WEBVTT 00:00.120 --> 00:07.800 Hello again! In this video, we are going to look at nested maps. A nested map is a map which contains another 00:07.800 --> 00:09.840 map, usually as its value. 00:10.740 --> 00:15.360 As a very simple example, we are going to look at the structure of an adventure game. 00:15.840 --> 00:21.300 So this has a series of levels, and the player has a different adventure in each level. 00:22.740 --> 00:26.100 We are going to use a map to represent the objects in each level. 00:26.550 --> 00:32.730 So in the first level, we have a player at position 1, and a door at position 10. So the player 00:32.730 --> 00:33.930 is just looking at the door. 00:35.640 --> 00:41.370 In the second level, we have the player at position 5, and the monster at position 10. So 00:41.370 --> 00:46.980 the player has walked towards the door and the door opens and there's a monster standing there. (Help!) 00:48.750 --> 00:53.940 So if we were writing the complete game, which I am not going to do, then it would probably have quite 00:53.940 --> 00:56.730 a lot of these levels. And we need some way to organize them. 00:57.300 --> 00:59.460 So we are going to create a game map. 01:00.000 --> 01:05.910 This will have the number of the level as its key and then the map for that level 01:06.210 --> 01:06.870 as the value. 01:07.260 --> 01:14.070 So we have level one with the level one map, and so on. And the value of this is going to be a map with 01:14.550 --> 01:17.550 keys which are ints, and values which are strings. 01:18.660 --> 01:21.870 If we were doing this in older C++, we would have to put a space in there. 01:22.590 --> 01:26.010 Otherwise, the compiler gets confused. But we no longer need to do that. 01:26.580 --> 01:33.030 If we write a lot of code which manipulates this game map type, then we can end up with a lot of angle brackets. 01:33.570 --> 01:40.640 So good thing to do is to create a type alias. And we can do that with a "using" statement, or a 01:40.650 --> 01:40.920 typedef. 01:41.400 --> 01:47.700 So when we put level_map, the compiler will know that we actually mean a map with int keys and string 01:47.700 --> 01:48.180 values. 01:49.470 --> 01:51.750 And this makes the code for the game map a bit simpler. 01:53.610 --> 01:59.790 So what happens if we intervene over this game map? We will get the elements of this game map, 01:59.790 --> 02:08.520 which are pairs, with the level number as the key and the level map as the value, so we can then iterate 02:08.520 --> 02:10.560 over all the elements in the level map. 02:10.920 --> 02:13.380 And if we keep doing that, we will get all the elements in the game. 02:15.060 --> 02:19.090 So our loop will get the level data for each level. 02:19.560 --> 02:21.330 The first member is the number of the level. 02:21.690 --> 02:25.710 And then, if we iterate over the second member, we get all the objects in that level. 02:27.550 --> 02:31.270 If we have C++17, then the code is a bit easier to read. 02:31.990 --> 02:35.200 We iterate over the game maps, and we get the level number and the level map. 02:35.620 --> 02:39.580 Then we iterate over the level map, and we get the positions and the objects. 02:42.990 --> 02:46.320 And there we are, we have our two levels and the objects within those levels. 02:47.190 --> 02:53.340 If we want to add an object to a level, or even add a completely new level, then we can just call the 02:53.460 --> 02:54.540 insert() member function. 02:55.830 --> 03:02.670 I have moved the loop iteration which prints out the elements into a separate function, to shorten the code a bit. 03:03.810 --> 03:09.150 Then we create and populate our map, as we did before. We call this function to print it out. 03:10.050 --> 03:13.320 And then we are going to insert a new object into level two. 03:13.590 --> 03:17.880 So we need to get an iterator to this element, in the game map. 03:18.720 --> 03:19.620 So we call find(). 03:20.820 --> 03:23.460 And then the second member will be the level two map. 03:23.730 --> 03:29.880 So if we insert this element into the second member, that will add the element to the level two. 03:30.930 --> 03:34.920 So we now have a magic wand at position three, which is just behind us. 03:34.980 --> 03:39.380 If you are keeping track of this. So the player is at 5, the wand is at 3, the monster is at 10. 03:39.720 --> 03:42.000 So you have to look behind to get the magic wand. 03:45.200 --> 03:50.900 And then to insert a new level, we just create a level map for it, and then we insert it with the 03:50.900 --> 03:52.010 appropriate level number. 03:57.060 --> 04:04.170 So there is our initial map. Then we insert the magic wand into level two, at position 3. 04:05.580 --> 04:08.640 And then we insert our new level, at level 3. 04:13.450 --> 04:19.000 And for removing objects we can just call erase(). So, same code again. 04:22.280 --> 04:25.910 And now we are going to remove the element with key 10 from level 2. 04:26.150 --> 04:27.710 So we are going to remove the monster. 04:28.730 --> 04:33.770 So we need an iterator to the level two entry in the game map. 04:35.130 --> 04:36.960 But we already have one from earlier on. 04:37.120 --> 04:38.100 So we can just use that. 04:39.350 --> 04:43.640 We need to use reference to auto, because we are going to modify this level 2 map. 04:44.730 --> 04:49.050 Then we call find() to get the element in level two, which has the key 10. 04:50.010 --> 04:51.120 And then we just erase it. 04:54.500 --> 04:59.030 So we have the maps that we had from the last time we ran this program, with the magic wand, and 04:59.030 --> 05:02.420 the level 3, and then we remove the monster from level two. 05:03.050 --> 05:05.090 So we have waved the magic wand at the monster. 05:05.090 --> 05:07.890 The monster has disappeared, and now we are right next to a bomb. 05:08.450 --> 05:09.710 (This game just keeps getting worse!) 05:10.430 --> 05:11.810 Okay, so that is it for this video. 05:12.170 --> 05:12.950 I will see you next time. 05:13.220 --> 05:15.020 But until then, keep coding!