1 00:00:08,760 --> 00:00:10,368 So we've made it to the end of 2 00:00:10,368 --> 00:00:14,690 Module ten, about iteration, what some people call looping. 3 00:00:14,690 --> 00:00:18,360 So, let's look at the module learning objectives. 4 00:00:22,200 --> 00:00:26,460 So last time, we finished our discussion about for and for each loops. 5 00:00:26,460 --> 00:00:29,115 The loops that we use when we know how many times we're 6 00:00:29,115 --> 00:00:32,590 going to loop when we get to the loop as the program executes. 7 00:00:32,590 --> 00:00:34,900 This time, we're going to look at while loops. 8 00:00:34,900 --> 00:00:36,524 Which are loops that we use when we 9 00:00:36,524 --> 00:00:39,660 don't know how many times we're going to iterate. 10 00:00:39,660 --> 00:00:42,348 So, just to see if you've paid attention to what I 11 00:00:42,348 --> 00:00:46,260 said over the last 30 seconds, here's an in-lecture quiz for you. 12 00:00:48,550 --> 00:00:53,730 So, I've started a program that we're going to start looking at. 13 00:00:53,730 --> 00:00:57,058 Before I do, I want to talk about sort of 14 00:00:57,058 --> 00:01:01,654 a standard problem that we solve using while loops. 15 00:01:01,654 --> 00:01:06,230 So, input validation means making sure that whoever is using 16 00:01:06,230 --> 00:01:10,370 our software has entered valid data when we ask for it. 17 00:01:10,370 --> 00:01:13,714 And the reason we need a while loop, here rather than 18 00:01:13,714 --> 00:01:16,374 a for, or for each loop, or even just an if 19 00:01:16,374 --> 00:01:20,890 statement is because we don't know how stupid the user is. 20 00:01:20,890 --> 00:01:26,460 They could enter invalid inputs 10 times before they finally enter a valid one. 21 00:01:26,460 --> 00:01:31,210 And so a while loop is a great choice for doing input validation. 22 00:01:31,210 --> 00:01:36,190 So, I'll open up our project. I've already added a little bit of code. 23 00:01:36,190 --> 00:01:39,339 So this is a console application, darned no graphics or 24 00:01:39,339 --> 00:01:44,550 explosions, or anything like that. So we have a menu here. 25 00:01:44,550 --> 00:01:48,670 And the user can enter J for Jump, C for Crouch, or Q for Quit. 26 00:01:50,420 --> 00:01:53,720 We have the standard prompt and reading in that we've done before. 27 00:01:53,720 --> 00:01:57,330 I will point out that the console read line gives us a string. 28 00:01:58,710 --> 00:02:04,974 And I'm calling the ToUpper Method on that string, so that we turn all the characters 29 00:02:04,974 --> 00:02:09,890 in the string to upper case. And hopefully, that's only one character. 30 00:02:09,890 --> 00:02:14,280 because we're going to do a char.Parse on it, and put it into this choice variable. 31 00:02:14,280 --> 00:02:17,980 So we know, when we get here, as long as the user doesn't do 32 00:02:17,980 --> 00:02:23,336 something super stupid, we're going to end up with an upper case single character. 33 00:02:23,336 --> 00:02:27,340 So I'm going to Ctrl+F5 to run this, just to show that I can do that. 34 00:02:27,340 --> 00:02:29,980 But I can also enter, you know, D and it's okay 35 00:02:29,980 --> 00:02:31,820 it gets to the end. 36 00:02:31,820 --> 00:02:33,885 So for this example, we're not actually 37 00:02:33,885 --> 00:02:36,380 going to do anything with their menu choice. 38 00:02:36,380 --> 00:02:41,930 We're just going to make sure that they've entered the valid one so that when we get 39 00:02:41,930 --> 00:02:44,555 to this point in the code where this cursor 40 00:02:44,555 --> 00:02:47,720 is we know that we have a valid input. 41 00:02:47,720 --> 00:02:50,200 So how do we validate input? 42 00:02:50,200 --> 00:02:55,156 Well, what we need to do is we need to make sure that, as long as the user 43 00:02:55,156 --> 00:02:57,750 has entered invalid input. 44 00:02:57,750 --> 00:03:00,975 That we can in fact give them an error message, give 45 00:03:00,975 --> 00:03:04,610 them the menu again and get a new input from them. 46 00:03:04,610 --> 00:03:07,550 And so, we're going to use a while loop for that. 47 00:03:07,550 --> 00:03:14,849 And I put the structure in first. Like that. 48 00:03:14,849 --> 00:03:20,645 So, what goes here is a Boolean expression that when it evaluates to true, 49 00:03:20,645 --> 00:03:26,370 we want to go into the loop body and do whatever happens in there. 50 00:03:26,370 --> 00:03:30,873 So, let's say, we know what the valid choices are. 51 00:03:30,873 --> 00:03:34,288 The valid choices are capital J, C, or Q. 52 00:03:34,288 --> 00:03:40,129 So, the Boolean expression here needs to evaluate to true when that, 53 00:03:40,129 --> 00:03:45,765 when the input is invalid. Which means that 54 00:03:45,765 --> 00:03:50,935 choice is not equal to J and 55 00:03:50,935 --> 00:03:56,575 choice is not equal to C, and 56 00:03:56,575 --> 00:04:01,560 choice is not equal to Q. 57 00:04:01,560 --> 00:04:05,824 So, if it's the case, that they haven't entered a J and they 58 00:04:05,824 --> 00:04:10,990 haven't entered a C, and they haven't entered a Q, then we know they've 59 00:04:10,990 --> 00:04:15,710 given us invalid input. And we need to get valid input from them. 60 00:04:15,710 --> 00:04:18,390 So, the first thing we'll do is print an error message. 61 00:04:20,670 --> 00:04:24,129 And you can be as nice or as mean with this error message as you want. 62 00:04:25,540 --> 00:04:31,430 I'll be reasonably nice and just say 63 00:04:31,430 --> 00:04:36,940 invalid input, must be J, C or Q. 64 00:04:38,210 --> 00:04:40,450 You can column name if you want to and so on. 65 00:04:40,450 --> 00:04:42,880 We're going to add a couple of lines in 66 00:04:42,880 --> 00:04:45,796 the output to set the error message off from 67 00:04:45,796 --> 00:04:49,320 the menu and from where they entered their choice. 68 00:04:55,440 --> 00:05:00,840 And now I need to print the menu again and get their choice again. 69 00:05:00,840 --> 00:05:04,410 And I'm going to copy and paste, even though I hate doing that. 70 00:05:04,410 --> 00:05:08,067 Once we learn how to write our own methods, we'll see that this 71 00:05:08,067 --> 00:05:12,750 whole copy paste thing doesn't really need to be done anymore by us. 72 00:05:12,750 --> 00:05:15,690 And of course I have to not declare the choice variable again. 73 00:05:16,950 --> 00:05:20,674 So now, what will happen is, I'm going to print the menu and 74 00:05:20,674 --> 00:05:22,670 get their choice. 75 00:05:22,670 --> 00:05:24,350 And then I'm going to check to see if 76 00:05:24,350 --> 00:05:27,470 their choice is invalid with this Boolean expression. 77 00:05:27,470 --> 00:05:32,000 If it is, I'm going to print the err, error message. 78 00:05:32,000 --> 00:05:35,280 I'm going to give them the menu and ask for their choice again. 79 00:05:35,280 --> 00:05:40,897 And then when I we, When I reach this closing curly brace, 80 00:05:40,897 --> 00:05:46,326 I'm going to loop back to the top of the while loop and I'm going to evaluate 81 00:05:46,326 --> 00:05:49,570 this Boolean expression again. 82 00:05:49,570 --> 00:05:51,880 And if it's true again, because the user is 83 00:05:51,880 --> 00:05:54,870 an idiot, then I'm going to go into the loop again. 84 00:05:54,870 --> 00:05:59,059 So I will keep looping through this while loop until they've entered a 85 00:05:59,059 --> 00:06:03,810 valid input, at which point I'll get here and I'll know it's valid. 86 00:06:03,810 --> 00:06:10,109 So let's Ctrl+F5. I'll enter some invalid 87 00:06:10,109 --> 00:06:12,550 inputs, D, D, D. 88 00:06:12,550 --> 00:06:17,356 But then when I finally enter a valid one J, it gets past the while 89 00:06:17,356 --> 00:06:22,680 loop, because this Boolean expression here now evaluates to false. 90 00:06:23,680 --> 00:06:26,500 And so it gets past the while loop and I get to the end of my program. 91 00:06:27,510 --> 00:06:35,460 So, before we continue, you should go do an in-lecture quiz, that's, talks about or 92 00:06:35,460 --> 00:06:38,360 asks you about infinite loops. 93 00:06:39,610 --> 00:06:44,110 And now you should do another in-lecture quiz that asks you 94 00:06:44,110 --> 00:06:49,190 about an acronym that can help us solve the infinite loop problem. 95 00:06:50,860 --> 00:06:51,590 Great. 96 00:06:51,590 --> 00:06:54,550 Now, infinite loops are not really infinite. 97 00:06:54,550 --> 00:06:54,880 Right? 98 00:06:54,880 --> 00:06:58,580 At some point the sun will explode, or our computer will burn up. 99 00:06:58,580 --> 00:07:00,532 Or, or we'll get so angry that we'll 100 00:07:00,532 --> 00:07:03,200 you know, throw it against the wall or something. 101 00:07:03,200 --> 00:07:05,479 But they're really called infinite loops because 102 00:07:05,479 --> 00:07:07,690 they don't terminate when they really should. 103 00:07:07,690 --> 00:07:10,430 You know, with sort of normal behavior. 104 00:07:10,430 --> 00:07:15,880 So, let's show how initialize, test, and modify both, 105 00:07:15,880 --> 00:07:21,980 bold, all three matter when avoiding infinite loops. 106 00:07:21,980 --> 00:07:26,110 It actually turns out that in this particular case, the initializing 107 00:07:26,110 --> 00:07:30,840 doesn't lead to an infinite loop, it just leads to nasty behavior. 108 00:07:30,840 --> 00:07:35,350 So, let's say that I have in fact declared my choice variable and 109 00:07:35,350 --> 00:07:41,330 initialized it to some invalid choice at the beginning of the main method. 110 00:07:41,330 --> 00:07:46,000 And then I don't actually initialize it by getting input from the user. 111 00:07:46,000 --> 00:07:51,550 What happens when I, when I run, is that it immediately says that I'm a moron. 112 00:07:51,550 --> 00:07:54,310 And gives me the message again, which is really bad for users. 113 00:07:54,310 --> 00:07:54,580 Right? 114 00:07:54,580 --> 00:07:56,800 That's not the appropriate way to do it. 115 00:07:56,800 --> 00:08:00,400 That's kind of a poor example of how initialize leads 116 00:08:00,400 --> 00:08:04,710 to an infinite loop, because in fact it does not here. 117 00:08:04,710 --> 00:08:11,370 So just trust me that sometimes initialize is the problem. 118 00:08:11,370 --> 00:08:13,821 Let's actually look at test and modify, because those 119 00:08:13,821 --> 00:08:16,728 are way better examples here in this particular example that 120 00:08:16,728 --> 00:08:20,730 we're working through. So let's say we got this test wrong. 121 00:08:20,730 --> 00:08:24,210 Let's say we've gotten confused about and versus or. 122 00:08:25,290 --> 00:08:28,530 And so here we've put or's instead of ands. 123 00:08:28,530 --> 00:08:29,980 So we've messed up the test. 124 00:08:29,980 --> 00:08:32,500 The test is invalid for the loop. 125 00:08:32,500 --> 00:08:34,600 And this is going to lead us to an infinite loop. 126 00:08:34,600 --> 00:08:38,950 So I'll Ctrl+F5, and I will enter an invalid input. 127 00:08:38,950 --> 00:08:41,770 And I say, oh, darn, I should have 128 00:08:41,770 --> 00:08:46,180 read the menu and I enter j. And wow. 129 00:08:46,180 --> 00:08:47,110 I'm looping again. 130 00:08:47,110 --> 00:08:49,611 So, like any good programmer, I'll do exactly 131 00:08:49,611 --> 00:08:52,290 the same thing and expect a different result. 132 00:08:52,290 --> 00:08:54,560 That's the sign of madness. 133 00:08:54,560 --> 00:08:56,720 Except, in the context of software, it's not 134 00:08:56,720 --> 00:08:59,320 the sign of madness, because something is different. 135 00:08:59,320 --> 00:09:03,100 The state of the computer, the memory, something is in fact different. 136 00:09:03,100 --> 00:09:07,480 We're mad as programmers, but not because we try the same thing multiple times. 137 00:09:07,480 --> 00:09:09,020 Okay. So now I'm stuck. 138 00:09:09,020 --> 00:09:09,290 Right? 139 00:09:09,290 --> 00:09:12,750 Even though I'm entering valid inputs, I can't get out. 140 00:09:12,750 --> 00:09:16,790 So I'll use the red X. Here's what's happening. 141 00:09:16,790 --> 00:09:19,870 When I entered J, this part of the Boolean 142 00:09:19,870 --> 00:09:23,630 expression, J not equal to J, was in fact false. 143 00:09:23,630 --> 00:09:24,386 [INAUDIBLE] 144 00:09:24,386 --> 00:09:29,450 good. The problem is, then I got to this part. 145 00:09:29,450 --> 00:09:35,120 And J not equal to C is true. And the way that or's work. 146 00:09:35,120 --> 00:09:35,530 Right? 147 00:09:35,530 --> 00:09:37,900 Is that any of them can be true to make 148 00:09:37,900 --> 00:09:42,070 the whole thing true, which pushes us into the loop again. 149 00:09:42,070 --> 00:09:45,424 So that's how a test can, an in, an incorrect 150 00:09:45,424 --> 00:09:49,552 test in our Boolean expression for the while loop can lead 151 00:09:49,552 --> 00:09:51,880 to an infinite loop. 152 00:09:51,880 --> 00:09:56,630 Let's hit the M part of the acronym, so if we forgot to modify. 153 00:09:56,630 --> 00:09:58,020 So I'll, I'll comma this out. 154 00:09:59,050 --> 00:10:03,768 If we forgot to modify the loop control variable, let's watch what happens. 155 00:10:03,768 --> 00:10:08,350 I'll Ctrl+F5, and now I'll enter a bad one, D. 156 00:10:08,350 --> 00:10:14,180 And oh dear, loop, loop, loop, loop, loop forever until I reach quit. 157 00:10:14,180 --> 00:10:14,648 So what's 158 00:10:14,648 --> 00:10:18,050 happening there, I'm going to fix this before I forget. 159 00:10:18,050 --> 00:10:21,445 What's happening there is, if I got into 160 00:10:21,445 --> 00:10:26,280 the loop, this Boolen expression evaluated to true. 161 00:10:26,280 --> 00:10:30,900 That's the only way I got into the loop body was because this is true. 162 00:10:30,900 --> 00:10:36,240 So if I don't change choice inside the loop. 163 00:10:36,240 --> 00:10:39,724 When I get to the end of the loop here, and I come back around and 164 00:10:39,724 --> 00:10:43,090 check it again, it's true again, because nothing changed. 165 00:10:43,090 --> 00:10:45,060 I've never changed choice. 166 00:10:45,060 --> 00:10:48,102 So, I'm stuck in this infinite loop because I 167 00:10:48,102 --> 00:10:52,140 didn't modify the variable that I'm using in my test. 168 00:10:52,140 --> 00:10:56,230 Sometimes we have multiple variable in our test, but the concept still applies. 169 00:10:56,230 --> 00:10:59,930 At least one of those variables has to get changed inside the 170 00:10:59,930 --> 00:11:05,110 loop body, otherwise we're guaranteed to end up with an infinite loop. 171 00:11:05,110 --> 00:11:06,908 Because, if we got into the loop body 172 00:11:06,908 --> 00:11:10,370 the first time, we're going to get into it forever. 173 00:11:10,370 --> 00:11:11,100 Okay. 174 00:11:11,100 --> 00:11:14,850 So one last. That's, that's it for the code for today. 175 00:11:14,850 --> 00:11:17,688 Those are while loops, and input validation tends 176 00:11:17,688 --> 00:11:20,210 to be the most common place we use them. 177 00:11:20,210 --> 00:11:23,180 But there are other places we find to use them as well. 178 00:11:23,180 --> 00:11:28,570 So go do one more in-lecture quiz about actually testing while loops. 179 00:11:31,770 --> 00:11:32,310 Okay. 180 00:11:32,310 --> 00:11:36,622 So, when we test while loops, it makes sense to make 181 00:11:36,622 --> 00:11:41,080 it, to run it so that the loop body executes 0 times. 182 00:11:41,080 --> 00:11:42,200 And note, I did that. 183 00:11:43,246 --> 00:11:45,358 And may not have done that, but I meant 184 00:11:45,358 --> 00:11:48,590 to do that by entering a valid input right away. 185 00:11:48,590 --> 00:11:53,078 So then the while, the Boolean expression in the while loop evaluated to false 186 00:11:53,078 --> 00:11:57,360 when we got to it the first time so we just skip the loop body. 187 00:11:57,360 --> 00:12:00,267 You also should execute it once, which we saw numerous times 188 00:12:00,267 --> 00:12:03,750 where it enters D and the J and something like that. 189 00:12:03,750 --> 00:12:05,860 And then you should execute it multiple times. 190 00:12:05,860 --> 00:12:10,110 Try a few invalid input followed by a valid one. 191 00:12:10,110 --> 00:12:16,182 The reason for this is that the sort of most common errors that people find in the 192 00:12:16,182 --> 00:12:23,120 while loops that they write, occur because they got the test not quite right. 193 00:12:23,120 --> 00:12:25,334 And so when you go for a zero run, 194 00:12:25,334 --> 00:12:28,990 basically, you end up going into the loop instead. 195 00:12:28,990 --> 00:12:30,279 And so you modify your test. 196 00:12:31,800 --> 00:12:35,045 You might, trying it once, we'd certainly catch the you forgot 197 00:12:35,045 --> 00:12:37,840 to modify it thing and you end up with an infinite loop. 198 00:12:37,840 --> 00:12:40,765 So if you have a test case that runs that while loop body 199 00:12:40,765 --> 00:12:45,350 loop that goes into the while loop or body once, you'll catch that. 200 00:12:45,350 --> 00:12:47,750 And sometimes much more rarely. 201 00:12:47,750 --> 00:12:48,206 We have 202 00:12:48,206 --> 00:12:52,310 a few iterations of the loop and then the loop breaks. 203 00:12:52,310 --> 00:12:55,690 But, that is more rare, certainly, than the first two. 204 00:12:55,690 --> 00:12:58,665 So that's a way to actually test of, to do 205 00:12:58,665 --> 00:13:02,650 a good test, of code that has a while loop. 206 00:13:02,650 --> 00:13:08,128 So to recap for today, we actually covered how we can use a while loop to iterate 207 00:13:08,128 --> 00:13:13,274 an unknown number of times, because it's based on some condition rather 208 00:13:13,274 --> 00:13:14,570 than a count. 209 00:13:14,570 --> 00:13:17,035 It's not how many things are in the 210 00:13:17,035 --> 00:13:20,660 collection, it's how times is the user a moron. 211 00:13:22,000 --> 00:13:25,950 That's the end of module ten. So we're all done with iteration. 212 00:13:25,950 --> 00:13:28,220 Again, what a lot of people call looping. 213 00:13:28,220 --> 00:13:32,252 We're all done with iteration, and you now have all three control 214 00:13:32,252 --> 00:13:36,950 structures you need to solve all kinds of problems in your game development. 215 00:13:37,990 --> 00:13:38,670 Next time, 216 00:13:38,670 --> 00:13:42,070 we're going to move on to the next module, where we're going to 217 00:13:42,070 --> 00:13:46,140 start building, designing and building a customed class of our own.