1 00:00:04,680 --> 00:00:09,180 Alright, so continuing on from the last video, make sure you've still got the while 2 00:00:09,180 --> 00:00:13,289 loop.py program open in your editor. So we've got a while loop on 3 00:00:13,289 --> 00:00:17,400 lines five through three seven, and you can see that it's functionally identical to the 4 00:00:17,400 --> 00:00:21,599 for loop that I commented out on the lines one and two. If you've programmed 5 00:00:21,600 --> 00:00:26,720 in C or Java, that while loop is how a for loop really works in those languages. 6 00:00:26,720 --> 00:00:31,410 They both compile to the same code. That's not true in Python. Although we're 7 00:00:31,410 --> 00:00:35,190 getting the same result, the two loops are doing very different things behind 8 00:00:35,190 --> 00:00:39,690 the scenes. I mention that because you wouldn't use a while loop just to iterate 9 00:00:39,690 --> 00:00:43,350 through a range of values, but it did provide a good example to see what's 10 00:00:43,350 --> 00:00:47,040 going on. Before we look at more realistic examples of while loops, 11 00:00:47,040 --> 00:00:50,520 there's a couple of things I want to say about this one. We've seen that the 12 00:00:50,520 --> 00:00:54,570 condition i < 10 controls execution of the block of code in the 13 00:00:54,570 --> 00:00:59,400 loop. As long as i is less than 10 the code will be executed. Once i becomes 14 00:00:59,400 --> 00:01:03,030 equal to or greater than 10, the condition becomes False and the loop 15 00:01:03,030 --> 00:01:08,430 terminates. So that leads to two other important things to consider. First, the 16 00:01:08,430 --> 00:01:13,140 condition must be initialized before the loop starts. With the for loop, we just 17 00:01:13,140 --> 00:01:17,760 used i without setting a value first. Python knows to give it the first value 18 00:01:17,760 --> 00:01:22,260 from the range and continue from there. With the while loop though, we have to 19 00:01:22,260 --> 00:01:27,450 give i a value first and we do that on line 4. If we hadn't, we'd get an error 20 00:01:27,450 --> 00:01:31,770 that i is not defined. The second thing is that we need some way within the loop, 21 00:01:31,770 --> 00:01:37,780 for the condition to become False. So remove the indentation, as I'm doing here, 22 00:01:37,780 --> 00:01:44,240 before i plus 1, then we'll run the program again. We run the program. 23 00:01:45,360 --> 00:01:48,900 Now because we're not increasing i inside the loop, the condition never 24 00:01:48,900 --> 00:01:53,700 becomes False and the loop will keep running forever. And we need to go up to 25 00:01:53,700 --> 00:01:59,180 the run menu in IntelliJ and choose stop to stop this. 26 00:01:59,180 --> 00:02:03,260 I could have clicked on the little red button to the left, as well. Now if you're using idle, 27 00:02:03,270 --> 00:02:07,530 control C will stop the program as well. When we stopped incrementing i in the 28 00:02:07,530 --> 00:02:11,580 loop, there's no way for the condition to become False, so the loop just kept going, 29 00:02:11,580 --> 00:02:15,090 as you saw. We could just as well, in that scenario, have written 30 00:02:15,090 --> 00:02:18,680 the loop as follows: 31 00:02:20,380 --> 00:02:25,160 while True, as you can see there, and that would get us the same undesirable 32 00:02:25,160 --> 00:02:28,940 situation. In some of our other examples, you will see loops that just use 33 00:02:28,940 --> 00:02:32,360 True as the condition, but the body of the loop will then contain a way to 34 00:02:32,360 --> 00:02:36,260 break out of the loop. It's easier to see why our for loop is preferable in this 35 00:02:36,260 --> 00:02:41,450 case. The codes on lines 1 & 2 are easier to understand and there's less chance of 36 00:02:41,450 --> 00:02:44,959 introducing an error. There are many reasons for looping around a block of 37 00:02:44,960 --> 00:02:48,500 code that don't involve just carry through items in a sequence. For example, 38 00:02:48,500 --> 00:02:52,820 in an adventure game, players have to choose a direction in order to exit from 39 00:02:52,820 --> 00:02:56,480 a location. If they don't choose one of the available exits, they stay where they 40 00:02:56,480 --> 00:03:03,580 are, and that's an ideal use of a while loop. So lets create a new file here. 41 00:03:03,580 --> 00:03:14,260 We're gonna call this one adventure.py. Alright, we're gonna start by typing available 42 00:03:14,260 --> 00:03:21,080 underscore exits is equal to square brackets and in double quotes, north, 43 00:03:21,080 --> 00:03:25,310 double quote comma. We're going to do south, east and west all in double quotes, 44 00:03:25,310 --> 00:03:34,000 separated by commas. We've got our right square bracket on the right hand side. 45 00:03:34,000 --> 00:03:39,880 Line three, chosen underscore exit equals and two double quotes, meaning an empty 46 00:03:39,890 --> 00:03:47,810 string. Now our while loop is going to be while chosen exit not in available exits 47 00:03:47,810 --> 00:03:53,890 colon, and indented on the next line, chosen underscore exit is equal to input 48 00:03:53,890 --> 00:03:58,940 parentheses double quotes Please choose a direction 49 00:03:59,980 --> 00:04:03,880 colon space then we've got our double quote and right parenthesis. Then on line 50 00:04:03,890 --> 00:04:10,609 seven print and double quotes in parentheses, aren't you glad you 51 00:04:10,609 --> 00:04:18,829 got out of there. So in this particular room in the game, the available exits are in 52 00:04:18,829 --> 00:04:24,110 the available underscore exits list; north, south, east and west. In order to 53 00:04:24,110 --> 00:04:28,220 enter the loop at all, we have to initialize chosen underscore exit to a 54 00:04:28,220 --> 00:04:32,390 value that isn't in the list. We could have used anything except one of 55 00:04:32,390 --> 00:04:36,380 those directions, but an empty string makes sense here. Because an empty string 56 00:04:36,380 --> 00:04:40,970 is not in the list, the condition evaluates to True and the program enters 57 00:04:40,970 --> 00:04:45,200 the loop. And it's a very simple loop - it just requests input from the player, 58 00:04:45,200 --> 00:04:49,580 and assignes what they type to the chosen underscore exit variable. The 59 00:04:49,580 --> 00:04:53,570 loop then tests the condition again. If the direction they typed is not one of 60 00:04:53,570 --> 00:04:57,530 the ones in the list, then the code block and the loop is executed again. This 61 00:04:57,530 --> 00:05:01,880 continues until they type one of the valid directions and escape. So if you run the 62 00:05:01,880 --> 00:05:07,040 program now, and we'll enter various strings when prompted to see what 63 00:05:07,040 --> 00:05:11,960 happens. I'm gonna start by entering up. Now that's not in the list so we get the 64 00:05:11,960 --> 00:05:18,240 input prompt again, down, also not in the list. Then we go around the loop again. 65 00:05:18,240 --> 00:05:27,080 Then when I enter east here, the condition evaluates to True and we leave the loop 66 00:05:27,080 --> 00:05:30,620 One of the important features of while loops is that they can be used when you 67 00:05:30,639 --> 00:05:34,990 can't determine, in advance, how many times you will need to loop. A for loop 68 00:05:34,990 --> 00:05:38,680 will repeat for each item in a predetermined sequence, whereas with a 69 00:05:38,680 --> 00:05:42,480 while loop you don't need to know how many times the loop will execute. 70 00:05:42,480 --> 00:05:45,900 One particular application of this is reading data from a file or an internet 71 00:05:45,910 --> 00:05:50,650 stream. It's difficult, not impossible but difficult, to know in advance how much 72 00:05:50,650 --> 00:05:54,880 data there is to read. A while loop can be used to keep reading until there's no 73 00:05:54,880 --> 00:05:59,020 more data left. And we'll see this soon when we look at file I/O, input and 74 00:05:59,020 --> 00:06:02,139 output, in Python. In the next video, we'll look at break 75 00:06:02,139 --> 00:06:07,650 and continue with while loops. See you in the next video.