WEBVTT 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.