1 00:00:04,680 --> 00:00:09,629 Now that we can perform calculations and display the results, it's time to move on 2 00:00:09,629 --> 00:00:15,940 to see how we can get Python to make decisions and perform tasks repeatedly. 3 00:00:15,940 --> 00:00:20,440 Like many programming languages, Python works in blocks of code. One of the 4 00:00:20,440 --> 00:00:23,960 design principles of Python, though, was that it should be uncluttered and 5 00:00:23,960 --> 00:00:29,540 easy to read. Because of this, Python doesn't use delimiters around blocks of code. 6 00:00:29,540 --> 00:00:33,040 If you've used other programming languages, you may be used to seeing braces, 7 00:00:33,080 --> 00:00:37,980 that's left and right curly braces, or BEGIN and END around code. Python doesn't use 8 00:00:37,980 --> 00:00:45,200 anything like that, but instead uses indentation to indicate when a new block starts. 9 00:00:45,200 --> 00:00:48,800 I've opened the exchange_chart program in IntelliJ 10 00:00:48,800 --> 00:00:53,040 so that I've got some code to talk about. Now you won't need this file in your project. 11 00:00:53,040 --> 00:00:56,000 We're not going to be running it in this section. This is one of the few 12 00:00:56,000 --> 00:00:59,700 times in the course when you won't be typing along. So relax, sit back and 13 00:00:59,700 --> 00:01:03,880 watch the video, as l show you blocks of code in this program. 14 00:01:03,880 --> 00:01:09,540 Alright, so let's look at the blocks of code in this program, firstly, starting on line 66, 15 00:01:09,540 --> 00:01:14,620 we've got a for loop. That starts a new code block and we can see that line 67 16 00:01:14,620 --> 00:01:18,120 through to lines 82, which is just off screen, well it's actually on-screen but 17 00:01:18,120 --> 00:01:23,910 I'll move up a little bit more, like so. So you can see those lines 67 to 82, 18 00:01:23,910 --> 00:01:29,480 they're all part of that code block, which is the for loop defined on line 66. 19 00:01:29,480 --> 00:01:32,550 Now I don't want you to try and understand the code here, because most of 20 00:01:32,550 --> 00:01:36,120 it probably won't make sense at this point in the course. You should be able 21 00:01:36,120 --> 00:01:41,010 to work out that line 67, though, is using replacement fields, and I'll just move that 22 00:01:41,010 --> 00:01:45,810 over a little bit more so we can see it, and that's using a replacement field to 23 00:01:45,810 --> 00:01:49,890 put the year and month at the end of that URL. But most of this code in 24 00:01:49,890 --> 00:01:55,020 general will be confusing, and that's fine. What we're interested in now at the 25 00:01:55,020 --> 00:01:59,640 moment are the various levels of indentation. So concentrate on the shape 26 00:01:59,640 --> 00:02:04,380 of the code and we'll learn what it all means in due course. Alright, so we've got 27 00:02:04,380 --> 00:02:10,979 a block of code from line 67, through to and including lines 82. We can make it 28 00:02:10,979 --> 00:02:16,650 easier to see the blocks by turning on indentation guides. Now most modern IDEs 29 00:02:16,650 --> 00:02:21,040 will have a similar feature. In IntelliJ, you go into Settings. 30 00:02:21,040 --> 00:02:25,180 Now that's Preferences if you're using a Mac. In my case, I'm on a Windows machine, 31 00:02:25,180 --> 00:02:30,500 and it'll be the same for Linux, File, I'm going to select Settings. 32 00:02:30,500 --> 00:02:38,760 You want to then navigate to Editor, then down to General and then here into Appearance. 33 00:02:38,760 --> 00:02:42,700 You want to make sure that the Show indent guides is actually checked, so I'm 34 00:02:42,700 --> 00:02:47,480 going to check that now. Click OK, 35 00:02:47,480 --> 00:02:50,460 and you can see now that IntelliJ draws guides 36 00:02:50,460 --> 00:02:54,800 to show the indentation levels. You can see that slight, really gray, light gray 37 00:02:54,800 --> 00:03:02,260 line now in the indentation. So line 67 and line 69 are indented one level. 38 00:03:02,260 --> 00:03:09,720 So below that, line 70 is indented another level, line 72 is also at that second level 39 00:03:09,720 --> 00:03:14,300 and so are all the lines 74 to 79, and you can see there's a third level 40 00:03:14,300 --> 00:03:21,540 of indentation on lines 80 through 82. So each level of indentation is a new block of code. 41 00:03:21,540 --> 00:03:25,680 So let's see them again. The main code has no indentation and starts 42 00:03:25,690 --> 00:03:30,340 up against the left margin. Lines 67 to 82 are all part of another block of code, 43 00:03:30,340 --> 00:03:35,980 and are indented one level. The convention is to indent four spaces at a time. 44 00:03:35,980 --> 00:03:38,859 Within that second block we have other blocks for lines 45 00:03:38,860 --> 00:03:47,560 70 and 72 and also lines 74 to 82, then finally there's a fourth block, lines 80 to 82. 46 00:03:47,560 --> 00:03:50,900 So that's how we create blocks of code in Python. 47 00:03:50,900 --> 00:03:54,430 Now I haven't explained why you want to do that. At the moment we're looking at 48 00:03:54,430 --> 00:03:59,700 what a code block is. So as you can see, a block of code can contain other blocks. 49 00:03:59,700 --> 00:04:03,780 We start a new code block by indenting the code. Now have you noticed anything 50 00:04:03,780 --> 00:04:06,860 else about the lines that start a new code block? 51 00:04:06,860 --> 00:04:15,520 Well lines 66, 69, 71, 73 and 79 all start a new code block. If you examine those lines carefully, 52 00:04:15,520 --> 00:04:18,060 hopefully you can spot something that they all have in common. 53 00:04:18,060 --> 00:04:24,920 Did you spot it? Well all those lines end in a colon. Whenever a line ends with a colon, 54 00:04:24,920 --> 00:04:27,840 Python will expect a new code block to follow it. 55 00:04:27,840 --> 00:04:31,510 That means that the next line must be indented. When you've finished 56 00:04:31,510 --> 00:04:34,620 all the code for a block, you remove the indentation 57 00:04:34,620 --> 00:04:40,420 for the next line. The blocks of code on lines 67 to 82 finished with line 82, 58 00:04:40,460 --> 00:04:43,320 because the indentation for the next line is removed, and you can see that 59 00:04:43,320 --> 00:04:48,630 line 84, right towards the bottom of the screen, isn't indented. Line 72 is a 60 00:04:48,630 --> 00:04:54,360 code block containing only one line. Line 73 is indented back one level. 61 00:04:54,360 --> 00:04:58,100 There's another code block a bit further down and I'll scroll that so you can see it, 62 00:04:58,100 --> 00:05:03,700 and the the block of code on lines 87 to 88. You can see those there. 63 00:05:03,700 --> 00:05:08,660 Only those two lines are indented, which means only those two lines are part of the for loop. 64 00:05:08,660 --> 00:05:12,840 Once again, don't worry about the code, just concentrate on how indentation is 65 00:05:12,840 --> 00:05:17,560 used to create a new code block. We're learning why in the rest of this section. 66 00:05:17,560 --> 00:05:21,580 Alright so that's the end of the demonstration and what code blocks are. 67 00:05:21,580 --> 00:05:25,980 Next we'll see how to create them. So I've created a new project for this section 68 00:05:25,980 --> 00:05:30,320 and I've called the project ProgramFlow, which probably isn't very imaginative. 69 00:05:30,320 --> 00:05:32,960 You may want to call your project something else, like flow control or 70 00:05:32,960 --> 00:05:36,260 program flow or something of that nature, or something that makes sense. I'm going 71 00:05:36,260 --> 00:05:41,000 to close this file now because we are done with it for now. Close it down. 72 00:05:41,000 --> 00:05:43,940 So what I'm going to do is click on my project to open it. I'm going to right 73 00:05:43,940 --> 00:05:51,700 click it, select New, Python file and let's go with the name, blocks. 74 00:05:51,740 --> 00:05:56,190 Alright and I'll just close down the project pane to make a bit of space. Now I'm going to start with 75 00:05:56,190 --> 00:06:00,240 some code that we've seen before. It still won't make sense but it will let us 76 00:06:00,240 --> 00:06:07,740 see how to create a code block. So I'm going to type on the first line, for i in range 77 00:06:07,740 --> 00:06:14,010 parentheses 1 comma 13 right parentheses and a colon. Now when I type 78 00:06:14,010 --> 00:06:19,140 that colon, IntelliJ IDEA is showing an error. If I hover the mouse over it, 79 00:06:19,140 --> 00:06:23,720 you can see a description that says indent expected. So if I then press ENTER here, 80 00:06:23,720 --> 00:06:28,640 you see that the error has now moved to the next line, and that can be very confusing. 81 00:06:28,640 --> 00:06:33,120 You've hardly typed anything and you've already got an error. Don't worry, 82 00:06:33,120 --> 00:06:36,500 you will sometimes get things like this until you finish typing the code. 83 00:06:36,500 --> 00:06:40,680 IntelliJ's trying to make sense of code that isn't finished yet, and it gets 84 00:06:40,680 --> 00:06:46,000 a bit confused, and most IDEs will do the same thing if they provide syntax checking 85 00:06:46,000 --> 00:06:48,860 while your type, like IntelliJ is doing. 86 00:06:48,860 --> 00:06:52,600 Helpfully though, IntelliJ has automatically indented that new line, 87 00:06:52,600 --> 00:06:56,820 when I pressed enter. So I'm going to start typing print. You can see there that was 88 00:06:56,820 --> 00:07:02,060 already indented for us. I'm gonna finish this line, print left parenthesis, 89 00:07:02,060 --> 00:07:11,300 double quotes, No, we'll put number, or No for short, period, then left and right curly braces, 90 00:07:11,300 --> 00:07:16,840 squared is, and left and right curly braces again, 91 00:07:16,840 --> 00:07:20,540 and cubed is. 92 00:07:20,540 --> 00:07:24,760 Then we want our left and right curly braces again, but within those we're going to put : 4. 93 00:07:24,760 --> 00:07:29,780 We have talked about this in a previous section, finishing off the double quote and put 94 00:07:29,780 --> 00:07:41,460 dot format parentheses i comma i ** 2 comma i ** 3 95 00:07:41,460 --> 00:07:47,160 and two right parentheses to finish the line. Note that that's now cleared the error, and notice 96 00:07:47,160 --> 00:07:51,260 also that the cursor is automatically positioned when I press ENTER now, 97 00:07:51,260 --> 00:07:55,580 at the same level of indentation. Now if we wanted more code inside the loop, 98 00:07:55,580 --> 00:07:59,700 we just keep typing. There's only one line I want in this block, so we're going to use 99 00:07:59,700 --> 00:08:04,040 the backspace key now to delete the indentation, and you can see just one 100 00:08:04,040 --> 00:08:08,100 backspace went right back to the start. Now if you want to indent again, 101 00:08:08,100 --> 00:08:12,800 press the tab key. Notice that the IDE adds four spaces 102 00:08:12,800 --> 00:08:17,700 and removes four spaces, when I use backspace again. So it knows, in other words, 103 00:08:17,700 --> 00:08:20,580 that we're in a code block and automatically adds the correct number of 104 00:08:20,580 --> 00:08:25,500 spaces for us. Now you can also select a line of code and use shift tab to 105 00:08:25,500 --> 00:08:31,220 remove a level of indentation. To see what I mean I'm going up to line 2. 106 00:08:31,220 --> 00:08:35,820 I'm gonna hold down shift and press tab. I can indent it again with tab, 107 00:08:35,820 --> 00:08:41,059 shift tab to remove the indentation and tab again to edit. Alright, so moving on to the 108 00:08:41,059 --> 00:08:48,720 next line now, line 3, I'm going to type print parentheses double quotes, 109 00:08:48,720 --> 00:08:55,880 and put an asterix in the double quotes, then * 80, right parenthesis. 110 00:08:55,880 --> 00:08:58,600 Now if I run this program, 111 00:09:02,940 --> 00:09:07,300 we can see we get the squares and cubes printed, followed by a line of asterixes. 112 00:09:07,300 --> 00:09:12,030 So the block of code inside that for loop executes 12 times. but we only get a 113 00:09:12,030 --> 00:09:17,980 single line of asterixes at the end, and that's because line 3 isn't part of the block. 114 00:09:17,980 --> 00:09:22,020 However, we can make it part of the block by tabbing it across. So come over 115 00:09:22,020 --> 00:09:26,610 here on line 3 and tab it over, so it's now indented at the same line or same 116 00:09:26,610 --> 00:09:31,580 level as line 2. If we run the program again, 117 00:09:31,580 --> 00:09:34,760 notice that we get a very different result this time. 118 00:09:34,760 --> 00:09:37,660 The output's very different, as you can see there. 119 00:09:37,660 --> 00:09:41,040 Both lines are now part of the same block and therefore both lines get 120 00:09:41,040 --> 00:09:45,600 executed 12 times. Alright, so that's how we create a code block using the tab 121 00:09:45,600 --> 00:09:50,400 key to indent the code. We're also starting to see why we might want to do that. 122 00:09:50,400 --> 00:09:53,580 We get very different results when we include line 3 in the block, 123 00:09:53,580 --> 00:09:57,560 compared to having it outside of the block. The code still won't make a lot of sense. 124 00:09:57,560 --> 00:10:01,280 What are for and range? Well that's some of the things we'll be looking at in the rest 125 00:10:01,280 --> 00:10:04,740 of this section, but now that we understand what code blocks are, 126 00:10:04,740 --> 00:10:08,340 we can start to use them in our code. We'll make a start on that when we look at 127 00:10:08,340 --> 00:10:13,160 telling Python how to make decisions, in the next video.