1 00:00:04,240 --> 00:00:09,000 Alright, let's talk more about using if, elif and else in the debugger. 2 00:00:09,000 --> 00:00:14,200 At the moment, our program only allows one guess as its answer. Now we're going to change that in an 3 00:00:14,200 --> 00:00:17,320 upcoming video, but first, let's run it in the debugger 4 00:00:17,320 --> 00:00:22,280 just to confirm what's happening. We're gonna set a breakpoint over here, on line 6. 5 00:00:22,280 --> 00:00:27,880 I'm going to right click and choose Debug guessinggame. Now if you're using a 6 00:00:27,890 --> 00:00:31,789 different IDE, just start your debugger in the usual way. Alright, so at the moment, 7 00:00:31,789 --> 00:00:35,390 you can see down the bottom of the screen, our program's waiting for our input. 8 00:00:35,390 --> 00:00:39,470 So I'm going to click into here and enter the answer 3, and that's obviously less than 9 00:00:39,470 --> 00:00:44,720 our answer 5. Now when I press enter, the debugger stops on line 6. That's going to 10 00:00:44,720 --> 00:00:48,230 test 'if guess is less than the answer', which of course it is. We can see the 11 00:00:48,230 --> 00:00:53,080 values of guess and answer in the variables pane. Now IntelliJ also shows 12 00:00:53,080 --> 00:00:57,820 the values in the code pane. We can see answer 5 at the end of line 1, but also, 13 00:00:57,820 --> 00:01:03,940 we can see 3 as the guess entered at the end of line 4. So work out in your mind 14 00:01:03,940 --> 00:01:08,000 which line's going to be executed next, before using the step-over button. So I'm 15 00:01:08,000 --> 00:01:15,040 going to come over and do that. As expected, we get to line 7. Stepping over again, 16 00:01:15,040 --> 00:01:18,240 the program's executed, and did you note that the little console 17 00:01:18,259 --> 00:01:22,130 tab has got the little yellow circle and a down arrow, indicating there was some 18 00:01:22,130 --> 00:01:25,700 output. You might have seen that flashing, so we click onto that. We can see we've 19 00:01:25,700 --> 00:01:29,300 now got some output below where we entered 3, "Please guess higher", and the program's 20 00:01:29,300 --> 00:01:33,020 terminated. So basically, Python won't test the code 21 00:01:33,020 --> 00:01:37,759 in an elif or an else when the if condition is true. And obviously, in this 22 00:01:37,760 --> 00:01:42,640 case, guess < answer was True. It printed "Please guess higher" and the program terminated. 23 00:01:42,640 --> 00:01:45,260 Alright, so now we're about to test the condition guess is greater 24 00:01:45,260 --> 00:01:48,960 than answer, which will be true. So let's rerun, 25 00:01:48,960 --> 00:01:55,180 or re-debug I should say, by clicking on debug. I'm going to enter 8 as my guess. 26 00:01:55,180 --> 00:01:57,780 We've stopped on line 6 again and this time the condition is going to be 27 00:01:57,800 --> 00:02:02,320 false because 8 is obviously not less than 5. So I'm going to step over. 28 00:02:02,320 --> 00:02:06,220 Next we're about to test the condition guess greater than answer, which will be true. 29 00:02:06,220 --> 00:02:11,090 So I'm going to step over again, stepping over one more time. You can see the flashing 30 00:02:11,090 --> 00:02:14,980 little down arrow in the console tab at the top there. Click on that and we can 31 00:02:14,980 --> 00:02:18,680 see the output "Please guess lower". Alright, so let's run the program one 32 00:02:18,680 --> 00:02:23,320 more time, in the debugger this time. We're going to enter the guess as 5. 33 00:02:23,320 --> 00:02:27,110 Alright, so now that we've entered 5, so the condition on line 6 will be 34 00:02:27,110 --> 00:02:31,000 false, and so will the condition on line 8. Those lines will still get 35 00:02:31,000 --> 00:02:35,940 executed. Python has to evaluate the conditions to know what the results will be. 36 00:02:35,940 --> 00:02:37,940 It's the code in the blocks after the if 37 00:02:37,940 --> 00:02:42,980 and elif that will get skipped. So let's use step over, to see the first 38 00:02:42,980 --> 00:02:47,810 condition getting evaluated, and then we'll click step over again to evaluate 39 00:02:47,810 --> 00:02:52,520 line 8. Execution has jumped to line 11. Note that the else isn't 40 00:02:52,520 --> 00:02:57,220 evaluated. There's no condition to evaluate. else blocks will be executed if nothing 41 00:02:57,220 --> 00:03:03,520 else evaluated to true. The rules I mentioned a couple of videos ago are simple. 42 00:03:03,520 --> 00:03:05,080 So an if-statement begins with the 43 00:03:05,100 --> 00:03:11,709 keyword if, followed by condition. The if condition will always be evaluated. 44 00:03:11,709 --> 00:03:15,980 Next ,you can have one or more elif blocks. You don't have to include elif 45 00:03:16,000 --> 00:03:21,160 and we will be seeing code that doesn't, but if you have any elif lines, then they 46 00:03:21,160 --> 00:03:28,520 come after the if. elif also has to come before else, if there is an else. 47 00:03:28,520 --> 00:03:32,840 And finally, you may have an else block. You don't have to use else, but if you do, 48 00:03:32,840 --> 00:03:38,600 it must come after the if, and it must also come after any elifs, if there are any. 49 00:03:38,660 --> 00:03:41,980 Alright so we're back in the code here and I'm going to click on step over 50 00:03:41,980 --> 00:03:47,180 one more time. Program's ended, click on the console tab and you can see "You've got it first time" 51 00:03:47,180 --> 00:03:50,600 is outputted to the screen. Alright, in the next video we're going to 52 00:03:50,600 --> 00:03:54,410 improve the program slightly. It's still not going to be very exciting. We've got 53 00:03:54,410 --> 00:03:58,340 a few more things to learn first but we have to learn to walk before we can run. 54 00:03:58,340 --> 00:04:01,520 See you in the next video.