WEBVTT 00:00:00.340 --> 00:00:05.360 Okay, so now we know what pointers are, but what is the point of pointers? 00:00:05.370 --> 00:00:09.780 The most basic thing you can do with a pointer is to dereference it. 00:00:09.790 --> 00:00:13.620 This is also known as indirection because you are not accessing the 00:00:13.620 --> 00:00:17.530 variable directly, you are getting access through the pointer. To 00:00:17.530 --> 00:00:21.270 dereference a pointer variable, you can just prepend it with this 00:00:21.280 --> 00:00:26.020 asterisk character. In this context, this character is known as 00:00:26.020 --> 00:00:27.560 dereference operator. 00:00:27.570 --> 00:00:31.090 So if I put this operator next to the addr_of_x pointer, 00:00:31.100 --> 00:00:35.400 I'm basically saying that I want to get the value stored at the memory 00:00:35.400 --> 00:00:40.150 address contained in the pointer, the address that this pointer points to. 00:00:40.940 --> 00:00:44.360 In this case, this would resolve to a number 2. 00:00:45.040 --> 00:00:49.260 We are accessing the value from the variable x through the pointer. 00:00:50.440 --> 00:00:54.020 So basically, when you add this dereference operator to the pointer, 00:00:54.030 --> 00:00:57.550 you can use this pointer just like a variable that it points to. 00:00:58.340 --> 00:01:01.200 If you put this on the left side of the assignment, 00:01:01.210 --> 00:01:04.300 you can update the value of the pointed place in memory. 00:01:04.360 --> 00:01:09.760 So if I assign 5, this will update the value of the variable x to 5. 00:01:10.440 --> 00:01:14.420 We should not confuse this operator with the asterisk we use to declare the 00:01:14.420 --> 00:01:19.170 variable as a pointer. Only in the case of pointer declaration this character 00:01:19.170 --> 00:01:23.890 is declaring this variable as a pointer. In any other context, when this 00:01:23.890 --> 00:01:26.000 character is next to the pointer variable, 00:01:26.010 --> 00:01:29.130 it is used to the reference it so that we can read and write 00:01:29.130 --> 00:01:31.560 to the memory address that it contains. 00:01:33.840 --> 00:01:37.650 I wrote this simple program as an exercise so that you can use it 00:01:37.660 --> 00:01:40.320 to see if you understand how pointers work. 00:01:40.340 --> 00:01:43.450 You can pause the video and try to guess what will be 00:01:43.450 --> 00:01:46.510 printed out from each one of these statements and then 00:01:46.510 --> 00:01:48.560 come back to check if you're right. 00:01:49.540 --> 00:01:53.870 Of course, I could have made this harder by using the variable x in the same 00:01:53.870 --> 00:01:58.610 statement where I dereference the pointer which points to the variable x, but 00:01:58.610 --> 00:02:01.260 the point of this exercise is not to trick you. 00:02:01.840 --> 00:02:04.060 Okay, let's quickly go through this code. 00:02:04.640 --> 00:02:06.720 These first two lines are easy. 00:02:06.920 --> 00:02:10.460 We are just printing out the values stored in each variable. 00:02:11.340 --> 00:02:16.740 The third line is dereferencing the pointer p. Since p is pointing to 00:02:16.740 --> 00:02:20.980 the address of variable x, that means that the value printed out here 00:02:20.980 --> 00:02:24.450 will be the value stored in x, which is 3. 00:02:25.440 --> 00:02:26.760 Let's go to the next line. 00:02:27.340 --> 00:02:31.260 This expression, in case you didn't know, is the same as this one. 00:02:32.340 --> 00:02:35.540 You are adding something to the value from this variable and 00:02:35.540 --> 00:02:38.460 then storing that result to the same variable. 00:02:39.240 --> 00:02:44.640 The right side of the assignment always gets executed first. Dereferenced 00:02:44.640 --> 00:02:49.740 pointer p will get the value from the variable x, which is 3, and it will 00:02:49.750 --> 00:02:52.360 add it to the value from the variable y, 00:02:52.370 --> 00:02:58.240 which is 5. Five plus three is eight, and this value gets stored 00:02:58.250 --> 00:03:01.750 inside of the dereferenced pointer, which is x. 00:03:03.240 --> 00:03:06.860 So the variable x is now holding the number 8. 00:03:07.740 --> 00:03:11.620 Then we are reassigning this pointer to the address of the 00:03:11.620 --> 00:03:15.180 variable y. In this case, we are not changing the value 00:03:15.180 --> 00:03:16.650 that this pointer points to. 00:03:16.660 --> 00:03:19.660 We are changing the value from the pointer itself, 00:03:19.670 --> 00:03:21.860 the memory address it contains. 00:03:22.540 --> 00:03:26.630 And now in this expression, the dereferenced pointer is the 00:03:26.630 --> 00:03:33.000 value from y, which is 5. Five plus 5 is 10, and we store that 00:03:33.000 --> 00:03:35.150 number inside of the variable y. 00:03:36.340 --> 00:03:42.900 So y will now be 10, x will be 8, and the dereferenced pointer will also be 00:03:42.900 --> 00:03:50.130 10 because this is basically the same thing as writing y here. Let's compile 00:03:50.130 --> 00:03:52.560 this and make sure that we did everything right. 00:03:53.640 --> 00:03:58.500 And it seems like we did. You can make your own small exercises 00:03:58.500 --> 00:04:01.950 like this one and move things around just so that you can see if 00:04:01.950 --> 00:04:05.840 you understand what is going on. Now we know how pointers work, but 00:04:05.840 --> 00:04:07.530 with this dereference operator, 00:04:07.530 --> 00:04:11.010 it seems that their only purpose is to indirectly access 00:04:11.010 --> 00:04:14.730 some other variable, like an alias, two variables that can 00:04:14.730 --> 00:04:16.459 access the same data in memory. 00:04:16.740 --> 00:04:20.440 And this might not seem like something useful, but to really see 00:04:20.440 --> 00:04:24.120 how essential this is in programming, we need to first cover one 00:04:24.120 --> 00:04:26.430 more thing, and that is scope. 00:04:26.440 --> 00:04:27.960 I'll see you in the next lesson.