1 00:00:04,500 --> 00:00:08,180 We've seen how we can store values in variables, and print them out. 2 00:00:08,180 --> 00:00:12,900 We've also looked at getting input from the keyboard, in the previous section in this course. 3 00:00:12,900 --> 00:00:16,680 We'll now start getting Python to make decisions, based on the values of 4 00:00:16,690 --> 00:00:21,640 variables and input. Just to quickly recap, I'm going to just type in a quick 5 00:00:21,640 --> 00:00:25,640 program. I'm going to delete the last three lines from the last video. 6 00:00:25,640 --> 00:00:34,200 I'm going to type name equals input parentheses and double quotes, Please enter your name, 7 00:00:34,200 --> 00:00:41,320 colon and a space there, and on line 2, age equals input parentheses double 8 00:00:41,320 --> 00:00:48,700 quotes again, How old are you comma space. Then we want left and right curly braces, 9 00:00:48,700 --> 00:00:52,420 replacement fields, zero in the middle, question mark to the right of the right 10 00:00:52,420 --> 00:00:59,500 curly brace and a space. Then a closing double quote dot format name and two 11 00:00:59,500 --> 00:01:02,980 right parenthesis. Then line three, we're going to print out the value of age, 12 00:01:02,980 --> 00:01:08,200 so print parentheses age. Alright, just as a quick recap to what we've done 13 00:01:08,200 --> 00:01:11,290 previously, I'm gonna run that code now, and remember to click into the run 14 00:01:11,290 --> 00:01:15,100 window as I did, before typing your name if you're using an IDE like IntelliJ. 15 00:01:15,100 --> 00:01:19,100 So come down here and click on that and I'll just move this down a little bit. 16 00:01:19,100 --> 00:01:25,080 I'm going to type in my name, enter, How are you Tim, like to think I'm 21, 17 00:01:25,080 --> 00:01:29,980 press enter, 21 is shown in the window. So note that we can use 18 00:01:29,980 --> 00:01:33,490 replacement fields in any string, including the one we're using as a 19 00:01:33,490 --> 00:01:39,660 prompt to the input function. The input function returns as str or string datatype. 20 00:01:39,660 --> 00:01:43,680 We have to convert it to an int if we want age to represent a number, 21 00:01:43,680 --> 00:01:48,660 rather than the string, and we do that with the int function. So if we change 22 00:01:48,660 --> 00:01:55,160 line 2 slightly. Instead of putting input parentheses we'll put int, left parentheses. 23 00:01:55,160 --> 00:01:57,360 Then right at the end, now we've done that, we need to go to the end 24 00:01:57,360 --> 00:02:02,300 of the line and add a right parenthesis there, to keep Python happy. 25 00:02:02,300 --> 00:02:06,900 So using int and input together like this is very common when you want to get a number 26 00:02:06,910 --> 00:02:11,080 from the user. It does have a serious problem though. Your code will crash if 27 00:02:11,080 --> 00:02:15,069 you type anything that can't be converted to an integer. Now we'll be 28 00:02:15,069 --> 00:02:17,420 looking at how to handle that later in the course. 29 00:02:17,420 --> 00:02:19,780 Until then, make sure your only enter valid 30 00:02:19,780 --> 00:02:26,580 numbers when our programs expect a number. Alright, so if we just run this to confirm, 31 00:02:26,580 --> 00:02:33,099 enter your name again, Tim, 25, we're getting the value of 25. 32 00:02:33,099 --> 00:02:37,120 And let's just quickly show what would happen if we do insist on typing 33 00:02:37,120 --> 00:02:42,420 something that's not a number. I'll type in the text twenty. 34 00:02:42,420 --> 00:02:44,420 You can see we've got an error there, 35 00:02:44,420 --> 00:02:47,940 and again, we'll talk about how to fix that later in the course. 36 00:02:47,940 --> 00:02:50,760 Right, at this point though, we've got a number. We saw that that was working, 37 00:02:50,760 --> 00:02:54,960 and we can make, at this point, some decisions based on the person's age. 38 00:02:54,960 --> 00:02:59,500 So let's start by checking if they're old enough to vote. In many countries, 39 00:02:59,500 --> 00:03:05,320 including Australia, the legal age for voting is 18. So let's come up and on line 5, 40 00:03:05,320 --> 00:03:13,880 type if space age is greater than, and the equal sign, 18 colon press enter. 41 00:03:13,880 --> 00:03:18,840 So we started with the keyword if and followed it with a condition, and here the condition is 42 00:03:18,840 --> 00:03:24,020 age greater than equal to 18. Now notice the colon that I placed on the end of that line. 43 00:03:24,020 --> 00:03:26,980 That means we're about to start a new code block, and in fact, we did 44 00:03:26,980 --> 00:03:31,000 start a new code block when I pressed enter, and the IDE indented the next line 45 00:03:31,000 --> 00:03:35,720 for us automatically, as we've seen previously. The indentation is very important. 46 00:03:35,720 --> 00:03:39,340 That's how Python knows that we're starting a new block of code. 47 00:03:39,340 --> 00:03:42,340 Alright, so if they're old enough to vote we'll print out a suitable message. 48 00:03:42,340 --> 00:03:47,480 So let's do that on line 6 and put print parentheses double quotes, 49 00:03:47,480 --> 00:03:54,780 You're old enough to vote. Okay. So if the condition is true, 50 00:03:54,780 --> 00:03:57,920 in other words, the age entered is greater than or equal to 18, 51 00:03:57,920 --> 00:04:03,500 then we're printing out the message, You are old enough to vote, 52 00:04:03,500 --> 00:04:08,100 and I'll just press backspace there to remove the indentation. Let's run it. 53 00:04:08,100 --> 00:04:12,760 We'll do it a few times just to make sure, with different ages, that it is working as expected. 54 00:04:12,760 --> 00:04:18,700 So if I start by typing my name firstly, then the age is 18, 55 00:04:18,700 --> 00:04:26,110 You are old enough to vote, so that's working. Tim and I'll try 17. We get just 56 00:04:26,110 --> 00:04:28,940 the value 17, we don't get a message saying we're old enough to vote, 57 00:04:28,940 --> 00:04:36,760 and lastly, do one 50, Tim 50, You are old enough to vote. 58 00:04:36,760 --> 00:04:39,039 And again, remember to type a valid number, or you'll get the error that I 59 00:04:39,039 --> 00:04:42,639 showed earlier in the video, if you're not using a number. Alright, so what 60 00:04:42,639 --> 00:04:46,990 we've done there on line 5, that's how to test a condition in Python, keyword 61 00:04:46,990 --> 00:04:50,620 if followed by condition, in this case, age greater than equal to 18, 62 00:04:50,620 --> 00:04:54,800 and finishing the first, and finally ending the line with a colon. 63 00:04:54,800 --> 00:04:58,779 Now conditions can be a lot more complex than that, as we'll see. We know that a 64 00:04:58,780 --> 00:05:04,420 colon tells python to expect a new code block, so line 6 is indented by 4 spaces. 65 00:05:04,420 --> 00:05:09,060 If age is less than 18, nothing happens, which is a bit unfriendly. We can cater 66 00:05:09,069 --> 00:05:14,340 for that by adding an else clause to our if keyword. Now we don't want any more 67 00:05:14,340 --> 00:05:19,700 code inside the if block, which means we remove the indentation on the else line. 68 00:05:19,700 --> 00:05:22,800 So I've already done that. You saw me do that earlier, but we'll start typing on 69 00:05:22,809 --> 00:05:29,529 line 7, else : press Enter. Notice how IntelliJ has helpfully 70 00:05:29,529 --> 00:05:34,599 indented again. We'll type print parentheses double quotes, Please come 71 00:05:34,599 --> 00:05:39,099 back in, and we use a replacement field with a zero in between the left and 72 00:05:39,100 --> 00:05:48,340 right curly braces, space years, double quote comma, and we'll put format parenthesis 18 minus age, 73 00:05:48,340 --> 00:05:53,460 and the two right closing parentheses. So if the person's age is greater than or 74 00:05:53,469 --> 00:05:58,569 equal to 18, the first block of code is executed, otherwise the second one is 75 00:05:58,569 --> 00:06:02,949 executed, asking them to come back when they're 18, or more specifically, in a 76 00:06:02,949 --> 00:06:05,980 number of years, asking them to come back when they're 18. 77 00:06:05,980 --> 00:06:14,600 So we run this a few times just to make sure that it works. Let's do that, Tim, 18, 78 00:06:14,600 --> 00:06:18,720 still get the same result, 18, You are old enough to vote. 79 00:06:18,720 --> 00:06:23,580 Tim, 50, You are old enough to vote. That's working, and we'll do one more, this time 80 00:06:23,580 --> 00:06:30,740 if they're less than 18, Tim, we'll say 9, Please come back in, and we made a typo there. 81 00:06:30,740 --> 00:06:34,839 I'll put a comma there. Notice I put a comma there instead of a period, which is 82 00:06:34,840 --> 00:06:40,100 a subtle little bug. I'll do that. We didn't see that until we entered in the value, 83 00:06:40,100 --> 00:06:43,860 so Tim, How old are you? 9. There it is again, 84 00:06:43,870 --> 00:06:46,120 Please come back in nine years and that's because I enter the age of 85 00:06:46,120 --> 00:06:52,780 9, and the calculation on line 18 was 18 minus what we entered, in this case, 9. 86 00:06:52,780 --> 00:06:56,680 So it says come back in 9 years, when of course you'll be 18. Alright, so as we saw 87 00:06:56,680 --> 00:07:01,320 in the last video, a block can contain several lines of code to execute. 88 00:07:01,320 --> 00:07:06,280 So let's go ahead and add something, a second line under line 6, so on line 7 89 00:07:06,280 --> 00:07:12,920 I'm going to type print parenthesis double quotes, Please put an X 90 00:07:12,920 --> 00:07:17,280 in the box, and we'll close off the string and then 91 00:07:17,289 --> 00:07:21,880 right parenthesis. Because line 6 and 7 are in the same block, in other 92 00:07:21,880 --> 00:07:25,659 words, they're indented to the same level, we should find that both messages will 93 00:07:25,659 --> 00:07:29,259 be printed if the age is greater than equal to 18. So let's just run that to 94 00:07:29,260 --> 00:07:38,280 confirm that is the case. Please enter your name, Tim, 19, You are old enough to vote. 95 00:07:38,280 --> 00:07:44,020 Please put an X in the box. There's often several ways to do the same thing when programming. 96 00:07:44,020 --> 00:07:46,900 It may seem natural to deal with the voters first, 97 00:07:46,900 --> 00:07:50,360 but we can do that the other way around as well. So to see what 98 00:07:50,360 --> 00:07:54,620 I mean I'm going to add some code. I'll add that down here. 99 00:07:54,620 --> 00:08:03,720 We'll start on line 11. We'll leave line 10 blank. We're going to put if age is less than 18: 100 00:08:03,720 --> 00:08:08,940 and let's just copy the line, here, we'll copy that line. Notice when you 101 00:08:08,940 --> 00:08:12,000 moved the cursor down it automatically moved over to the indentation level. We're 102 00:08:12,040 --> 00:08:18,680 going to paste that as if the age is less than 18, else: 103 00:08:18,680 --> 00:08:24,860 I'm going to grab the two lines up here. I'm going to paste those in as well. 104 00:08:24,860 --> 00:08:29,200 Notice that IntelliJ helpfully indents at the right line, or to the right 105 00:08:29,200 --> 00:08:32,659 indentation level, and again we're doing things the other way around. We're not 106 00:08:32,659 --> 00:08:35,799 dealing with the voters first this time. We're dealing with people who, in fact, aren't 107 00:08:35,799 --> 00:08:39,578 old enough to vote, first, and the people who are, second, just to show you there's 108 00:08:39,578 --> 00:08:45,040 another way of doing the same thing. So we run this. I'm going to enter the same 109 00:08:45,040 --> 00:08:51,880 thing twice now for Tim, 15, Please come back in 3 years, as you can see. 110 00:08:51,880 --> 00:08:54,790 We've got exactly the same output two times and that's because we've got two 111 00:08:54,790 --> 00:08:58,820 conditions in there, two code blocks if you will. So if I enter that again, 112 00:08:58,820 --> 00:09:07,860 Tim, 21, and you can see we've got the output again, duplicated, 113 00:09:07,860 --> 00:09:12,100 because effectively, we've duplicated, we've got two if code blocks with the appropriate 114 00:09:12,100 --> 00:09:15,670 else clauses entered. So the code, ultimately, that I've added does exactly 115 00:09:15,670 --> 00:09:19,260 the same thing, but it's checking the condition the other way around. 116 00:09:19,260 --> 00:09:22,640 If the person is less than 18, we print the message asking them to come back, 117 00:09:22,640 --> 00:09:26,660 otherwise, we print the two lines asking them to tick the box. 118 00:09:26,660 --> 00:09:31,680 So it doesn't matter which way round you phrase the conditions? Well in this example, no it doesn't, 119 00:09:31,680 --> 00:09:34,980 but in the next video, we'll see why it can matter. 120 00:09:34,980 --> 00:09:37,880 See you in the next video.