1 00:00:04,990 --> 00:00:09,130 Let's talk about debugging with parameters. If you've used functions in 2 00:00:09,130 --> 00:00:12,700 other languages, you may want to play this video at double speed. It's 3 00:00:12,700 --> 00:00:16,689 something that a lot of new programmers find confusing at first, which is why I'm 4 00:00:16,689 --> 00:00:20,520 going to take the time to demonstrate what happens. 5 00:00:57,640 --> 00:01:01,120 and the link's really good for a description of call by sharing that was 6 00:01:01,120 --> 00:01:06,200 first introduced in the programming language, CLU. The Python documentation 7 00:01:06,200 --> 00:01:10,580 uses the term call by assignment, but it's basically the same thing. 8 00:01:10,580 --> 00:01:14,460 Now this entry in the Python 3 FAQ, which I'm about to put on screen, is also 9 00:01:14,460 --> 00:01:18,400 interesting, if you wanted to produce the effect of call by reference. 10 00:01:18,400 --> 00:01:21,740 Check that out as well. If none of that makes any sense, 11 00:01:21,759 --> 00:01:25,420 and you've never written a function before, let's step through the code 12 00:01:25,420 --> 00:01:30,740 and see what's going on. Alright, so we've still got a breakpoint set on line 6, 13 00:01:30,740 --> 00:01:34,480 So let's run the program in the debugger, and see how our parameters get replaced, 14 00:01:34,480 --> 00:01:38,700 with the values we pass as arguments. 15 00:01:41,140 --> 00:01:43,780 The debugger the stops on line 6, ready to enter 16 00:01:43,780 --> 00:01:47,830 our multiply function. We're going to use the Step into my code button, to enter 17 00:01:47,830 --> 00:01:52,130 the function. The debugger's now stopped on line two. 18 00:01:52,130 --> 00:01:56,600 Before we go any further, have a look now at the variables pane down the bottom. 19 00:01:56,600 --> 00:02:01,340 On line six, we passed the arguments 10.5 and 4 to the function. 20 00:02:01,340 --> 00:02:07,020 On line one, and in the variables pane, you can see that x has the value 10.5, 21 00:02:07,020 --> 00:02:13,220 and y is 4. What we have here, are two ordinary variables that only exist 22 00:02:13,220 --> 00:02:18,530 inside the function. That's all that a parameter is - it's a variable that exists 23 00:02:18,530 --> 00:02:22,940 inside the function. Within the function, you can use them just like you'd use any 24 00:02:22,940 --> 00:02:27,170 other variable- there's nothing special about them. When an argument is passed to 25 00:02:27,170 --> 00:02:32,280 the function, like we do on line six, the parameter gets the value of the argument 26 00:02:32,280 --> 00:02:37,580 that's passed to the function. On line six, the first argument is 10.5. 27 00:02:37,580 --> 00:02:43,040 The first parameter on line one is x, so x is therefore bound to the value 10.5. 28 00:02:43,040 --> 00:02:47,660 The second argument passed to the function is 4, the second parameter 29 00:02:47,700 --> 00:02:53,660 is y, so y is bound to the value 4. the values of the arguments are assigned to 30 00:02:53,700 --> 00:02:58,600 the parameters in order. Each argument is assigned to the parameter in the same 31 00:02:58,600 --> 00:03:03,720 position. Our first argument is 10.5, and that gets assigned to the first 32 00:03:03,739 --> 00:03:10,270 parameter x. For that reason, these arguments are positional arguments. 33 00:03:30,500 --> 00:03:32,750 Alright, so let's continue step you through the code. I'm going to use 34 00:03:32,750 --> 00:03:37,430 Step over, to check that the result had the value 42, 35 00:03:37,430 --> 00:03:42,760 and you can see that it has there. Now watch what happens when I step over again. 36 00:03:42,760 --> 00:03:46,980 The variables x, y and result no longer exist. 37 00:03:46,980 --> 00:03:50,349 IntelliJ is no longer showing their value on line one, and they also 38 00:03:50,349 --> 00:03:56,440 don't appear in the variables pane. Their scope is the function they're defined in. 39 00:03:56,440 --> 00:03:59,980 That's all scope means when you come across that term. It's where a variable 40 00:03:59,980 --> 00:04:04,959 exists. I'll be talking more about scope, later in the course. Alright, so let's step 41 00:04:04,959 --> 00:04:08,700 over twice more to get to line 9. 42 00:04:09,110 --> 00:04:13,370 This time, our arguments are six and seven. You should be able to work out the 43 00:04:13,370 --> 00:04:17,149 values that x and y will have, when we step into the function. Make sure you work 44 00:04:17,149 --> 00:04:22,400 that out before clicking Step into my code. So we're passing the values six and 45 00:04:22,400 --> 00:04:26,540 seven in that order. That means x should get the first value, because x appears 46 00:04:26,540 --> 00:04:32,920 first in the parameter list, on line one. I'm going to click Step into my code, 47 00:04:33,920 --> 00:04:40,419 and we were right; x is 6 and y is 7. The function's about to multiply x by y, and bind a 48 00:04:40,419 --> 00:04:43,840 variable called result, to the answer. We're going to click Step over 49 00:04:43,840 --> 00:04:49,760 to execute that line. We now have x and y and result in the variables pane. 50 00:04:49,760 --> 00:04:54,680 We're going to step over four more times now, to take us to line 14. 51 00:04:55,840 --> 00:05:00,940 Before we continue, check out the variables pane, now. answer was created, on 52 00:05:00,940 --> 00:05:05,110 line six, when it was bound to the value returned by the first call to the 53 00:05:05,110 --> 00:05:10,419 multiply function. forty_two was created on line nine. These two 54 00:05:10,420 --> 00:05:15,460 variables exist throughout our program. Their scope is the entire program. 55 00:05:15,460 --> 00:05:20,020 The variables x, y and result don't appear in the variables pane, because they no 56 00:05:20,020 --> 00:05:25,400 longer exist. Their scope is the multiply function, and they don't exist outside it. 57 00:05:25,400 --> 00:05:30,520 Alright, so we're about to create a new variable here, called val. The name val, 58 00:05:30,520 --> 00:05:36,060 will be bound to each of the numbers 1 to 4, in turn, as the for loop executes. 59 00:05:36,070 --> 00:05:40,360 I'm gonna click on Step over, and we can see val appears in our 60 00:05:40,360 --> 00:05:45,640 variables pane. IntelliJ is also showing its value at the end of line 14. 61 00:05:45,640 --> 00:05:50,820 We're now on line 15 and about to enter the multiply function again. This time we're 62 00:05:50,820 --> 00:05:55,480 passing a variable for the second argument. We've included this example, 63 00:05:55,480 --> 00:05:59,700 to make it clear that you don't have to pass just literal values. You can use any 64 00:05:59,710 --> 00:06:04,930 expression that evaluates to the correct type for the parameter. In this case, our 65 00:06:04,930 --> 00:06:10,100 function expects a number, so you can use anything that evaluates to be a number. 66 00:06:10,100 --> 00:06:14,680 We're using the variable val, which currently has the value 1. So we come 67 00:06:14,680 --> 00:06:19,000 over here and click on Step in to my code, and you shouldn't be surprised, 68 00:06:19,000 --> 00:06:25,060 therefore, to see that x got the value 2 and y got the value of val - that's 1. 69 00:06:25,060 --> 00:06:28,240 We don't need to see 2 being multiplied by 1, so I'm going to show you another one 70 00:06:28,240 --> 00:06:32,289 of the debugger functions - that's the upwards pointing arrow Step out, over 71 00:06:32,289 --> 00:06:36,669 here. It can be useful if you step into a function by mistake, or if you've seen 72 00:06:36,669 --> 00:06:40,380 everything you want in a function, and you want to go back to the main code. 73 00:06:40,380 --> 00:06:44,760 Step out will execute the remainder of the code in the function, and return to 74 00:06:44,770 --> 00:06:49,479 the point where the function was called. In our case, the function was called on 75 00:06:49,479 --> 00:06:53,979 line 15. We cannot currently see that, but when I click on Step out, we'll get back 76 00:06:53,979 --> 00:06:58,050 to line 15. Alright, so the value 2 will have 77 00:06:58,050 --> 00:07:03,379 been returned from the function, and when I step over once more now, 78 00:07:03,759 --> 00:07:09,699 the variable two_times, now, is bound to 2. I'm going to step over once 79 00:07:09,699 --> 00:07:13,010 more to print out two_times. 80 00:07:13,010 --> 00:07:18,940 We get back to line 40. We can check in the console that 2 has been output. 81 00:07:18,940 --> 00:07:23,249 Back to the debugger. Right, so we're on line 14, and we can see that's going 82 00:07:23,249 --> 00:07:27,270 to bind val to the next number in the sequence. I step over and val should now 83 00:07:27,270 --> 00:07:32,249 have the value 2, and we can see that is the case. This time when we step into 84 00:07:32,249 --> 00:07:36,340 my code, you can see that both parameters now 85 00:07:36,340 --> 00:07:39,550 have the value 2. You should now understand what happens when you call a 86 00:07:39,550 --> 00:07:43,990 function in your code. Python will execute the code in your function, then 87 00:07:43,990 --> 00:07:48,580 return to the point where your function was called. I'll stop this video here. 88 00:07:48,580 --> 00:07:53,060 Continue stepping into the multiply function, and check that the parameter y 89 00:07:53,060 --> 00:07:56,940 gets each value of val, each time around the loop. 90 00:07:56,940 --> 00:08:00,600 And I'll see you in the next video.