1 00:00:04,569 --> 00:00:09,530 For this video, we're going to make a change to our high-low game. So open up 2 00:00:09,530 --> 00:00:15,290 hilo.py, and that's the one that we used in an earlier video. Now we can use 3 00:00:15,290 --> 00:00:19,970 an else clause with our while loop, and there's our while loop, to make the game 4 00:00:19,970 --> 00:00:24,740 a bit more impressive. Now let's have a look at what we're going to do. 5 00:00:24,740 --> 00:00:29,000 Alright, so in the binary search slides, earlier, we worked through what happens when we 6 00:00:29,000 --> 00:00:32,630 try to guess the number 7. We were guessing in the range of 1 to 10. So I 7 00:00:32,630 --> 00:00:37,400 want to quickly review those earlier slides. We start off by guessing the 8 00:00:37,400 --> 00:00:42,380 mid-point between the low and high values. Our first guess is 5. We're told to guess 9 00:00:42,380 --> 00:00:46,550 higher - that means the answer must lie somewhere between 6 and 10. The new 10 00:00:46,550 --> 00:00:51,080 mid-point is 8, which becomes our second guess. We're told to guess lower - 11 00:00:51,080 --> 00:00:54,590 that means the answer must lie somewhere between 6 and 7. Our third guess 12 00:00:54,590 --> 00:01:00,230 will be 6. Where things get interesting, is in the next slide. 7 is greater than 13 00:01:00,230 --> 00:01:06,100 6, so we're told to guess higher. The low value becomes equal to the high value. 14 00:01:06,100 --> 00:01:10,100 When we went through this example earlier, I said at this point, a human 15 00:01:10,100 --> 00:01:14,750 would guess 7 right away. I also said that the computer has to do the 16 00:01:14,750 --> 00:01:18,770 calculation first, but that's not completely true. Rather than use our 17 00:01:18,770 --> 00:01:23,060 formula to calculate the new mid-point, the computer could test to see if low 18 00:01:23,060 --> 00:01:28,330 is equal to high. If low does equal high, the computer 19 00:01:28,330 --> 00:01:32,590 then doesn't have to ask us if the guess is correct. When low and high are equal. it 20 00:01:32,590 --> 00:01:36,700 must have been the correct number. Note that this is only the case because all 21 00:01:36,700 --> 00:01:41,320 numbers are present. In a normal binary search, when what we're looking for may 22 00:01:41,320 --> 00:01:47,140 not be present, this doesn't apply. We can make our program a bit more impressive, 23 00:01:47,140 --> 00:01:52,060 by not asking if the value is correct. We know it must be, when low is equal to 24 00:01:52,060 --> 00:01:56,020 high, and we can tell the player that we've guessed their number. And the 25 00:01:56,020 --> 00:01:59,200 changes to our code are quite simple. So let's swing over and get started with 26 00:01:59,200 --> 00:02:05,620 that. Alright, so on line 23, we break out of the loop when the player confirms that 27 00:02:05,620 --> 00:02:09,190 we've guessed the right number. In the cases where we know we've got the right 28 00:02:09,190 --> 00:02:13,510 number, we can cause the loop to terminate and use an else clause to tell 29 00:02:13,510 --> 00:02:17,140 the player what their number was. So we're gonna start by changing the 30 00:02:17,140 --> 00:02:22,420 condition for the while loop. Let's go up and do that. So instead of while true, 31 00:02:22,420 --> 00:02:30,480 we're gonna put while low is not equal to high. So we continue looping, as long 32 00:02:30,489 --> 00:02:35,080 as low doesn't equal high. When they are equal, the loop will terminate, and if 33 00:02:35,080 --> 00:02:38,620 that happens, and it won't happen for all numbers - just some of them - but when that 34 00:02:38,620 --> 00:02:42,280 happens, we know that we've found the correct number. So let's add the code for 35 00:02:42,280 --> 00:02:49,960 that. So there's going to be else down here on line 29, else : line 30 print parentheses 36 00:02:49,960 --> 00:02:54,680 double quotes, You thought of the number, 37 00:02:54,989 --> 00:03:01,630 left and right curly braces double quote dot format low in parentheses, and the 38 00:03:01,630 --> 00:03:07,630 right parenthesis to end the line. Then on the next line, print, I got it in, and left 39 00:03:07,630 --> 00:03:14,980 and right curly braces again, guesses, double quotes dot format guesses and two 40 00:03:14,980 --> 00:03:19,720 right parentheses. Now just before I run the program, I'm going to comment out up 41 00:03:19,720 --> 00:03:25,520 here, the diagnostic information that we're printing on line 9. So let's do that. 42 00:03:25,520 --> 00:03:28,600 Alright, so let's now run the program. 43 00:03:29,300 --> 00:03:33,519 So my number's going to be a hundred. Press ENTER to start. So with my number being 44 00:03:33,520 --> 00:03:37,760 a hundred, the first guess is obviously too high, so we're going to put lower 45 00:03:37,760 --> 00:03:49,540 lower, lower, higher, higher, lower, lower, higher. 46 00:03:49,540 --> 00:03:54,470 And as you can see, the last guess there is 99. If we tell the computer to go higher, it 47 00:03:54,470 --> 00:03:57,380 doesn't have to ask us if a hundred is correct, because you can see it's already 48 00:03:57,380 --> 00:04:02,880 guessed 101, and now it's guessing 99. So if I press h here, which is correct 49 00:04:02,880 --> 00:04:07,940 because it's higher than 99, You thought of the number 100. I got it in 10 guesses. 50 00:04:07,940 --> 00:04:12,040 So at that point, when I selected that, low and high both equalled a hundred, and 51 00:04:12,040 --> 00:04:18,040 the while loop terminated. It terminated normally and the else clause was invoked, 52 00:04:18,040 --> 00:04:22,740 and we got the output on the screen, as you can see. 53 00:04:22,740 --> 00:04:26,759 So that's a bit more impressive. What would be really good is if we could get 54 00:04:26,759 --> 00:04:31,169 the computer to do that for any guess, but we can't. It only knows it has the 55 00:04:31,169 --> 00:04:36,020 correct answer when high and low converge to the same value. With a high value 56 00:04:36,020 --> 00:04:40,740 that's close to an exact power of two, that happens about half the time. 1000 is 57 00:04:40,740 --> 00:04:44,310 closer to 1024, and the computer can tell it has the correct 58 00:04:44,310 --> 00:04:50,250 answer for 489 numbers that we think of. If you'd like to run this code in the 59 00:04:50,250 --> 00:04:53,250 debugger, to help understand what's happening, you'll find it a bit 60 00:04:53,250 --> 00:04:57,780 frustrating. It can be annoying debugging code that waits for input. Setting 61 00:04:57,780 --> 00:05:01,530 breakpoints can help a bit, but there's another technique you can use and I'll 62 00:05:01,530 --> 00:05:05,419 show you what that is in the next video.