1 00:00:05,060 --> 00:00:09,300 Alright, so make sure you've got the Hello World project open from the previous video, 2 00:00:09,300 --> 00:00:12,900 and opened in your IDE. In my case, of course, we're using IntelliJ. 3 00:00:12,900 --> 00:00:15,960 And I'm just going to double click this helloworld.py tab, 4 00:00:15,960 --> 00:00:20,040 to give us the full screen, and we're going to try and do that in the course, but you may or may not want to do that. 5 00:00:20,040 --> 00:00:24,300 depending on the amount of screen real estate you've got, and whether you want to have the project pane open. 6 00:00:24,300 --> 00:00:29,200 So at the moment we've got a very simple program, that just prints out the text Hello World! 7 00:00:29,200 --> 00:00:34,560 There's a few important things to know about the code, and I've already mentioned that print is a function. 8 00:00:34,560 --> 00:00:40,140 When you call a function in Python, you provide the things you want to print inside parentheses. 9 00:00:40,140 --> 00:00:44,840 So we've provided a string literal, Hello World!, and that's what got printed. 10 00:00:44,840 --> 00:00:47,700 A string literal is a sequence of characters, 11 00:00:47,700 --> 00:00:52,280 anything you can type on the keyboard, basically, enclosed in quotes. 12 00:00:52,280 --> 00:00:56,420 You can use either a single quote or double quotes to enclose a string. 13 00:00:56,420 --> 00:01:00,600 I used single quotes but you could use double quotes if you wanted to. 14 00:01:00,600 --> 00:01:03,500 It's important, though, to use the same ones, though. 15 00:01:03,500 --> 00:01:08,960 So if you start a string with double quotes, then it has to be terminated with a double quote. 16 00:01:08,960 --> 00:01:14,660 So if I change my single quote, on the end of Hello World, to a double quote, 17 00:01:14,660 --> 00:01:16,760 suddenly we've got an error there. 18 00:01:16,760 --> 00:01:19,500 You can't start with one style of quote and end with the other. 19 00:01:19,500 --> 00:01:23,260 You have to use the same type, at the start and at the end. 20 00:01:23,260 --> 00:01:28,180 So I'm going to fix the error by just doing a ctrl Z on my keyboard, 21 00:01:28,180 --> 00:01:33,860 twice to undo the change and go back to what we had before, which was a single quote at the end. 22 00:01:33,860 --> 00:01:40,140 And if I run it again, we've got no errors and we're getting the same output we got in the previous video. 23 00:01:40,140 --> 00:01:44,500 So when you've done that for the first time, as we did in the previous video, then again just now, 24 00:01:44,500 --> 00:01:47,300 IntelliJ remembers which program you want to run. 25 00:01:47,300 --> 00:01:51,700 Now in the drop down on the menu, in the right top of the screen, you can see it's going to run Hello World, 26 00:01:51,700 --> 00:01:56,300 it's showing is that, and you can use the green triangle icon, just to the right of that drop-down 27 00:01:56,300 --> 00:02:00,040 to run the same program again. So I can come over and click on that 28 00:02:00,040 --> 00:02:03,180 So as you work through the course, you'll have several files open at once. 29 00:02:03,180 --> 00:02:08,919 If you run them, using this green triangle icon, check that you're running the program that you expect. 30 00:02:08,919 --> 00:02:12,580 I'll talk about it again later, when we've got more than one file open. 31 00:02:12,580 --> 00:02:16,660 Alright, so you saw me use the green triangle to the right of the Hello World text there, 32 00:02:16,660 --> 00:02:21,460 the drop down to run the program again, we got the same output as we did previously. 33 00:02:21,460 --> 00:02:27,440 Alright, so a bit more jargon now. The string literal "Hello World!" is called an argument, 34 00:02:27,440 --> 00:02:29,840 at least in the context of what we're doing here. 35 00:02:29,840 --> 00:02:33,300 We're passing the argument to the print function. 36 00:02:33,300 --> 00:02:36,160 Functions can take more than one argument, 37 00:02:36,160 --> 00:02:41,600 and we'll be looking at how to define those when we write our own functions, later in the course. 38 00:02:41,600 --> 00:02:48,960 So we're passing a string argument to the print function, but we could also print, for example, the result of calculations. 39 00:02:48,960 --> 00:02:52,080 So let's go ahead and add some calculations. 40 00:02:52,080 --> 00:02:56,200 So I'm going to press ENTER there, I'm going to type print 41 00:02:56,200 --> 00:03:02,380 left parenthesis, and there's the right one automatically, and in the parentheses I'm going to type 1 + 2. 42 00:03:02,380 --> 00:03:09,480 Then on the next line i'm going to type print, add my parentheses and add 7 * 6. 43 00:03:09,480 --> 00:03:15,320 On the next line I'm just going to type print, and have left and right parentheses with nothing in there. 44 00:03:15,320 --> 00:03:21,680 Then on the line 5, the last one, I'm going to put print and parentheses again, double quotes this time. 45 00:03:21,680 --> 00:03:30,680 I'm going to type The end. Alright, so let's run this, 46 00:03:30,680 --> 00:03:33,820 and move this up a little bit, the run window, so we can see the output. 47 00:03:33,820 --> 00:03:39,520 So you can see that now that we've done that, Python works out the result of our arithmetic expressions, 48 00:03:39,520 --> 00:03:45,480 and it prints the answers; 1 + 2 is obviously 3, 7 * 6 is obviously 42. 49 00:03:45,480 --> 00:03:49,500 Now printing with left and right parentheses without telling Python what to print, 50 00:03:49,500 --> 00:03:52,900 just produces a blank line, and you can see that blank line there. 51 00:03:52,900 --> 00:03:56,300 We still need the parentheses, even though we're not providing an argument. 52 00:03:56,300 --> 00:04:00,440 When calling a function, you must have parentheses after the function name, 53 00:04:00,440 --> 00:04:04,040 and finally the program finishes after we're printing the string The end. 54 00:04:04,040 --> 00:04:07,360 Functions often take a specific number of arguments, 55 00:04:07,360 --> 00:04:12,340 but it's also possible to write functions that can take a variable number of arguments. 56 00:04:12,340 --> 00:04:15,640 The print function that's built into Python has been written that way. 57 00:04:15,640 --> 00:04:18,800 What that means is, we can print several things at once. 58 00:04:18,800 --> 00:04:23,620 So let's change the last line to print several strings and a number. 59 00:04:23,620 --> 00:04:29,680 So I'm going to put my cursor at the end of the double quote, after "The end". 60 00:04:29,680 --> 00:04:34,740 I'm gonna put a comma there, double quotes "or is it?" 61 00:04:34,740 --> 00:04:39,000 I'm going to move on after the double quotes again, comma 62 00:04:39,000 --> 00:04:47,780 I'm going to type "Keep watching to learn more about Python", and a double quote and comma 63 00:04:47,780 --> 00:04:51,900 then a number 3, making sure that's all in our parentheses. 64 00:04:51,900 --> 00:04:55,340 If we run the program now, 65 00:04:55,340 --> 00:04:59,500 you can see now that we've got 3 strings and a number printed on that line. 66 00:04:59,500 --> 00:05:03,480 So each argument is separated from the previous one with a comma. 67 00:05:03,480 --> 00:05:06,960 We're passing four arguments on this line, to print. 68 00:05:06,960 --> 00:05:12,500 So the first argument is "The end", and that's followed by a comma to separate it from the second argument, 69 00:05:12,500 --> 00:05:13,920 or is it? 70 00:05:13,920 --> 00:05:19,760 Once again, we need a comma before we provide the third argument, "Keep watching to learn more about Python", 71 00:05:19,760 --> 00:05:27,380 and the final argument is the numeric literal 3, which is also separated from the previous argument with a comma. 72 00:05:27,380 --> 00:05:32,560 Alright, so there's still a few more things to learn about the print function, and I'll be coming back to it in later videos. 73 00:05:32,560 --> 00:05:38,100 For now, we've learnt enough to be able to print from our programs, to check that they produce the correct results. 74 00:05:38,100 --> 00:05:44,900 Now I've introduced quite a few jargon words there, so let's finish this video with a slide to summarize them. 75 00:05:44,900 --> 00:05:54,340 So a literal, well that's a value of some type. An example of a numeric literal are the numbers 1, 42 and 98.04, 76 00:05:54,340 --> 00:05:56,460 and an example of a string literal 77 00:05:56,460 --> 00:06:01,440 are "Hello World!", "Giudo van Rossum" and "Python", all in quotes, as you can see there. 78 00:06:01,440 --> 00:06:06,280 Moving on, a function, well that's a named block of code that we can call by using its name. 79 00:06:06,280 --> 00:06:10,840 We can write our own functions, or we can use functions that are built into Python, 80 00:06:10,840 --> 00:06:13,500 such as we've been doing here, with the print function. 81 00:06:13,500 --> 00:06:16,900 In Python, all functions return a value. 82 00:06:16,900 --> 00:06:21,840 Argument: a value passed to a function, in order to give it values to work with. 83 00:06:21,840 --> 00:06:25,220 Now there may be no arguments, or there may be one or more. 84 00:06:25,220 --> 00:06:28,800 Arguments appear in parentheses, after the function name. 85 00:06:28,800 --> 00:06:33,980 If there are no arguments, you still have to type in the left and right parentheses. 86 00:06:33,980 --> 00:06:38,500 Calling a function: using the function name to execute the function's code. 87 00:06:38,500 --> 00:06:43,460 When you call a function, you have to provide the arguments that the function expects. 88 00:06:43,460 --> 00:06:48,020 If it doesn't expect any arguments, don't put anything between the parentheses. 89 00:06:48,020 --> 00:06:51,940 Return value: well that's the value that a function returns. 90 00:06:51,940 --> 00:06:56,500 We haven't talked about that yet, but it belongs in this slide to make the slide complete. 91 00:06:56,500 --> 00:07:01,560 So more on that later, and parameter, also called a formal parameter. 92 00:07:01,560 --> 00:07:03,620 This is something else we haven't discussed yet, 93 00:07:03,620 --> 00:07:07,420 and we'll learn about parameters when we start writing our own functions. 94 00:07:07,420 --> 00:07:10,040 Alright, so let's end the video here. In the next one, 95 00:07:10,040 --> 00:07:12,180 we're gonna start talking about strings. 96 00:07:12,180 --> 00:07:15,020 See you in the next video.