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These are the user uploaded subtitles that are being translated: 1 00:00:00,640 --> 00:00:05,590 The first concept I want to introduce you to is something called functions. 2 00:00:06,250 --> 00:00:11,200 And just as we perform many different functions could be mother, father, 3 00:00:11,220 --> 00:00:12,490 son, daughter, 4 00:00:12,970 --> 00:00:17,470 we can also get our code to perform different pieces of functionality. 5 00:00:18,250 --> 00:00:20,110 Now, if we think back, 6 00:00:20,230 --> 00:00:25,230 we've actually already come across functions and I've been referring to them 7 00:00:25,750 --> 00:00:27,610 every time we've needed to use one. 8 00:00:28,510 --> 00:00:31,960 If you take a look at this link in the course resources 9 00:00:32,020 --> 00:00:34,750 which takes you to the Python documentation, 10 00:00:35,290 --> 00:00:39,340 then you can see that Python has a whole bunch of built in functions that we've 11 00:00:39,340 --> 00:00:42,370 been using. For example, the len function, 12 00:00:42,400 --> 00:00:47,400 which gives us the number of items in a collection or the int function 13 00:00:47,860 --> 00:00:52,420 which turns something into an integer or the print function, 14 00:00:52,420 --> 00:00:55,840 the range function, and other ones that we haven't seen yet. 15 00:00:56,770 --> 00:00:58,720 If you head over to the Day 6 16 00:00:58,750 --> 00:01:03,070 starting Repl.it and go ahead and forward a copy of it. 17 00:01:03,910 --> 00:01:08,290 Then we can start typing one of the functions that we've been using a lot, 18 00:01:08,320 --> 00:01:09,850 which is the print function. 19 00:01:10,480 --> 00:01:14,680 And the reason why we know it's a function is because it's the name of a 20 00:01:14,680 --> 00:01:18,010 function followed by a set of parentheses. 21 00:01:18,550 --> 00:01:22,960 Now we know that if we put something inside these parentheses, 22 00:01:23,290 --> 00:01:27,070 say, for example, hello, then when I run the code, 23 00:01:27,430 --> 00:01:29,110 whatever is inside those parentheses 24 00:01:29,110 --> 00:01:32,650 will get outputted over here in the console. 25 00:01:33,820 --> 00:01:37,360 Now all of the functions work pretty much the same. 26 00:01:37,450 --> 00:01:41,560 So if I wanted to use the len function to see, well, 27 00:01:41,560 --> 00:01:45,580 how many characters are there in this word Hello? I, again, 28 00:01:45,640 --> 00:01:50,260 put my hello inside the parentheses because of the parentheses 29 00:01:50,290 --> 00:01:55,290 we know that this is a function and then I can save the outcome of this length 30 00:01:55,870 --> 00:01:59,680 function to a variable, say, 31 00:01:59,740 --> 00:02:02,230 let's call it number of characters, num_char. 32 00:02:02,740 --> 00:02:07,270 And then I can go ahead and print it in order to view it inside the console. 33 00:02:07,270 --> 00:02:10,870 So let's print the num_char, 34 00:02:11,560 --> 00:02:15,610 and now you can see that this len function gets to work, calculates the 35 00:02:15,610 --> 00:02:18,970 number of characters, it's five, and then it gets printed. 36 00:02:19,300 --> 00:02:24,300 So all of this functionality has been achieved by these built-in functions from 37 00:02:25,240 --> 00:02:27,430 Python, like print and len. 38 00:02:28,810 --> 00:02:32,350 Now what if we wanted to make our own functions? 39 00:02:32,410 --> 00:02:37,360 How would we do that? Well, if we want to make our own function, 40 00:02:37,750 --> 00:02:40,780 we first start out with a keyword, 41 00:02:41,260 --> 00:02:43,030 which is called def. 42 00:02:43,540 --> 00:02:48,070 And this is because we're creating or defining our function. 43 00:02:49,090 --> 00:02:50,740 Now, after the def keyword, 44 00:02:51,070 --> 00:02:56,070 we can give our function a name and I'm just going to call it my_function. 45 00:02:57,220 --> 00:03:00,910 Now, the thing that differentiates a function from a variable, however, 46 00:03:01,330 --> 00:03:05,980 is the parentheses. So after the name comes the parentheses, 47 00:03:06,580 --> 00:03:08,170 and now the final thing, 48 00:03:08,170 --> 00:03:13,170 the finishing touch to our function definition is a colon because that says 49 00:03:13,720 --> 00:03:18,720 everything that comes after that line and is indented belongs with the 50 00:03:19,180 --> 00:03:20,013 function. 51 00:03:20,710 --> 00:03:24,280 Let's make a really simple function that just has two lines of code. 52 00:03:24,340 --> 00:03:29,080 Maybe it prints Hello and then it prints Bye. 53 00:03:29,980 --> 00:03:32,590 So now I've created my function, 54 00:03:32,740 --> 00:03:37,090 I've placed the lines of code that are associated with this function all indented 55 00:03:37,090 --> 00:03:40,600 after the definition. But notice how, 56 00:03:40,660 --> 00:03:45,370 if I go ahead and run my code, actually nothing will happen. 57 00:03:45,580 --> 00:03:49,930 And the reason is because we haven't yet executed the function. 58 00:03:50,590 --> 00:03:54,460 Now we can define lots and lots of functions ahead of time. 59 00:03:54,850 --> 00:03:57,100 And it's only when we actually need it 60 00:03:57,280 --> 00:04:02,280 do we go ahead and trigger it and to trigger it or in programming lingo, 61 00:04:02,320 --> 00:04:04,720 we would say to call the function, 62 00:04:05,050 --> 00:04:08,140 all we have to do is type the name of the function, 63 00:04:08,530 --> 00:04:10,180 which is my_function 64 00:04:10,720 --> 00:04:15,720 and then to add the parentheses and any necessary inputs. In our case, 65 00:04:16,060 --> 00:04:20,860 our function doesn't require any inputs so we can leave the parentheses blank 66 00:04:20,950 --> 00:04:25,120 again. And now if I go ahead and run my code once more, 67 00:04:25,480 --> 00:04:29,470 then you'll see that by the time the computer reaches line 5, 68 00:04:29,770 --> 00:04:32,230 it's going to search for this thing called my_ function. 69 00:04:32,590 --> 00:04:35,470 It sees that it was defined right here on line 1 70 00:04:35,770 --> 00:04:40,770 and it goes through all of the content and executes them in turn, line by line. 71 00:04:41,650 --> 00:04:44,890 And that's why we get hello and bye being printed here. 72 00:04:46,420 --> 00:04:50,710 So just to recap, this is how we create a function in Python. 73 00:04:51,190 --> 00:04:56,140 There's two steps to it. The first step is to actually define the function, 74 00:04:56,440 --> 00:04:58,480 to specify what it should do. 75 00:04:58,990 --> 00:05:03,990 And we do that by first using the def keyword and then we give our function a 76 00:05:04,390 --> 00:05:06,370 name. So for example, in this case, 77 00:05:06,370 --> 00:05:11,370 I called it my_function and then comes a set of parentheses and a colon. 78 00:05:12,670 --> 00:05:16,030 And then after that, we get to put the lines of code 79 00:05:16,030 --> 00:05:20,260 which will be included in this function. And remember that these lines of code 80 00:05:20,260 --> 00:05:22,180 which goes into the function 81 00:05:22,210 --> 00:05:24,880 which will be carried out when this function is triggered 82 00:05:25,150 --> 00:05:27,790 must be indented like this. 83 00:05:28,720 --> 00:05:32,410 So once you've defined the function, you've created the recipe, 84 00:05:32,440 --> 00:05:36,610 the next step is to actually use it, which in programming lingo, 85 00:05:36,640 --> 00:05:41,640 we would say calling the function. And we call the function just by specifying 86 00:05:42,520 --> 00:05:45,130 the name and a set of parentheses. 87 00:05:45,640 --> 00:05:50,640 And once the computer sees this line of code it'll know to go and carry out all 88 00:05:50,770 --> 00:05:52,570 the instructions inside 89 00:05:52,600 --> 00:05:56,560 where we defined our function. And to learn about functions, 90 00:05:56,890 --> 00:06:01,190 I'm to introduce you to something that's very similar to Karel the robot. 91 00:06:02,030 --> 00:06:07,030 The robot is going to be something that is just going to perform the tasks that 92 00:06:07,070 --> 00:06:10,550 we want it to, and no more and no less. 93 00:06:11,060 --> 00:06:12,290 So for example, 94 00:06:12,470 --> 00:06:17,470 if we wanted a robot to go to the store and pick up some milk for us, 95 00:06:18,410 --> 00:06:20,510 we can't simply just tell it, Oh yeah, 96 00:06:20,540 --> 00:06:24,620 just go and buy some milk because it won't know how to do that 97 00:06:24,800 --> 00:06:25,580 unless 98 00:06:25,580 --> 00:06:29,810 we give it specific instructions such as leave the house, 99 00:06:29,840 --> 00:06:34,550 walk two blocks to the right, walk four blocks back and then two blocks to the right. 100 00:06:34,940 --> 00:06:36,140 When you go to the store, 101 00:06:36,140 --> 00:06:39,590 give them some money and then take the milk and come back. 102 00:06:40,130 --> 00:06:44,030 We have to program each and every step. But say, 103 00:06:44,030 --> 00:06:49,010 if this functionality of getting the robot to go and pick up some milk for us is 104 00:06:49,010 --> 00:06:51,110 needed every single day 105 00:06:51,500 --> 00:06:55,970 then we have to write out all of those instructions day after day, 106 00:06:55,970 --> 00:07:00,260 day after day. And at some point our fingers are going to hurt, right? 107 00:07:00,350 --> 00:07:05,350 We're typing so much code that's repeated because we don't have a way of bundling 108 00:07:05,450 --> 00:07:08,870 all of those instructions together. And that's where functions come in. 109 00:07:09,260 --> 00:07:12,890 Functions will give us a way of referring to all those instructions 110 00:07:13,010 --> 00:07:14,240 at the same time. 111 00:07:14,690 --> 00:07:19,610 That way we can give our robot a single instruction and it will carry out all of 112 00:07:19,610 --> 00:07:22,430 those little steps for us and get us some milk. 113 00:07:23,510 --> 00:07:25,460 Now to try this out yourself, 114 00:07:25,490 --> 00:07:30,490 I want you to head over to the course resources and click on the link that takes 115 00:07:31,070 --> 00:07:32,690 you to Reeborg's World. 116 00:07:33,290 --> 00:07:37,610 Now Reeborg's World is very similar to Karel the robot that I showed you 117 00:07:37,640 --> 00:07:38,473 earlier on, 118 00:07:38,630 --> 00:07:43,630 but it allows us to write Python code. And it's very easy to get started. 119 00:07:44,570 --> 00:07:48,710 Now we've got this area here which we're going to use to write our Python code. 120 00:07:49,370 --> 00:07:52,340 And then once we're done, we can click on play to 121 00:07:52,340 --> 00:07:57,340 run the Python code and see the instructions carried out by our robot. 122 00:07:58,340 --> 00:08:00,650 But if you click on Reeborg's keyboard, 123 00:08:00,980 --> 00:08:05,480 you can see all of the functions that we can use to command our robot, 124 00:08:05,930 --> 00:08:09,950 like move or turn left, or build a wall, et cetera. 125 00:08:11,270 --> 00:08:15,170 Let's see if we go ahead and run this line of code here, move. 126 00:08:16,280 --> 00:08:18,380 What happens? When I click play 127 00:08:18,710 --> 00:08:23,710 you can see that our robot moves forward in the direction that it's facing by 128 00:08:23,720 --> 00:08:28,550 one step. So if we wanted it to move, say for example, three steps, 129 00:08:28,880 --> 00:08:32,270 all we have to do is the call the move function three times. 130 00:08:32,900 --> 00:08:37,280 Now let's reset by clicking the return button to return our robot back to the 131 00:08:37,280 --> 00:08:38,300 starting position. 132 00:08:38,750 --> 00:08:42,950 Now let's click play again in order for our robot to carry out the new 133 00:08:42,950 --> 00:08:43,790 instructions. 134 00:08:44,840 --> 00:08:49,640 Notice how all the instructions were carried out very quickly. To execute each 135 00:08:49,640 --> 00:08:52,850 instruction one step at a time, line by line, 136 00:08:53,030 --> 00:08:55,200 we can use this step-through button. 137 00:08:55,680 --> 00:09:00,680 This button allows us to see how each instruction is executed and after each 138 00:09:01,380 --> 00:09:05,490 instruction, it pauses and highlights the next instruction. 139 00:09:05,760 --> 00:09:07,530 This is very helpful for debugging. 140 00:09:08,370 --> 00:09:13,080 So notice how we're highlighting line 2. So if I click on step again, 141 00:09:13,380 --> 00:09:17,220 it's going to carry out that function and then it's going to carry out line 142 00:09:17,220 --> 00:09:20,430 3 and finally it reaches the end. 143 00:09:20,730 --> 00:09:25,710 And that's the end of our code. Now, 144 00:09:25,740 --> 00:09:29,760 in addition, we can use some of the other commands in the keyboard 145 00:09:30,120 --> 00:09:31,890 for example, turn left. 146 00:09:32,550 --> 00:09:37,550 If I decide to say move three times and then turn left and then maybe move three 147 00:09:38,250 --> 00:09:42,660 times again, can you predict which square the robot is going to end up in? 148 00:09:44,250 --> 00:09:49,230 Are you ready? Let's go ahead and run the code. There we go. 149 00:09:49,440 --> 00:09:51,810 We end up on the square 4,4. 150 00:09:51,990 --> 00:09:56,160 It moves three forwards turns left and then moves three steps again, 151 00:09:56,430 --> 00:09:59,250 because that's what we told it to do with our Python code. 152 00:10:00,150 --> 00:10:04,440 So now here's a question. If you take a look inside the set of commands, 153 00:10:04,500 --> 00:10:08,790 you'll notice that there is move and there is turn left. But there is no turn 154 00:10:08,790 --> 00:10:11,460 right and there is no turn around. 155 00:10:12,030 --> 00:10:16,680 So how could we use a function to create those commands? 156 00:10:17,310 --> 00:10:20,340 Because, of course, we could just simply write, 157 00:10:20,400 --> 00:10:25,140 turn left and then turn left again. 158 00:10:27,180 --> 00:10:31,470 And this would effectively turn the robot around, right? 159 00:10:32,280 --> 00:10:35,700 But if we want to do that many times, say for example, 160 00:10:35,700 --> 00:10:39,510 if we wanted to go to three, turn around, go back to one, turn around, 161 00:10:39,510 --> 00:10:44,340 go back to three, then we would have to write code that looks a bit like this. 162 00:10:44,340 --> 00:10:45,960 We would have to say move 163 00:10:47,550 --> 00:10:51,360 two times to get to square three, turn left, turn left, 164 00:10:51,480 --> 00:10:56,480 and then move two times again to get to one and then turn left, 165 00:10:57,120 --> 00:10:59,640 turn left again in order to turn around. 166 00:11:00,390 --> 00:11:05,390 This is all the code we would have to write to achieve this kind of effect. 167 00:11:06,900 --> 00:11:11,430 But instead of all of this, we could just define a new function. 168 00:11:11,970 --> 00:11:15,930 So we use the def keyword to say that this code is defining a new function 169 00:11:16,200 --> 00:11:17,880 called turn_around 170 00:11:19,410 --> 00:11:24,410 and this function is simply going to be turn left twice. And remember, all the 171 00:11:26,190 --> 00:11:29,640 code that's going to go inside the function has to be indented. 172 00:11:30,330 --> 00:11:33,480 And now instead of writing, turn left, turn left, 173 00:11:33,510 --> 00:11:37,950 we can simplify our code by telling it to call the function, 174 00:11:38,250 --> 00:11:42,960 turn_around, and we can do that here and we can do that here. 175 00:11:43,710 --> 00:11:45,780 So now when we run the code, 176 00:11:45,840 --> 00:11:48,930 you'll see that it does exactly the same thing as before, 177 00:11:49,500 --> 00:11:52,590 but this time we've cut down two lines of code. 178 00:11:53,320 --> 00:11:56,350 So instead of eight lines of code for our instructions, 179 00:11:56,680 --> 00:11:58,870 we only now need six lines. 180 00:11:59,410 --> 00:12:02,950 And you can see that the more that you need to use this function, 181 00:12:03,310 --> 00:12:08,080 the more typing it will save you. But more important than reducing 182 00:12:08,080 --> 00:12:13,080 the number of lines of code is that our code has also become a lot more 183 00:12:13,090 --> 00:12:13,923 readable. 184 00:12:14,350 --> 00:12:19,350 So the turn_around function is much more clear than having turn left twice. 185 00:12:20,260 --> 00:12:23,140 Now let's see what happens when we use the step through button. 186 00:12:23,560 --> 00:12:26,830 Look closely at how each line in our code is executed. 187 00:12:27,850 --> 00:12:32,320 So now what we're going to carry out the move by one, move by one 188 00:12:32,740 --> 00:12:37,740 and now it's going to call turn_around, that function. And it jumps over to find 189 00:12:38,020 --> 00:12:42,610 this function and carries out the code inside in turn from the top to the 190 00:12:42,610 --> 00:12:46,870 bottom. So turn left, turn left. And now once that function's done, 191 00:12:46,900 --> 00:12:51,280 it goes back to where it was before and continues running, move, move, 192 00:12:51,700 --> 00:12:53,770 and then turn around again. 193 00:12:55,330 --> 00:12:57,310 So here's your challenge. 194 00:12:57,610 --> 00:13:02,560 Try and create a separate function that turns our robot to the right. 195 00:13:03,310 --> 00:13:04,240 Pause the video, 196 00:13:04,360 --> 00:13:09,070 head over to the course resources and click on the link that takes you to 197 00:13:09,070 --> 00:13:10,030 Reeborg's World 198 00:13:10,420 --> 00:13:14,500 and then go ahead and create your own function called a turn_right. 199 00:13:15,860 --> 00:13:16,693 Right. 200 00:13:16,780 --> 00:13:17,140 All right. 201 00:13:17,140 --> 00:13:22,030 So this should be pretty simple because all we have to do is to define a new 202 00:13:22,030 --> 00:13:23,770 function called turn_right 203 00:13:24,580 --> 00:13:29,470 and this function is going to simply be turn left three times 204 00:13:30,100 --> 00:13:30,933 Right? 205 00:13:33,630 --> 00:13:37,890 because turning left three times is going to effectively turn our robot 206 00:13:37,890 --> 00:13:38,723 right. 207 00:13:38,940 --> 00:13:43,940 So even though we only have this one way of turning our robot, 208 00:13:44,700 --> 00:13:47,790 we can define our own functions to give it many, 209 00:13:47,790 --> 00:13:50,280 many other commands that it could use. 210 00:13:50,940 --> 00:13:54,360 So now using this turn right function, 211 00:13:54,810 --> 00:13:59,810 see if you can write some code that makes our robot draw a little square going 212 00:14:00,450 --> 00:14:04,410 from here to here then to here and back to the beginning. 213 00:14:05,340 --> 00:14:09,720 Once you're done and you hit play, this is what should happen. 214 00:14:12,350 --> 00:14:15,710 So pause the video now and see if you can complete that challenge. 215 00:14:19,730 --> 00:14:20,460 Right? 216 00:14:20,460 --> 00:14:23,010 All right. So let's go from the beginning. 217 00:14:23,430 --> 00:14:26,730 We start out facing right at the position 1,1 218 00:14:26,730 --> 00:14:30,600 one in order to draw the square, starting from this way, 219 00:14:30,840 --> 00:14:32,520 we have to first turn left 220 00:14:34,110 --> 00:14:38,220 and then we have to move one step and that should take us to here. 221 00:14:38,970 --> 00:14:43,740 And now we have to turn right, move 222 00:14:43,740 --> 00:14:46,770 one more step, turn right again, 223 00:14:49,080 --> 00:14:54,080 move one more step and finally turn right for the last time and move one more 224 00:14:55,580 --> 00:14:56,413 step. 225 00:14:57,770 --> 00:15:02,770 So using our awesome function means that we don't have to right out turn left 226 00:15:03,110 --> 00:15:05,780 three times each time we need to turn right. 227 00:15:06,260 --> 00:15:11,260 So our code can just be eight lines long instead of 14 lines long 228 00:15:12,200 --> 00:15:16,070 and we're using our function to cut down on a lot of typing. 229 00:15:16,970 --> 00:15:19,370 But not only are there fewer lines of code, 230 00:15:19,670 --> 00:15:24,020 look how much more readable our code has become thanks to our turn right 231 00:15:24,020 --> 00:15:28,670 function. It's now so much easier to read, turn right, 232 00:15:28,820 --> 00:15:33,740 and understand what it needs to do rather than seeing turn left three times in 233 00:15:33,740 --> 00:15:34,573 the code. 234 00:15:35,210 --> 00:15:39,710 So now it's easier to read and it's easier to follow the logic in our code. 235 00:15:40,400 --> 00:15:41,600 In the next lesson, 236 00:15:41,690 --> 00:15:46,640 I've got a challenge for you. Head over there and apply what you've learned 237 00:15:46,670 --> 00:15:50,120 about functions to try and solve it. I'll see you there. 22894