WEBVTT 0 00:00.530 --> 00:00.980 All right. 1 00:00.980 --> 00:05.510 So the first thing I want to talk about is Dictionaries in Python. 2 00:05.510 --> 00:11.510 Now dictionaries in Python work kind of similarly to dictionaries in real life. 3 00:11.510 --> 00:11.990 Right? 4 00:11.990 --> 00:19.610 So if you were to look up a word in the dictionary, say the word code, then you might find the definition 5 00:19.610 --> 00:24.260 as something along the lines of, "program instructions for the computer." 6 00:24.260 --> 00:31.220 And dictionaries are really useful because they allow us to group together and tag related pieces of 7 00:31.220 --> 00:32.390 information. 8 00:32.540 --> 00:37.310 The way I like to think about dictionaries is in the form of a table. 9 00:37.370 --> 00:40.910 Every dictionary has two parts to it. 10 00:40.910 --> 00:47.330 On the left-hand side is the key, and that is the equivalent of the word in the dictionary, 11 00:47.540 --> 00:54.670 and then it's also got an associated value. That would be the equivalent of the actual definition of 12 00:54.670 --> 00:55.420 the word. 13 00:55.630 --> 01:02.500 Now let's say that we took this very simple table of definitions of programming words that we've come 14 01:02.500 --> 01:03.730 across so far, 15 01:03.730 --> 01:06.700 and we go ahead and we try to convert it into a dictionary. 16 01:06.730 --> 01:08.080 How would we do that? 17 01:08.080 --> 01:12.910 Let me firstly drop the last two rows and let's just start with the first one. 18 01:12.940 --> 01:20.740 The first thing we want to do is we want to create a dictionary and to do that in Python, this is what 19 01:20.740 --> 01:22.210 the syntax looks like. ({Key: Value}) 20 01:22.210 --> 01:24.670 We have a set of curly braces. 21 01:24.670 --> 01:29.500 And everything that's inside the curly brace is the content of our dictionary. 22 01:29.620 --> 01:36.220 The key goes first, followed by a colon and then followed by the value. 23 01:36.250 --> 01:41.110 In our table we've got this word, "Bug" which is the first key. 24 01:41.110 --> 01:44.680 So we can replace that over here in our dictionary. 25 01:44.680 --> 01:49.510 And the value that's associated with this key is the definition for a bug. 26 01:49.510 --> 01:56.050 So, "An error in a program that prevents the program from running as expected." That becomes the value and 27 01:56.050 --> 01:58.690 can be replaced here after the colon. 28 01:58.690 --> 02:03.310 So now we've created an actual dictionary using Python code. 29 02:03.910 --> 02:07.630 What if you wanted to have more than one entry in your dictionary? 30 02:07.630 --> 02:15.060 Well, you would separate each of the key value pairs using a comma, and then you can continue adding 31 02:15.060 --> 02:20.190 key and value pairs until you get to the end of your dictionary. 32 02:21.330 --> 02:21.750 All right. 33 02:21.750 --> 02:24.600 Let's take a look at dictionaries in action. 34 02:24.630 --> 02:31.800 If you head over to the Day 9 start code file, then you can see that in here I've already added 35 02:31.920 --> 02:38.010 the dictionary that we were defining earlier on, and I've stored it inside a variable called, 36 02:38.010 --> 02:38.910 programming_dictionary. 37 02:39.180 --> 02:45.180 Now currently I've only got two entries in this dictionary, the definition for bug and the definition 38 02:45.180 --> 02:46.020 for function. 39 02:46.740 --> 02:52.620 Now the first thing I want to highlight is when you create a dictionary that has more than one element, 40 02:52.620 --> 02:59.460 such as in this case, then you want to take care to format it properly so that it's more easily readable. 41 02:59.610 --> 03:05.880 So what you'll see Python programmers do by convention is they will start off the dictionary with the 42 03:05.880 --> 03:13.140 open curly brace at the top, and then every subsequent entry is indented by one indent. 43 03:13.560 --> 03:17.760 And then at the very end of that entry there's a comma. 44 03:17.760 --> 03:22.730 And then we hit enter so that the next item goes on to the next line. 45 03:22.730 --> 03:31.040 And finally, the last curly brace should go at the very beginning in line with the start of the dictionary. 46 03:31.220 --> 03:38.390 And another thing that's quite nice to do is to cap off all entries in your dictionary or list with 47 03:38.390 --> 03:39.080 a comma. 48 03:39.110 --> 03:44.630 This means that if you needed to add more items into the dictionary, you can simply just hit Enter 49 03:44.630 --> 03:47.130 and continue typing the next thing. 50 03:47.460 --> 03:54.420 And if we wanted to add another entry, it's as simple as adding in the key, a colon, and then the value. 51 03:54.660 --> 03:58.140 And to cap it off again with a comma. 52 03:58.350 --> 04:06.900 So now our dictionary represents exactly the same data as we saw in our table over here with a bunch 53 04:06.900 --> 04:11.350 of key/value pairs and a total of three entries. 54 04:11.650 --> 04:18.100 Now, the next thing I want to do is what if I wanted to retrieve an item from the dictionary? 55 04:18.100 --> 04:23.800 Because we know that if we had a list, what we would do is we would use a set of square brackets, 56 04:23.800 --> 04:27.730 and then we would give the index of the item that we wanted. 57 04:27.730 --> 04:32.830 So the item at index 0 or 1 or 2 and so on and so forth. 58 04:32.860 --> 04:39.760 Now for dictionaries it's kind of similar in terms of syntax, but the only difference is that dictionaries 59 04:39.760 --> 04:43.690 have elements which are identified by their key. 60 04:44.080 --> 04:50.860 If we wanted this piece of information, for example, then all we have to do is tap into the dictionary 61 04:50.860 --> 04:53.920 and then add a set of square brackets, 62 04:53.920 --> 04:57.940 and inside the square brackets, we're going to provide the key. 63 04:57.970 --> 05:02.920 So here the key is a string and it's the string, "Bug". 64 05:02.920 --> 05:05.380 So let's go ahead and put Bug in here. 65 05:06.100 --> 05:13.420 And now if I go ahead and print this then you can see that it's going to give me the value, which is 66 05:13.420 --> 05:17.500 "An error in a program that prevents the program from running as expected." 67 05:17.530 --> 05:25.570 Now it's really, really important that you make sure that when you're fetching something out of a dictionary 68 05:25.570 --> 05:29.470 by its key, that you actually spell the key correctly. 69 05:29.830 --> 05:35.950 A really, really common error is when you're trying to retrieve something out of a dictionary and you've 70 05:35.950 --> 05:37.870 just made a very simple typo. 71 05:37.870 --> 05:42.220 So instead of u, I'm typing o here, and you'll see we get an error. 72 05:42.790 --> 05:47.950 And the error tells us that it's a KeyError referring to this particular key, 73 05:47.950 --> 05:53.580 and it highlights this Line 7, where we're trying to retrieve something out of this dictionary 74 05:53.580 --> 05:54.720 by this key. 75 05:54.720 --> 06:00.090 Basically, it's telling you that this key doesn't actually exist and it can't be found, so it doesn't 76 06:00.090 --> 06:01.950 know what it is that you want. 77 06:02.580 --> 06:09.270 Remember how when we had lists and when we tried to retrieve something that was not inside the list? 78 06:09.270 --> 06:15.390 So, for example, this particular list at index[4] doesn't actually exist because this is 0, 79 06:15.390 --> 06:20.910 1, 2, 3, and 4 is actually not a piece of data inside this list. 80 06:21.030 --> 06:27.930 Similarly with dictionaries, if we try to put in a key that doesn't exist, then we get this KeyError. 81 06:28.800 --> 06:37.200 Now another common pitfall that students fall down into is they don't actually use the correct data 82 06:37.200 --> 06:37.710 type. 83 06:37.740 --> 06:47.270 So for example, if when we defined this dictionary without putting a string around each of these keys, 84 06:47.270 --> 06:51.050 then it's going to error out and it won't even let us run. 85 06:51.050 --> 06:56.270 It's going to tell us undefined name bug, because it thinks that this is a variable that you've declared 86 06:56.270 --> 07:01.850 somewhere, but it's not, in fact, what you wanted are these strings for keys. 87 07:01.850 --> 07:07.910 And so when you have a key that is a string, when you're trying to retrieve the data from that key, 88 07:07.940 --> 07:13.340 you also have to make sure that you provide the key in its actual data type. 89 07:13.340 --> 07:19.760 So for example, if this was just a number say, 1, 2, 3, then of course all you have to write 90 07:19.760 --> 07:21.920 in here is just 1, 2, 3. 91 07:21.920 --> 07:26.270 And it would know that this piece of data is what you wanted. 92 07:27.590 --> 07:34.220 So that's how you retrieve items from a dictionary, by adding a square bracket and then giving it the 93 07:34.220 --> 07:34.880 key, 94 07:35.150 --> 07:40.550 and it will look for the key inside the dictionary and give you back the value. 95 07:40.850 --> 07:47.030 Now what if you wanted to add a piece of data such as that loop that we had earlier on, but you want 96 07:47.030 --> 07:48.470 to do it programmatically. 97 07:48.470 --> 07:55.180 So instead of doing it when you were defining the dictionary at the beginning, what if at some later 98 07:55.180 --> 07:58.480 stage in your program, you needed to add a new entry? 99 07:58.630 --> 08:01.690 Well, to do this, it's also really simple. 100 08:01.720 --> 08:08.770 All you have to do is to tap into the dictionary, which is called programming_dictionary in our case, 101 08:08.770 --> 08:12.580 and again using square brackets we define the key. 102 08:12.790 --> 08:16.060 The key I'm going to add is "Loop", 103 08:16.060 --> 08:20.650 and then after an equal sign I get to assign the value. 104 08:21.160 --> 08:25.360 So in my case the value is going to be the definition for a loop. 105 08:25.360 --> 08:34.060 And now when this line of code is executed and we go ahead and just print the programming_dictionary 106 08:34.060 --> 08:39.310 after this has happened, then we'll see that it's actually different from what we had before, 107 08:39.310 --> 08:40.750 so let's hit Run. 108 08:40.750 --> 08:47.110 You can see that previously we had a programming_dictionary that had only two items, Bug and Function, 109 08:47.110 --> 08:53.230 and after this line where we print our programming_dictionary again, you can see it's now got three 110 08:53.230 --> 08:55.420 items Bug, Function and Loop. 111 08:56.230 --> 09:03.400 Now very often when you're writing code, it can be really helpful to start out with an empty dictionary. 112 09:03.490 --> 09:08.910 Just as you saw previously, you can create an empty list by simply having a set of square brackets 113 09:08.910 --> 09:10.470 with nothing inside. 114 09:10.500 --> 09:19.050 You can also create an empty dictionary by simply creating a set of curly braces with nothing inside, 115 09:19.050 --> 09:25.590 and then add a later stage you can add to your dictionary by using this method that you saw here. 116 09:26.490 --> 09:31.560 Now on the other hand, you might actually want to wipe an entire dictionary. 117 09:31.770 --> 09:38.250 Here I'm creating a new empty dictionary by creating this pair of curly braces with nothing inside, 118 09:38.250 --> 09:44.160 but I can also wipe an existing dictionary by simply doing the same thing. 119 09:44.160 --> 09:51.150 We know that this dictionary, programming_dictionary has three items in it, but on Line 17 I'm going 120 09:51.150 --> 09:54.480 to say programming_dictionary equals empty dictionary. 121 09:54.480 --> 10:01.650 And now if I move this print statement down here, then you can see that when it prints out, it's actually 122 10:01.650 --> 10:03.480 going to be completely empty. 123 10:04.740 --> 10:09.930 That can be really useful if you wanted to clear out a user's progress, 124 10:09.930 --> 10:16.800 or for example, if a game restarts, then all the scores and stats will probably have to be wiped empty. 125 10:16.800 --> 10:19.140 So this is one way that you could do that. 126 10:19.440 --> 10:29.150 Now this method of tapping into a dictionary, using the key to fetch the relevant item from it, and 127 10:29.150 --> 10:32.840 then doing something with it, goes beyond just adding. 128 10:32.840 --> 10:37.400 You can also use this to edit an item in a dictionary. 129 10:38.780 --> 10:44.600 For example, let me go ahead and comment out these two lines of code so we don't wipe our programming 130 10:44.600 --> 10:45.410 dictionary. 131 10:45.410 --> 10:52.520 And instead I'm going to tap into the programming_dictionary, and I'm going to fetch the item that 132 10:52.520 --> 10:54.530 has a key of Bug. 133 10:54.530 --> 10:57.230 And I'm going to redefine its value. 134 10:57.230 --> 11:03.230 So currently it should be, "An error in a program that prevents the program from running as expected." 135 11:03.230 --> 11:12.070 And I can prove that if I just wrap this around a print statement. If instead I wanted this to be a 136 11:12.070 --> 11:17.860 different value, then I can simply use the same syntax as I did for adding new items, 137 11:17.860 --> 11:23.950 but in this case, I'm actually editing this entry because it's going to look through the dictionary, 138 11:23.950 --> 11:30.400 find a value with this key, and then assign it to whatever I put on the right hand side of the equals 139 11:30.400 --> 11:30.940 sign. 140 11:30.970 --> 11:36.970 Now on the other hand, if it finds nothing with that key then it's going to create a new entry with 141 11:36.970 --> 11:39.940 the value again on the right-hand side of the equal sign. 142 11:40.360 --> 11:45.310 Now let's say that a bug is instead, A moth in your computer." 143 11:46.390 --> 11:54.130 Now if I go ahead and print my programming_dictionary once more, then you can see that the definition 144 11:54.130 --> 11:57.190 for our bug has now just been changed. 145 11:58.750 --> 12:04.810 The final thing with regards to dictionaries that I think is really, really useful is how you loop 146 12:04.810 --> 12:09.970 through a dictionary and using your knowledge from lists, 147 12:09.970 --> 12:13.300 you might think that to loop through a dictionary you would, 148 12:13.300 --> 12:20.770 let's say we're using a for loop. And we say for thing in programming_dictionary, 149 12:20.770 --> 12:25.170 let's go ahead and print this thing each time. 150 12:25.500 --> 12:31.410 Here's a good moment to play computer and think what do you expect to be printed? 151 12:31.410 --> 12:35.250 So let's comment out all the other print statements so we don't get confused. 152 12:35.250 --> 12:40.140 And when I click run, what do you think will be printed? 153 12:42.300 --> 12:42.780 All right. 154 12:42.780 --> 12:44.310 Is that what you expected? 155 12:44.310 --> 12:46.500 Because it certainly wasn't what I expected 156 12:46.500 --> 12:52.590 when I first learned Python. I thought it would give me each of the items in the dictionary with this 157 12:52.590 --> 12:59.280 key and its value, but instead this code actually just gives you the keys. 158 13:00.150 --> 13:06.540 Now, of course, once you do have access to the key instead of thing, I should actually really say, 159 13:06.540 --> 13:12.540 'for key in programming_dictionary', if I wanted to print the key, then, of course, I could just write 160 13:12.540 --> 13:13.450 print key. 161 13:13.450 --> 13:20.080 But if I wanted to get hold of the value, I could equally just as easily tap into my dictionary, use 162 13:20.080 --> 13:23.170 the square bracket, and pass in that key. 163 13:23.200 --> 13:30.550 So now when I hit Run, you can see it's giving me first the key from this line and then secondly the 164 13:30.550 --> 13:33.100 value based on this line. 165 13:33.190 --> 13:37.790 I'm using that retrieval code in order to get the value. 166 13:38.690 --> 13:44.690 Of course, remember that you can get access to all the code that I'm writing here, and if you want 167 13:44.690 --> 13:50.810 to have a quick review of everything that we've gone through so far, because a lot of this knowledge 168 13:50.810 --> 13:57.590 is going to be tested in the next lesson, where we do a coding exercise that will really solidify everything 169 13:57.590 --> 13:59.720 you've learned in this lesson so far. 170 13:59.720 --> 14:03.200 So for all of that and more, I'll see you on the next lesson.