1 00:00:00,130 --> 00:00:01,890 Hi guys and welcome back! 2 00:00:01,890 --> 00:00:04,970 In this video we're going to talk about Loops in Python. 3 00:00:04,970 --> 00:00:06,220 There are two types of Loop. 4 00:00:06,220 --> 00:00:08,460 One that allows us to repeat some 5 00:00:08,460 --> 00:00:10,590 code a certain number of times. 6 00:00:10,590 --> 00:00:12,400 And one that allows us to repeat 7 00:00:12,400 --> 00:00:15,910 code for as long as a condition is true. 8 00:00:15,910 --> 00:00:18,636 Let's start with that one in this video. 9 00:00:18,636 --> 00:00:21,797 So, we've got our code from the last video here 10 00:00:21,797 --> 00:00:24,850 where we ask the user where they want to play. 11 00:00:24,850 --> 00:00:27,860 And then, if the user's input was y, 12 00:00:27,860 --> 00:00:29,390 we ask them to get our number. 13 00:00:29,390 --> 00:00:31,210 And we told them whether the number was correct. 14 00:00:31,210 --> 00:00:33,258 The number being defined up here. 15 00:00:33,258 --> 00:00:35,140 Now, what we're going to do is, 16 00:00:35,140 --> 00:00:37,940 we're going to repeat the game 17 00:00:37,940 --> 00:00:41,700 over and over until the user asks us to stop. 18 00:00:41,700 --> 00:00:43,360 So, what I'm going to do is, I'm going to change 19 00:00:43,360 --> 00:00:46,680 this prompt a little bit before we begin. 20 00:00:46,680 --> 00:00:48,960 And I will say, "Would you like to play?" 21 00:00:48,960 --> 00:00:50,650 And then, I would give them the 22 00:00:50,650 --> 00:00:55,650 options of y in capital, or n in lowercase. 23 00:00:56,020 --> 00:00:59,250 This is a common convention in terminal based programmes 24 00:00:59,250 --> 00:01:00,740 where you use one of the options 25 00:01:00,740 --> 00:01:02,800 as uppercase and one of them as lowercase. 26 00:01:02,800 --> 00:01:04,921 And that means that if they type anything 27 00:01:04,921 --> 00:01:09,510 other than n, y would be the default. 28 00:01:09,510 --> 00:01:12,210 So they have to specifically type n if they want to stop. 29 00:01:12,210 --> 00:01:13,570 This is a common convention, 30 00:01:13,570 --> 00:01:15,893 so you can use it in your own programmes as well. 31 00:01:15,893 --> 00:01:18,880 What that means is that now we want to repeat 32 00:01:18,880 --> 00:01:22,180 this code here, allowing the user to guess a number, 33 00:01:22,180 --> 00:01:24,280 and then telling them whether it's wrong or not, 34 00:01:24,280 --> 00:01:28,030 for as long as the user didn't type n. 35 00:01:28,030 --> 00:01:30,800 So, if the user types anything except n, 36 00:01:30,800 --> 00:01:32,590 we wanna run this code. 37 00:01:32,590 --> 00:01:35,800 So, we no longer need this if statement there. 38 00:01:35,800 --> 00:01:36,680 But what we want to do is, 39 00:01:36,680 --> 00:01:38,210 we want to use a while loop. 40 00:01:38,210 --> 00:01:41,501 So we will say, while the user input is not n, 41 00:01:41,501 --> 00:01:45,020 so while user input is not n, 42 00:01:45,020 --> 00:01:47,833 then we will repeat this code over and over. 43 00:01:49,580 --> 00:01:50,413 What happens here is that we've got 44 00:01:50,413 --> 00:01:52,696 the while keyword, then we've got a space, 45 00:01:52,696 --> 00:01:54,650 and then we have our condition. 46 00:01:54,650 --> 00:01:57,200 More or less likely if statement we have earlier on 47 00:01:57,200 --> 00:02:01,950 but now this code here, makes the indented code 48 00:02:01,950 --> 00:02:04,430 run over and over. 49 00:02:04,430 --> 00:02:06,350 And what happens is, it starts up at the top 50 00:02:06,350 --> 00:02:07,580 and it goes to the bottom, 51 00:02:07,580 --> 00:02:08,720 and then when it reaches the end 52 00:02:08,720 --> 00:02:10,560 of this block that's indented, 53 00:02:10,560 --> 00:02:14,410 it goes back to the top and evaluates this Boolean again. 54 00:02:14,410 --> 00:02:16,730 So, if this Boolean is true, 55 00:02:16,730 --> 00:02:18,030 then it will run this again, 56 00:02:18,030 --> 00:02:19,300 and then it will go back to the top, 57 00:02:19,300 --> 00:02:21,400 and evaluate the Boolean again, and so on. 58 00:02:22,340 --> 00:02:25,220 Of course, the first time that Python reaches line four, 59 00:02:25,220 --> 00:02:27,040 this Boolean is also evaluated. 60 00:02:27,040 --> 00:02:30,620 So, potentially the user could enter n to begin with 61 00:02:30,620 --> 00:02:33,440 and then this loop would not run at all. 62 00:02:33,440 --> 00:02:36,023 So, that is how the y loop works. 63 00:02:37,620 --> 00:02:40,800 Now, of course if the user enters y, 64 00:02:40,800 --> 00:02:42,676 then y would not be equal to n, 65 00:02:42,676 --> 00:02:45,280 that's true, so this would run. 66 00:02:45,280 --> 00:02:47,080 And then we we'll go back to the top, 67 00:02:47,080 --> 00:02:49,090 and this would still be y. 68 00:02:49,090 --> 00:02:50,450 And it still wouldn't be equal to n, 69 00:02:50,450 --> 00:02:51,700 so this would run again. 70 00:02:51,700 --> 00:02:53,520 And so on, forever. 71 00:02:53,520 --> 00:02:58,040 If you don't change the condition here, then you're gonna 72 00:02:58,040 --> 00:02:59,730 end up in an infinite loop. 73 00:02:59,730 --> 00:03:01,800 So, what we have to do to begin with is we have to ask 74 00:03:01,800 --> 00:03:04,330 the user whether they wanna play again. 75 00:03:04,330 --> 00:03:05,300 So, what we'll do is, will we copy 76 00:03:05,300 --> 00:03:07,363 this line here just for now. 77 00:03:08,590 --> 00:03:10,980 And we will put it inside the loop, 78 00:03:10,980 --> 00:03:13,640 but not inside the else. 79 00:03:13,640 --> 00:03:15,930 So, just down here in the same indentation level 80 00:03:15,930 --> 00:03:17,080 as all the other lines, 81 00:03:17,080 --> 00:03:18,370 we ask the user whether they want to play again. 82 00:03:18,370 --> 00:03:20,980 And what that means is that when we go back 83 00:03:20,980 --> 00:03:22,855 to reevaluate this condition, 84 00:03:22,855 --> 00:03:25,116 the user can potentially affect that 85 00:03:25,116 --> 00:03:27,493 and potentially terminate the loop. 86 00:03:28,660 --> 00:03:32,000 However, there is a small issue in here, 87 00:03:32,000 --> 00:03:34,540 which is that you have these two lines duplicated. 88 00:03:34,540 --> 00:03:36,130 So, if you ever wanna change your prompt 89 00:03:36,130 --> 00:03:37,870 for example you're gonna have to change it in both 90 00:03:37,870 --> 00:03:40,713 places and that can be a little bit cumbersome. 91 00:03:41,670 --> 00:03:43,230 So, instead what we're gonna do is, 92 00:03:43,230 --> 00:03:45,349 we're going to get rid of it and introduce 93 00:03:45,349 --> 00:03:48,972 another Python keyword that allows us to do that. 94 00:03:48,972 --> 00:03:52,230 Here, I'm going to change this so it no longer 95 00:03:52,230 --> 00:03:54,030 uses the condition. 96 00:03:54,030 --> 00:03:56,595 Instead I'm going to do while true. 97 00:03:56,595 --> 00:04:00,200 This always creates an infinite loop. 98 00:04:00,200 --> 00:04:02,260 So, now this code will run and then we will 99 00:04:02,260 --> 00:04:03,150 go back to the top. 100 00:04:03,150 --> 00:04:05,230 This is still true, so it runs again. 101 00:04:05,230 --> 00:04:06,971 And so on, forever. 102 00:04:06,971 --> 00:04:09,980 If you use while true, 103 00:04:09,980 --> 00:04:13,160 then you must be able to terminate the loop 104 00:04:13,160 --> 00:04:16,500 from within the loop's code. 105 00:04:16,500 --> 00:04:17,730 So what I'm going to do, 106 00:04:17,730 --> 00:04:19,770 is I am going to move this line 107 00:04:20,910 --> 00:04:22,453 into the loop. 108 00:04:23,310 --> 00:04:25,400 And then after the line, 109 00:04:25,400 --> 00:04:26,450 I'm going to say, 110 00:04:26,450 --> 00:04:30,180 if user input is equal to n, 111 00:04:30,180 --> 00:04:32,490 and here comes the break keyword. 112 00:04:32,490 --> 00:04:35,940 The break keyword allows us to exit a loop. 113 00:04:35,940 --> 00:04:37,919 So, because we're inside the loop here 114 00:04:37,919 --> 00:04:41,990 we can break out of it and that terminates it and continues 115 00:04:41,990 --> 00:04:44,490 our Python code from the next line after the loop. 116 00:04:45,340 --> 00:04:46,470 So what we've done here is, 117 00:04:46,470 --> 00:04:48,190 within the loop, we ask the user whether 118 00:04:48,190 --> 00:04:51,600 they'd like to play, and if they type no, then we terminate. 119 00:04:51,600 --> 00:04:53,080 And if they type anything else, 120 00:04:53,080 --> 00:04:54,910 then we continue with the loop. 121 00:04:54,910 --> 00:04:56,860 And that happens over and over. 122 00:04:56,860 --> 00:04:59,490 This is an alternative way of doing the loop 123 00:04:59,490 --> 00:05:01,080 that you saw just a moment ago. 124 00:05:01,080 --> 00:05:04,128 You can do either this one does reduce duplication 125 00:05:04,128 --> 00:05:07,280 but it adds an extra if statement inside your loop. 126 00:05:07,280 --> 00:05:09,110 So, its up to you which one you wanna do. 127 00:05:09,110 --> 00:05:12,650 Usually though, I would go for something like this. 128 00:05:12,650 --> 00:05:13,650 That's everything I wanted to show 129 00:05:13,650 --> 00:05:15,530 you regarding the y loop. 130 00:05:15,530 --> 00:05:17,963 So, let's take a look at the For loop. 131 00:05:19,160 --> 00:05:21,700 Let's say you've got a list of your friends 132 00:05:21,700 --> 00:05:23,260 Rolf, Jen, Bob, and Anne. 133 00:05:23,260 --> 00:05:27,050 And you want to print out a simple stream for each friend. 134 00:05:27,050 --> 00:05:29,560 For example, you want to print out Rolf is my friend. 135 00:05:29,560 --> 00:05:30,393 Jen is my friend. 136 00:05:30,393 --> 00:05:31,226 Bob is my friend. 137 00:05:31,226 --> 00:05:32,061 Anne is my friend. 138 00:05:32,061 --> 00:05:35,510 The simple way to do that without loops, 139 00:05:35,510 --> 00:05:37,510 is of course to do something like print, 140 00:05:38,790 --> 00:05:41,633 friend zero is my friend. 141 00:05:42,567 --> 00:05:46,120 And then repeat that for one, two, and three. 142 00:05:46,120 --> 00:05:49,050 Obviously, as the list of friends grows, 143 00:05:49,050 --> 00:05:51,090 these get increasingly difficult. 144 00:05:51,090 --> 00:05:54,320 Plus, if you have this list as part of a programme, 145 00:05:54,320 --> 00:05:56,640 and you wanna be able to print out that all 146 00:05:56,640 --> 00:05:58,265 your friends, are your friends, 147 00:05:58,265 --> 00:06:00,210 you're not gonna be able to do that without 148 00:06:00,210 --> 00:06:01,640 changing the code. 149 00:06:01,640 --> 00:06:04,220 So, here's where the loop comes in handy. 150 00:06:04,220 --> 00:06:06,856 We're going to repeat this structure 151 00:06:06,856 --> 00:06:09,640 for each of your friends. 152 00:06:09,640 --> 00:06:11,000 So, what we'll do is we will say, 153 00:06:11,000 --> 00:06:13,915 for friend in friends. 154 00:06:13,915 --> 00:06:16,907 And then we will indent this piece of code. 155 00:06:16,907 --> 00:06:20,398 And here we will use the variable of friend. 156 00:06:20,398 --> 00:06:22,220 How the For loop works is, 157 00:06:22,220 --> 00:06:24,130 you have the for keyword, 158 00:06:24,130 --> 00:06:25,580 then you have your space, 159 00:06:25,580 --> 00:06:28,540 then you define a variable 160 00:06:28,540 --> 00:06:33,170 that will take the first value of this list. 161 00:06:33,170 --> 00:06:34,950 So, Rolf. 162 00:06:34,950 --> 00:06:37,440 And then you have in friends. 163 00:06:37,440 --> 00:06:39,600 That's the list that you wanna go over. 164 00:06:39,600 --> 00:06:43,500 And what this allows you to do, is to use this variable 165 00:06:43,500 --> 00:06:47,273 and make it equal to the first value in this list, 166 00:06:47,273 --> 00:06:49,417 and then run this code. 167 00:06:49,417 --> 00:06:53,040 When you reach the end of the indented code, it goes 168 00:06:53,040 --> 00:06:57,180 back to the top, and friend takes the second value. 169 00:06:57,180 --> 00:06:58,930 And then it runs this code and it then it goes 170 00:06:58,930 --> 00:07:00,100 right to the top, and friend takes 171 00:07:00,100 --> 00:07:01,630 the third value and so on. 172 00:07:01,630 --> 00:07:03,270 When you reach the end, 173 00:07:03,270 --> 00:07:06,300 the For loop structure knows that you are done. 174 00:07:06,300 --> 00:07:07,770 And then it continues running the Python 175 00:07:07,770 --> 00:07:10,200 code from the next line onwards. 176 00:07:10,200 --> 00:07:13,010 So, this is great when you wanna do something for each 177 00:07:13,010 --> 00:07:16,610 element in a list, or a tuple, or a set and so on. 178 00:07:16,610 --> 00:07:18,610 And also when you wanna repeat something 179 00:07:18,610 --> 00:07:20,373 a certain number of times. 180 00:07:21,700 --> 00:07:23,380 For example, here you know you have four 181 00:07:23,380 --> 00:07:25,110 elements in this list. 182 00:07:25,110 --> 00:07:28,480 So therefore this loop will run four times. 183 00:07:28,480 --> 00:07:31,100 If you wanted to run a loop four times, 184 00:07:31,100 --> 00:07:34,550 and you didn't care to create a list of four elements, 185 00:07:34,550 --> 00:07:37,670 you can actually do for variable 186 00:07:37,670 --> 00:07:40,730 in range four. 187 00:07:40,730 --> 00:07:43,990 And that is also going to allow you to repeat 188 00:07:43,990 --> 00:07:45,780 this code four times. 189 00:07:45,780 --> 00:07:49,504 Of course the friend variable still is used. 190 00:07:49,504 --> 00:07:51,340 So, if you run this code, you'll actually 191 00:07:51,340 --> 00:07:53,303 see that you'd get back. 192 00:07:54,170 --> 00:07:55,210 Zero is my friend. 193 00:07:55,210 --> 00:07:56,043 One is my friend. 194 00:07:56,043 --> 00:07:56,876 Two is my friend 195 00:07:56,876 --> 00:07:57,709 Three is my friend. 196 00:07:57,709 --> 00:07:59,740 Because when you do range four, that creates 197 00:07:59,740 --> 00:08:01,400 something like a list 198 00:08:01,400 --> 00:08:03,290 where the first element is zero, 199 00:08:03,290 --> 00:08:04,123 the second one is one, 200 00:08:04,123 --> 00:08:05,000 and so on. 201 00:08:05,000 --> 00:08:07,210 This is a construct that allows you to iterate 202 00:08:07,210 --> 00:08:11,560 over a list of numbers so that you can repeat the code 203 00:08:11,560 --> 00:08:13,550 a certain number of times. 204 00:08:13,550 --> 00:08:14,870 Usually though for a for loop, 205 00:08:14,870 --> 00:08:18,830 you'll be using something like an existing list and do 206 00:08:18,830 --> 00:08:20,230 something with each element. 207 00:08:21,880 --> 00:08:24,640 Let's use a For loop to calculate the average 208 00:08:24,640 --> 00:08:27,770 grade of this class. 209 00:08:27,770 --> 00:08:29,490 So, let's say we've got a list of grades 210 00:08:29,490 --> 00:08:30,323 where the numbers are 211 00:08:30,323 --> 00:08:33,370 35, 67, 98, and two 100s. 212 00:08:33,370 --> 00:08:34,970 And we have a variable total 213 00:08:34,970 --> 00:08:36,330 which is equal to zero. 214 00:08:36,330 --> 00:08:38,980 Then we also have an amount of grades, 215 00:08:38,980 --> 00:08:40,930 which is the length of the grades list 216 00:08:40,930 --> 00:08:42,860 which gives us five. 217 00:08:42,860 --> 00:08:44,750 What we wanna do is use a For loop 218 00:08:44,750 --> 00:08:46,700 to modify this total variable 219 00:08:46,700 --> 00:08:50,260 and add to it each of these grades. 220 00:08:50,260 --> 00:08:51,150 How we do that is we say, 221 00:08:51,150 --> 00:08:55,640 for grade for example in grades, and that creates 222 00:08:55,640 --> 00:08:57,820 a new variable that we're calling grade 223 00:08:57,820 --> 00:08:59,210 although you can call it whatever you want, 224 00:08:59,210 --> 00:09:02,890 and is going to help us go through each of the values 225 00:09:02,890 --> 00:09:04,600 in the grades list. 226 00:09:04,600 --> 00:09:05,433 Then we will say, 227 00:09:05,433 --> 00:09:08,060 total plus equal grade. 228 00:09:08,060 --> 00:09:09,950 And this by the way, I think we've not see it before 229 00:09:09,950 --> 00:09:13,186 but it's the same as doing total equal total plus grade. 230 00:09:13,186 --> 00:09:17,240 So, it adds up the existing total plus the new grade, 231 00:09:17,240 --> 00:09:18,640 and sets it back to total. 232 00:09:18,640 --> 00:09:22,406 You can do that or you can do plus equal is the same thing. 233 00:09:22,406 --> 00:09:25,420 And then, after the For loop, 234 00:09:25,420 --> 00:09:27,620 once we've added them all together, 235 00:09:27,620 --> 00:09:31,610 we can print out the total divided by the amount. 236 00:09:31,610 --> 00:09:33,560 And that's gonna give us the average. 237 00:09:33,560 --> 00:09:35,420 So if we run this piece of code here, 238 00:09:35,420 --> 00:09:37,873 you'll see that the average is 80.0. 239 00:09:40,500 --> 00:09:41,401 Now, say at this point that you don't actually 240 00:09:41,401 --> 00:09:42,967 need a For loop 241 00:09:42,967 --> 00:09:44,580 for this specific piece of code. 242 00:09:44,580 --> 00:09:46,940 And this'll help me introduce another function in Python 243 00:09:46,940 --> 00:09:48,460 which is the Sum function. 244 00:09:48,460 --> 00:09:51,690 Instead of doing all of this, 245 00:09:51,690 --> 00:09:55,860 you can actually say that the total is the sum of grades. 246 00:09:55,860 --> 00:09:59,600 Just as we have Len to calculate the length of a list, 247 00:09:59,600 --> 00:10:01,700 Sum calculates the addition of all 248 00:10:01,700 --> 00:10:03,400 the elements inside a list, 249 00:10:03,400 --> 00:10:05,362 or a tuple, or a set, and so on. 250 00:10:05,362 --> 00:10:07,637 So, here we have the total and the amount 251 00:10:07,637 --> 00:10:09,330 and that's exactly the same thing 252 00:10:09,330 --> 00:10:10,836 without the need for a For loop. 253 00:10:10,836 --> 00:10:14,470 So, you just kind of have to know that this exists 254 00:10:14,470 --> 00:10:15,360 if you wanna use it. 255 00:10:15,360 --> 00:10:17,510 But, my advice would be to pretty 256 00:10:17,510 --> 00:10:20,520 much search for everything in Google or your search 257 00:10:20,520 --> 00:10:21,488 engine of choice. 258 00:10:21,488 --> 00:10:24,110 I would search when I was starting, 259 00:10:24,110 --> 00:10:28,130 for how to add a list of numbers in Python. 260 00:10:28,130 --> 00:10:30,930 And that will give you many answers. 261 00:10:30,930 --> 00:10:33,696 And one of them will teach you about this Sum function. 262 00:10:33,696 --> 00:10:36,230 When you're doing anything else in Python, 263 00:10:36,230 --> 00:10:39,150 usually I would also search for everything. 264 00:10:39,150 --> 00:10:41,150 And that way, you're going to be constantly 265 00:10:41,150 --> 00:10:44,010 absorbing new information and learning 266 00:10:44,010 --> 00:10:45,920 different ways of doing things. 267 00:10:45,920 --> 00:10:48,330 It's very often that programmers will try 268 00:10:48,330 --> 00:10:51,040 a very complicated solution at first 269 00:10:51,040 --> 00:10:53,040 because that's what we can build 270 00:10:53,040 --> 00:10:55,360 based on the building blocks that we already know. 271 00:10:55,360 --> 00:10:57,730 But, there's usually better solutions out there. 272 00:10:57,730 --> 00:10:59,410 And by searching for them, 273 00:10:59,410 --> 00:11:03,050 you're going to be able to absorb that knowledge as I said. 274 00:11:03,050 --> 00:11:04,160 That's it for this video though. 275 00:11:04,160 --> 00:11:05,300 Thank you for joining me. 276 00:11:05,300 --> 00:11:06,950 And I'll see you in the next one.