1 00:00:04,840 --> 00:00:08,980 So I've created a new project in IntelliJ for this section, as I've been 2 00:00:09,020 --> 00:00:12,700 doing for each section of the course. I've called this one, as you can see, 3 00:00:12,700 --> 00:00:17,780 Functions_Intro. So to get started, we need a new python file for our code. 4 00:00:17,780 --> 00:00:21,240 I'm going to call mine functions. 5 00:00:27,220 --> 00:00:31,030 Okay, so we've used some functions already in the course. In this video, 6 00:00:31,030 --> 00:00:35,400 we'll start writing our own. Once you've seen a couple of examples, I'll discuss 7 00:00:35,400 --> 00:00:40,560 the formal definition of a function. A Python function definition starts with 8 00:00:40,570 --> 00:00:44,840 the keyword def. I'm going to call this one multiply, so I'm going to type def 9 00:00:44,840 --> 00:00:52,760 space multiply. Then I'm going to add left and right parentheses to that, and add a colon. 10 00:00:52,760 --> 00:00:56,360 Function names follow the same rules as variable names. They're written 11 00:00:56,379 --> 00:01:00,640 in lowercase and mustn't start with a digit. You can use an underscore 12 00:01:00,640 --> 00:01:05,290 to separate the words in the name, just like we did for our variables. After the 13 00:01:05,290 --> 00:01:09,580 function name, we have opening and closing parentheses. Finally, we end with 14 00:01:09,580 --> 00:01:12,800 a colon. Our function starts a new block of code, 15 00:01:12,800 --> 00:01:17,620 then any statement that starts a new block, always ends with a colon. Inside 16 00:01:17,620 --> 00:01:21,440 the function, we write the code that we want to be executed, when the function 17 00:01:21,440 --> 00:01:27,520 gets called. I'll keep this one simple to start with. We're just going to multiply two numbers: 18 00:01:27,540 --> 00:01:34,260 result is equal to 10.5 x 4. Press ENTER and then I'm 19 00:01:34,260 --> 00:01:42,280 going to type return result. So line 2 is simple - it just binds the variable result 20 00:01:42,280 --> 00:01:45,860 to the result of multiplying the two numbers. Our multiply function 21 00:01:45,860 --> 00:01:50,680 multiplies 10.5 by 4. Now that's not very useful if the function doesn't let 22 00:01:50,680 --> 00:01:54,550 us know what the answer is. So that we can find out what value our function 23 00:01:54,550 --> 00:01:58,930 calculated, we tell the function to return the result. So that's the return 24 00:01:58,930 --> 00:02:02,680 statement on line three. You've probably got a few 25 00:02:02,680 --> 00:02:05,860 questions at this point, but don't worry, we're just starting, and we will be 26 00:02:05,860 --> 00:02:09,639 covering more about functions in a moment. First though, let's see how to use 27 00:02:09,639 --> 00:02:12,120 the function. You already know how to do that, 28 00:02:12,120 --> 00:02:15,640 because we've used quite a few functions in the course already. The only 29 00:02:15,640 --> 00:02:19,900 difference is, this time we wrote the function ourselves. So I'm going to press 30 00:02:19,900 --> 00:02:23,740 ENTER a few times. Come down to line 6 - I'm going to type answer 31 00:02:23,740 --> 00:02:29,900 is equal to multiply, noting that IntelliJ helps us automatically, by giving us that. 32 00:02:29,900 --> 00:02:33,040 Press ENTER there. On the next line I'm going to print, and in 33 00:02:33,040 --> 00:02:38,659 parentheses, answer. So on line six, I'm calling the multiply 34 00:02:38,659 --> 00:02:42,440 function. It doesn't have any arguments, but we still need to include the 35 00:02:42,440 --> 00:02:46,879 parentheses, as I've done, and because our function returns a value, we can bind the 36 00:02:46,879 --> 00:02:52,190 variable answer to the return value. And by doing that, that lets us print it out 37 00:02:52,190 --> 00:02:57,040 on line seven. We run the program now, 38 00:02:58,120 --> 00:03:03,040 and we can see that the answer is 42, well actually, 42.0 to be precise. 39 00:03:03,040 --> 00:03:08,680 We multiplied an int by a float, so the result is a float. Now that's a very simple 40 00:03:08,690 --> 00:03:13,100 function, and it's not really doing anything useful. We'll change it soon, so 41 00:03:13,100 --> 00:03:17,210 that it does in fact, do something more useful. Before we do that, though, let's 42 00:03:17,210 --> 00:03:23,120 examine the structure of our function. It starts with the Python keyword def - def 43 00:03:23,120 --> 00:03:27,560 is short for define. That's followed by the name of the function - multiply, in 44 00:03:27,560 --> 00:03:32,900 this case. All function definitions have parentheses after the function name. You 45 00:03:32,900 --> 00:03:36,620 can also define parameters inside the parentheses, but at the moment, our 46 00:03:36,620 --> 00:03:41,540 function doesn't have any parameters. A function definition starts a new block, 47 00:03:41,540 --> 00:03:46,040 which means that we have to indent the body of the function. Lines two and three 48 00:03:46,040 --> 00:03:50,150 are indented one level, which makes them part of the function, and also notice 49 00:03:50,150 --> 00:03:54,740 that the function definition ends with a colon, at the end of line one. 50 00:03:54,740 --> 00:03:59,840 Our multiply function is performing a calculation. We return the answer to the 51 00:03:59,840 --> 00:04:05,090 calling code, using the return keyword that's on line three. The convention is 52 00:04:05,090 --> 00:04:09,890 to have two blank lines after a function. Now that's also mentioned in the Python 53 00:04:09,890 --> 00:04:15,290 style guide - pep 8 - and that's why our main code starts on line six, so that two 54 00:04:15,290 --> 00:04:20,089 blank lines separate it from the function. It makes your code much easier 55 00:04:20,089 --> 00:04:24,260 to read, especially once you start having several functions. Make sure you leave 56 00:04:24,260 --> 00:04:30,170 two black lines, before and after your functions. Our main code calls the 57 00:04:30,170 --> 00:04:35,120 function on line six. To call a function, you just use its name, with any arguments 58 00:04:35,120 --> 00:04:38,540 in parentheses. You've seen a lot of functions being called so far, so that's 59 00:04:38,540 --> 00:04:44,420 nothing new. Because our function returns a value, we bind the variable answer to 60 00:04:44,420 --> 00:04:49,590 the value that the function returns. If you're wondering why we can't just 61 00:04:49,590 --> 00:04:54,200 use the variable result from line two, I'll be talking about that very soon. 62 00:04:54,200 --> 00:04:57,360 Keep watching, because there are still a few more things that we need to know 63 00:04:57,360 --> 00:05:02,480 about functions. Alright, so that's the basic structure of a Python function. 64 00:05:02,480 --> 00:05:06,860 Let's see some of that on slides before I finish the video. 65 00:05:06,860 --> 00:05:10,800 So you can see that the function definition starts with the keyword def. 66 00:05:10,800 --> 00:05:16,419 Next we have the name of the function. If the function will take parameters, they're 67 00:05:16,420 --> 00:05:20,780 declared in parentheses. And keep in mind that the parentheses are required, 68 00:05:20,780 --> 00:05:25,090 even when there are no parameters. Alright, so in the next video, we'll have 69 00:05:25,090 --> 00:05:28,509 a look at what happens when our code calls a function. We'll step through the 70 00:05:28,509 --> 00:05:32,900 code in the debugger, to see how a function call alters the flow of execution. 71 00:05:32,900 --> 00:05:36,100 See you in the next video.