51.610 gives you two values for each student 148 00:06:51.610 --> 00:06:55.590 that you can get as two separate variables here in Python. 149 00:06:55.590 --> 00:06:57.321 Saving and running this 150 00:06:57.321 --> 00:06:59.073 line:15% will give you exactly the same as before. 151 00:07:00.080 --> 00:07:02.520 You can use the in keyword in a dictionary 152 00:07:02.520 --> 00:07:05.530 to check whether a value is one of the keys 153 00:07:05.530 --> 00:07:06.363 of the dictionary. 154 00:07:06.363 --> 00:07:10.880 So for example, if Bob in student attendance, 155 00:07:10.880 --> 00:07:11.910 you can print something like 156 00:07:11.910 --> 00:07:14.733 Bob attended this many lectures. 157 00:07:16.450 --> 00:07:17.760 Otherwise, you can print that 158 00:07:17.760 --> 00:07:21.363 Bob is not a student in this class. 159 00:07:23.140 --> 00:07:25.450 So, the in keyword allows you to check 160 00:07:25.450 --> 00:07:27.820 whether a value is one of the keys. 161 00:07:27.820 --> 00:07:30.640 Notice that you can't check whether one of the values 162 00:07:30.640 --> 00:07:33.903 is in the dictionary, you can only check the keys. 163 00:07:35.110 --> 00:07:38.850 Finally, you can also get just the values back if you like. 164 00:07:38.850 --> 00:07:41.210 If you wanted 96, 80, and 100 165 00:07:41.210 --> 00:07:43.050 without worrying about the keys, 166 00:07:43.050 --> 00:07:46.030 for example if wanted to calculate an average, 167 00:07:46.030 --> 00:07:49.010 you could have a variable called attendance values 168 00:07:49.010 --> 00:07:53.080 and make it equal to student attendance dot values. 169 00:07:53.080 --> 00:07:56.042 And then you can print the sum of attendance values 170 00:07:56.042 --> 00:08:01.042 divided by the length of attendance values. 171 00:08:01.210 --> 00:08:02.700 And what this is going to give you is 172 00:08:02.700 --> 00:08:07.200 add 96, 80, and 100 together and divide by three 173 00:08:07.200 --> 00:08:10.860 which is the length of what you get back which is a list. 174 00:08:10.860 --> 00:08:13.493 line:15% So if you run that you get back 92.0. 175 00:08:15.050 --> 00:08:17.350 Going back to what we had earlier, 176 00:08:17.350 --> 00:08:20.940 which is this syntax where you have two variables 177 00:08:20.940 --> 00:08:23.860 being defined, separated by a comma inside a for loop, 178 00:08:23.860 --> 00:08:26.150 we're going to learn more about that in the next video. 179 00:08:26.150 --> 00:08:28.370 So if that's unclear, don't worry about it, 180 00:08:28.370 --> 00:08:29.570 we'll look at it just now. 181 00:08:29.570 --> 00:08:31.153 I'll see you in the next video.