WEBVTT 0 00:00.350 --> 00:05.810 The first concept I want to introduce you to is something called functions. 1 00:05.810 --> 00:12.710 And just as we perform many different functions could be mother, father, son, daughter, 2 00:12.710 --> 00:17.660 we can also get our code to perform different pieces of functionality. 3 00:18.110 --> 00:25.580 Now if we think back we've actually already come across functions, and I've been referring to them 4 00:25.580 --> 00:27.920 every time we've needed to use one. 5 00:28.220 --> 00:34.960 If you take a look at this link in the course resources, which takes you to the Python documentation, 6 00:34.960 --> 00:40.270 then you can see that Python has a whole bunch of built in functions that we've been using. 7 00:40.270 --> 00:47.740 For example, the len() function, which gives us the number of items in a collection, or the int() function, 8 00:47.740 --> 00:54.540 which turns something into an integer, or the print() function, the range() function, and other ones 9 00:54.540 --> 00:56.010 that we haven't seen yet. 10 00:56.640 --> 01:02.550 We can start typing one of the functions that we've been using a lot, which is the print function. 11 01:02.550 --> 01:09.270 And the reason why we know it's a function is because it's the name of a function followed by a set 12 01:09.270 --> 01:10.650 of parentheses. 13 01:10.650 --> 01:17.300 Now we know that if we put something inside these parentheses say for example ("Hello"), 14 01:17.300 --> 01:24.410 then when I run the code, whatever is inside those parentheses will get outputted over here in the 15 01:24.410 --> 01:25.220 console. 16 01:25.940 --> 01:29.720 Now all the functions work pretty much the same. 17 01:29.720 --> 01:36.320 So if I wanted to use the len() function to see, well, how many characters are there in this word 18 01:36.320 --> 01:43.510 "Hello", I again put my hello inside the parentheses and because of the parentheses, we know that this 19 01:43.510 --> 01:44.530 is a function. 20 01:44.530 --> 01:51.550 And then I can save the outcome of this length function to a variable. 21 01:51.550 --> 01:54.610 Say let's call it number of characters, num_char. 22 01:54.820 --> 01:59.650 And then I can go ahead and print it in order to view it inside the console. 23 01:59.650 --> 02:03.310 So let's print the num_char, 24 02:03.640 --> 02:09.120 and now you can see that this len() function gets to work, calculates the number of characters, 25 02:09.120 --> 02:11.460 it's 5, and then it gets printed. 26 02:11.460 --> 02:19.410 So all of this functionality has been achieved by these built-in functions from Python like print() and 27 02:19.410 --> 02:19.980 len(). 28 02:20.940 --> 02:24.750 Now what if we wanted to make our own functions? 29 02:24.750 --> 02:26.610 How would we do that? 30 02:27.270 --> 02:35.600 Well, if we want to make our own function, we first start out with a keyword which is called, def, 31 02:35.600 --> 02:40.700 and this is because we're creating or defining our function. 32 02:41.360 --> 02:45.500 Now after the def keyword we can give our function a name. 33 02:45.500 --> 02:48.890 And I'm just going to call it my_function. 34 02:49.370 --> 02:55.260 Now the thing that differentiates a function from a variable however, is the parentheses. 35 02:55.260 --> 02:58.650 So after the name comes the parentheses. 36 02:58.650 --> 03:00.540 And now the final thing, 37 03:00.540 --> 03:07.080 the finishing touch to our function definition is a colon because that says everything that comes 38 03:07.080 --> 03:12.360 after that line and is indented belongs with the function. 39 03:12.870 --> 03:16.740 Let's make a really simple function that just has two lines of code. 40 03:16.740 --> 03:20.570 Maybe It prints "Hello", and then it prints. 41 03:20.960 --> 03:21.680 "Bye". 42 03:22.100 --> 03:25.040 So now I've created my_function. 43 03:25.040 --> 03:31.700 I've placed the lines of code that are associated with this function, all indented after the definition, 44 03:31.700 --> 03:37.820 but notice how if I go ahead and run my code, actually nothing will happen. 45 03:37.820 --> 03:42.640 And the reason is because we haven't yet executed the function. 46 03:42.730 --> 03:49.540 Now we can define lots and lots of functions ahead of time, and it's only when we actually need it 47 03:49.540 --> 03:51.730 do we go ahead and trigger it. 48 03:51.730 --> 03:57.310 And to trigger it, or in programming lingo, we would say to call the function, 49 03:57.310 --> 04:05.250 all we have to do is type the name of the function, which is my_function, and then to add the parentheses 50 04:05.250 --> 04:07.350 and any necessary inputs. 51 04:07.350 --> 04:14.160 In our case, our function doesn't require any inputs, so we can leave the parentheses blank again. 52 04:14.160 --> 04:21.120 And now if I go ahead and run my code once more, then you'll see that by the time the computer reaches 53 04:21.120 --> 04:24.780 Line 5, it's going to search for this thing called my function. 54 04:24.780 --> 04:31.200 It sees that it was defined right here on Line 1, and it goes through all of the content and executes 55 04:31.200 --> 04:33.690 them in turn, line by line. 56 04:33.690 --> 04:37.530 And that's why we get, Hello and Bye being printed here. 57 04:38.580 --> 04:43.350 So just to recap, this is how we create a function in Python. 58 04:43.350 --> 04:45.030 There's two steps to it. 59 04:45.030 --> 04:51.120 The first step is to actually define the function to specify what it should do, 60 04:51.120 --> 04:54.640 and we do that by first using the def keyword. 61 04:54.640 --> 04:57.400 And then we give our function a name, 62 04:57.400 --> 05:00.340 so for example in this case I called it my_function. 63 05:00.340 --> 05:04.450 And then comes a set of parentheses and a colon. 64 05:04.780 --> 05:10.570 And then after that we get to put the lines of code which will be included in this function. 65 05:10.570 --> 05:16.240 And remember that these lines of code which goes into the function which will be carried out when this 66 05:16.240 --> 05:20.370 function is triggered, must be indented, like this. 67 05:21.000 --> 05:24.810 So once you've defined the function, you've created the recipe, 68 05:24.810 --> 05:32.130 the next step is to actually use it, which in programming lingo we would say calling the function. And 69 05:32.130 --> 05:37.770 we call the function just by specifying the name and a set of parentheses. 70 05:37.770 --> 05:44.990 And once the computer sees this line of code, it'll know to go and carry out all the instructions inside 71 05:44.990 --> 05:46.730 where we defined our function. 72 05:47.360 --> 05:53.060 And to learn about functions, I'm going to introduce you to something that's very similar to Karel 73 05:53.060 --> 05:53.810 the Robot. 74 05:54.140 --> 06:01.430 The robot is going to be something that is just going to perform the tasks that we want it to, and 75 06:01.430 --> 06:03.170 no more and no less. 76 06:03.170 --> 06:11.680 So, for example, if we wanted a robot to go to the store and pick up some milk for us, we can't simply 77 06:11.680 --> 06:18.370 just tell it, "Oh yeah, just go and buy some milk," because it won't know how to do that unless we give 78 06:18.370 --> 06:25.540 it specific instructions such as leave the house, walk two blocks to the right, walk four blocks back, 79 06:25.540 --> 06:27.130 and then two blocks to the right. 80 06:27.130 --> 06:32.280 When you go to the store, give them some money and then take the milk and come back. 81 06:32.310 --> 06:35.430 We have to program each and every step. 82 06:35.430 --> 06:42.630 But say if this functionality of getting the robot to go and pick up some milk for us is needed every 83 06:42.630 --> 06:49.680 single day, then we have to write out all of those instructions day after day, day after day. 84 06:49.680 --> 06:52.740 And at some point our fingers are gonna hurt, right? 85 06:52.740 --> 06:58.850 We're typing so much code that's repeated because we don't have a way of bundling all those instructions 86 06:58.850 --> 06:59.510 together. 87 06:59.510 --> 07:01.460 And that's where functions come in. 88 07:01.460 --> 07:06.860 Functions will give us a way of referring to all those instructions at the same time. 89 07:06.860 --> 07:12.860 That way we can give our robot a single instruction, and it will carry out all of those little steps 90 07:12.860 --> 07:15.050 for us and get us some milk. 91 07:15.740 --> 07:22.870 Now to try this out yourself, I want you to head over to the Course Resources and click on the link 92 07:22.870 --> 07:25.390 that takes you to Reeborg's World. 93 07:25.390 --> 07:32.080 Now Reeborg's World is very similar to Karel the Robot that I showed you earlier on, but it allows 94 07:32.080 --> 07:36.700 us to write Python code and it's very easy to get started. 95 07:36.820 --> 07:41.400 Now we've got this area here which we're going to use to write our Python code, 96 07:41.490 --> 07:48.480 and then once we're done, we can click on play to run the Python code and see the instructions carried 97 07:48.480 --> 07:50.280 out by our Robot. 98 07:50.400 --> 07:57.210 Now if you click on Reebok's keyboard, you can see all of the functions that we can use to command 99 07:57.210 --> 08:02.520 our robot like move() or turn_left(), or build_wall, etc.. 100 08:03.480 --> 08:06.730 Let's see if we go ahead and run this line of code here, 101 08:06.730 --> 08:07.780 move(), 102 08:08.380 --> 08:10.930 what happens? When I click Play▶️, 103 08:10.930 --> 08:17.080 you can see that our robot moves forward in the direction that it's facing by one step. 104 08:17.200 --> 08:23.740 So if we wanted it to move say for example three steps, all we have to do is to call the move() function 105 08:23.740 --> 08:24.880 three times. 106 08:25.030 --> 08:30.870 Now let's reset by clicking the Return button to return our robot back to the starting position. 107 08:30.900 --> 08:36.360 Now let's click Play again in order for our robot to carry out the new instructions. 108 08:36.960 --> 08:43.590 Notice how all the instructions were carried out very quickly. To execute each instruction, one step 109 08:43.590 --> 08:45.300 at a time, line by line, 110 08:45.300 --> 08:47.820 we can use this step-through button ⏯️. 111 08:47.820 --> 08:55.790 This button allows us to see how each instruction is executed and after each instruction, it pauses 112 08:55.790 --> 08:57.980 and highlights the next instruction. 113 08:58.010 --> 09:00.110 This is very helpful for debugging. 114 09:00.680 --> 09:03.620 So notice how we're highlighting Line 2. 115 09:03.650 --> 09:07.820 So if I click on step again it's going to carry out that function, 116 09:07.820 --> 09:10.250 and then it's going to carry out Line 3, 117 09:10.250 --> 09:12.920 and finally it reaches the end. 118 09:12.920 --> 09:16.220 And that's the end of our code. 119 09:17.560 --> 09:22.360 Now, in addition, we can use some of the other commands in the keyboard, 120 09:22.360 --> 09:24.550 for example, turn_left(). 121 09:24.730 --> 09:31.690 If I decide to say move() three times, and then turn_left(), and then maybe move() three times again, 122 09:31.720 --> 09:35.320 can you predict which square the robot is going to end up in? 123 09:36.580 --> 09:37.480 Are you ready? 124 09:37.510 --> 09:39.340 Let's go ahead and run the code. 125 09:40.870 --> 09:41.730 There we go. 126 09:41.760 --> 09:44.370 We end up on the square 4, 4. 127 09:44.400 --> 09:48.660 It moves() three forwards, turns_left(), and then moves() three steps again, 128 09:48.660 --> 09:51.900 because that's what we told it to do with our Python code. 129 09:52.290 --> 09:54.570 So now here's a question, 130 09:54.570 --> 09:59.760 if you take a look inside this set of commands, you'll notice that there is move(), and there is turn_left(), 131 09:59.760 --> 10:04.140 but there is no turn_right(), and there is no turn_around. 132 10:04.140 --> 10:09.410 So how could we use a function to create those commands? 133 10:09.410 --> 10:17.690 Because of course, we could just simply write turn_left(), and then turn_left() again. 134 10:19.280 --> 10:24.200 And this would effectively turn the robot around, right? 135 10:24.380 --> 10:30.340 But if we wanted to do that many times, say for example, if we wanted to go to 3, turn around, 136 10:30.340 --> 10:33.130 go back to 1, turn around, go back to 3, 137 10:33.130 --> 10:36.760 then we would have to write code that looks a bit like this. 138 10:36.760 --> 10:45.520 We would have to say move() two times to get to square 3, turn_left(), turn_left(), and then move two 139 10:45.520 --> 10:49.540 times again to get to 1 and then turn_left(), 140 10:49.540 --> 10:52.300 turn_left() again in order to turn around. 141 10:52.600 --> 10:58.350 This is all the code we would have to write to achieve this kind of effect. 142 10:59.040 --> 11:04.080 But instead of all of this, we could just define a new function. 143 11:04.080 --> 11:10.440 So we use the def keyword to say that this code is defining a new function called turn-around(). 144 11:11.460 --> 11:19.700 And this function is simply going to be turn_left() twice. And remember all the code that's going to go 145 11:19.700 --> 11:20.150 inside 146 11:20.150 --> 11:22.370 the function has to be indented. 147 11:22.400 --> 11:25.850 And now instead of writing turn_left(), turn_left(), 148 11:25.850 --> 11:32.000 we can simplify our code by telling it to call the function turn_around(). 149 11:32.540 --> 11:34.220 And we can do that here. 150 11:34.220 --> 11:35.690 And we can do that here. 151 11:35.840 --> 11:41.590 So now when we run the code you'll see that it does exactly the same thing as before. 152 11:41.590 --> 11:45.250 But this time we've cut down two lines of code. 153 11:45.460 --> 11:51.490 So instead of eight lines of code for our instructions, we only now need six lines. 154 11:51.490 --> 11:57.910 And you can see that the more that you need to use this function, the more typing it will save you. 155 11:58.210 --> 12:05.080 But more important than reducing the number of lines of code is that our code has also become a lot 156 12:05.080 --> 12:12.420 more readable, so the turn_around() function is much more clear than having turn_left() twice. 157 12:12.420 --> 12:15.720 Now let's see what happens when we use the step through button. 158 12:15.720 --> 12:19.410 Look closely at how each line in our code is executed. 159 12:20.040 --> 12:24.900 So now we're going to carry out the move() by one, move() by one. 160 12:24.900 --> 12:28.320 And now it's going to call turn_around(), that function. 161 12:28.320 --> 12:35.000 And it jumps over to find this function and carries out the code inside in turn, from the top to the 162 12:35.000 --> 12:35.360 bottom. 163 12:35.360 --> 12:37.250 So turn_left(), turn_left(). 164 12:37.250 --> 12:42.650 And now once that function is done, it goes back to where it was before and continues running, 165 12:42.650 --> 12:46.340 move(), move(), and then turn_around() again. 166 12:47.480 --> 12:49.790 So here's your challenge, 167 12:49.790 --> 12:55.240 try and create a separate function that turns our robot to the right. 168 12:55.480 --> 13:02.020 Pause the video, head over to the Course Resources and click on the link that takes you to Reeborg's 169 13:02.020 --> 13:02.590 World. 170 13:02.590 --> 13:07.090 And then go ahead and create your own function called turn_right(). 171 13:09.040 --> 13:15.370 All right, so this should be pretty simple because all we have to do is to define a new function called 172 13:15.370 --> 13:16.390 turn_right(). 173 13:16.570 --> 13:22.040 And this function is going to simply be turn_left() three times. 174 13:25.790 --> 13:31.070 Because turning left three times is going to effectively turn our robot, right. 175 13:31.070 --> 13:39.440 So even though we only have this one way of turning our robot, we can define our own functions to give 176 13:39.440 --> 13:43.000 it many, many other commands that it could use. 177 13:43.000 --> 13:51.850 So now using this turn_right function(), see if you can write some code that makes our robot draw a little 178 13:51.850 --> 13:57.040 square going from here to here, then to here and back to the beginning. 179 13:57.520 --> 14:02.320 Once you're done and you hit play, this is what should happen. 180 14:04.480 --> 14:08.280 So pause the video now and see if you can complete that challenge. 181 14:12.690 --> 14:15.600 All right so let's go from the beginning. 182 14:15.600 --> 14:19.800 We start out facing right at the position 1, 1. 183 14:19.860 --> 14:25.140 In order to draw the square starting from this way we have to first turn_left(), 184 14:26.220 --> 14:28.680 and then we have to move() one step. 185 14:28.680 --> 14:30.860 And that should take us to here. 186 14:31.010 --> 14:33.350 And now we have to turn_right(), 187 14:35.540 --> 14:37.400 move() one more step, 188 14:37.640 --> 14:48.740 turn_right() again, move() one more step, and finally turn_right() for the last time and move() one more step. 189 14:49.910 --> 14:56.380 So using our awesome function means that we don't have to write out turn_left() three times, 190 14:56.380 --> 15:04.360 each time we need to turn_right(). So our code can just be eight lines long instead of 14 lines long. 191 15:04.360 --> 15:08.680 And we're using our function to cut down on a lot of typing. 192 15:09.100 --> 15:15.340 But not only are there fewer lines of code, look at how much more readable our code has become thanks 193 15:15.340 --> 15:17.290 to our turn_right() function. 194 15:17.670 --> 15:24.780 It's now so much easier to read, turn_right(), and understand what it needs to do rather than seeing turn_left() 195 15:24.780 --> 15:26.820 three times in the code. 196 15:27.390 --> 15:32.370 So now it's easier to read and it's easier to follow the logic in our code. 197 15:32.730 --> 15:36.390 In the next lesson, I've got a challenge for you. 198 15:36.420 --> 15:41.250 Head over there and apply what you've learned about functions to try and solve it. 199 15:41.670 --> 15:42.630 I'll see you there.