WEBVTT 00:00.110 --> 00:04.910 In this lesson we are going to work on loops and let's start with a problem. 00:04.910 --> 00:08.540 So imagine that we want to print something ten times. 00:08.570 --> 00:11.630 Let's say I have my print hello. 00:11.870 --> 00:14.420 And I want to do that ten times. 00:14.420 --> 00:20.240 So the first uh, kind of obvious solution is you would just write this ten times. 00:20.240 --> 00:24.110 So here just copy and paste ten times, okay. 00:24.170 --> 00:25.850 Eight, nine and ten. 00:26.090 --> 00:33.860 If I run this well we expect to have hello printed ten times and that's what we have. 00:33.860 --> 00:35.060 So this works. 00:35.060 --> 00:37.550 But it's very far from being optimal okay. 00:37.580 --> 00:41.150 Writing the same code many times is really not efficient. 00:41.150 --> 00:44.210 And it's quite a bad practice when programming okay. 00:44.240 --> 00:46.970 The best is not to repeat yourself too much. 00:46.970 --> 00:49.490 And also, what about if we want to print hello? 00:49.490 --> 00:53.570 Not ten times, but 100 times or 1000 times? 00:53.600 --> 00:56.570 Okay, you're not going to write this 1000 times. 00:56.570 --> 01:03.720 In this case you are going to use Loops, so a loop will be used to repeat a given block of code. 01:03.750 --> 01:05.040 Here, for example, the print. 01:05.070 --> 01:05.730 Hello. 01:05.760 --> 01:11.760 You only need to write the block of code once, and then it will be repeated as many times as it needs 01:11.760 --> 01:12.000 to. 01:12.030 --> 01:14.850 So I'm going to remove all of this for now. 01:14.850 --> 01:17.760 And there are two main ways to write loops. 01:17.760 --> 01:21.000 You can use a for loop and use a while loop. 01:21.000 --> 01:24.120 I'm going to show you both here in this lesson. 01:24.120 --> 01:25.830 And let's start with the for loop. 01:25.830 --> 01:30.300 So I'm going to write a very simple for loop and then explain to you what it does. 01:30.330 --> 01:32.820 Okay I'm going to start with the for keyword. 01:32.820 --> 01:36.690 You can see we have the syntax highlighting for the for keyword. 01:36.720 --> 01:41.070 And then I'm going to put I in range okay. 01:41.100 --> 01:48.180 Open parentheses between zero comma and ten okay I put the colon I go back to a new line. 01:48.180 --> 01:50.310 We have an indentation and I print. 01:50.790 --> 01:51.660 Hello. 01:52.350 --> 01:55.500 So that's the block of code that I want to execute. 01:55.530 --> 01:58.170 Let's just run this to see. 01:58.350 --> 02:00.330 And we have you can see. 02:00.360 --> 02:00.750 Hello. 02:00.750 --> 02:02.130 Printed ten times. 02:02.160 --> 02:02.310 Okay. 02:02.340 --> 02:05.160 So that's the exact same behavior as the previous one. 02:05.190 --> 02:07.620 Now let me explain what I did here. 02:07.620 --> 02:11.280 So first this line we have the for keyword okay. 02:11.310 --> 02:13.110 That's going to create a for loop. 02:13.230 --> 02:16.680 Then we have I here I is a variable. 02:16.680 --> 02:17.730 It's an index. 02:17.730 --> 02:22.650 So you create a local variable in this for loop usually for basic for loops. 02:22.680 --> 02:25.680 Or when you want to go through an index or a counter. 02:25.680 --> 02:27.840 We just use I as a convention okay. 02:27.840 --> 02:30.990 You could use any uh, variable okay. 02:31.020 --> 02:31.920 It doesn't really matter. 02:31.920 --> 02:35.730 But we use I then you have the in keyword okay. 02:35.730 --> 02:39.810 So for I in and here usually you will have a list. 02:39.840 --> 02:42.300 We're going to see list in the next lesson. 02:42.300 --> 02:46.800 So list or for example here a range that's the same thing. 02:46.800 --> 02:50.460 So this range it just means that it's going to be between zero. 02:50.460 --> 02:54.870 And so this value and then that value minus one okay. 02:54.900 --> 02:58.750 So 0123456789. 02:58.750 --> 02:59.350 And that's it. 02:59.350 --> 03:01.720 So we have ten values between 0 and 9. 03:01.720 --> 03:03.190 Then we have a colon. 03:03.190 --> 03:05.800 And that's it for the first line of the file. 03:05.830 --> 03:06.070 Okay. 03:06.100 --> 03:08.260 So with this we have the definition of the for loop. 03:08.260 --> 03:14.080 And inside you can see with an indentation we have the code that we want to repeat. 03:14.080 --> 03:18.970 So what's going to happen is that this code is going to be repeated as many times as we have. 03:18.970 --> 03:24.070 Well asked here which is basically a range between 0 and 10 minus one. 03:24.070 --> 03:25.540 So we have ten values total. 03:25.570 --> 03:27.820 Just one thing here I printed ten times. 03:27.820 --> 03:30.130 But let's come back to our previous problem. 03:30.130 --> 03:36.250 If I want to print it a thousand times, I just write 1000 here and I'm going to run this. 03:36.280 --> 03:43.660 You can see it's taking a bit more time, because executing this 1000 times is not going to be instant. 03:43.660 --> 03:44.590 And now we are done. 03:44.590 --> 03:50.140 We have hello printed 1000 times okay, so you could count but no need. 03:50.380 --> 03:54.370 Let's go back to ten and I'm going to talk about the indentation here okay. 03:54.400 --> 03:55.760 You can see I have an indentation. 03:55.760 --> 03:58.100 So the indentation is very important. 03:58.130 --> 04:04.070 Now you can see if I go back to a new line I am still with an indentation, which means I am still in 04:04.100 --> 04:04.790 the for loop. 04:04.790 --> 04:09.680 I need to go back here to the previous indentation so that I am out of the for loop. 04:09.710 --> 04:15.860 Let's just add a print end like this and let's run it. 04:15.860 --> 04:20.060 And you can see we have hello ten times and then end. 04:20.450 --> 04:20.780 All right. 04:20.810 --> 04:23.870 So the end is executed after the for loop. 04:23.900 --> 04:24.050 Okay. 04:24.080 --> 04:27.680 So you can see the for loop has a loop going through this block of code. 04:27.680 --> 04:29.780 And after ten times it's going out. 04:29.780 --> 04:31.250 And then we print end. 04:31.280 --> 04:37.610 If I put end like this you can see with an indentation even if we have an empty line here. 04:37.640 --> 04:37.820 Okay. 04:37.850 --> 04:38.570 Empty lines. 04:38.570 --> 04:39.710 They don't do anything. 04:39.740 --> 04:41.150 Look at what's going to happen. 04:41.150 --> 04:45.830 You can see we have hello and hello and printed ten times okay. 04:45.860 --> 04:50.390 Because this print is in the for loop because of the indentation. 04:50.390 --> 04:53.790 If I remove the indentation the print is out of the loop. 04:53.790 --> 04:55.770 So that's very important detail. 04:55.800 --> 04:56.070 Okay. 04:56.100 --> 04:59.550 Now let's come back to this AI variable that I created. 04:59.550 --> 05:05.610 I told you that we create a local variable inside the for loop that's going to go through this range. 05:05.610 --> 05:07.290 But we can actually print this. 05:07.290 --> 05:08.640 So we can say hello. 05:08.640 --> 05:13.200 And then plus and because it's going to be an integer number. 05:13.200 --> 05:17.340 And now you know that we cannot concatenate a string with an integer number. 05:17.340 --> 05:20.490 We need to cast this as a string just like that. 05:20.520 --> 05:23.640 Make sure you add the correct amount of parentheses. 05:23.640 --> 05:25.560 So I'm going to run this. 05:26.100 --> 05:29.730 And now you can see we have hello zero. 05:29.730 --> 05:34.920 And then hello 12345678 and nine. 05:34.950 --> 05:38.760 I can add a space if I want to to make it a bit cleaner. 05:39.510 --> 05:40.020 Okay. 05:40.050 --> 05:43.680 You can see here clearly that we are going between 0 and 9. 05:43.680 --> 05:45.330 So we have ten values total. 05:45.360 --> 05:45.690 All right. 05:45.690 --> 05:48.120 So that's how a for loop works. 05:48.150 --> 05:52.620 Now there is another way to do what we did here using the while loop. 05:52.620 --> 05:53.940 So let's see how that works. 05:53.940 --> 05:57.420 I'm going to comment this code here. 05:58.830 --> 06:01.230 So right click and toggle comment. 06:01.650 --> 06:07.020 And I'm going to write a while loop I will keep this print n here. 06:07.020 --> 06:09.210 And let's do the same with a while loop. 06:09.210 --> 06:12.030 So first let me write the code and then I will explain it to you. 06:12.030 --> 06:15.690 So here I'm going to create a variable I is equal to zero. 06:15.690 --> 06:20.670 And then I will do while I is strictly lower than ten. 06:21.060 --> 06:27.930 And then colon go back to a new line and we execute a block of code, which is this print that I'm just 06:27.930 --> 06:30.090 going to copy here is the same. 06:30.390 --> 06:32.880 So I copy and paste it right there. 06:33.000 --> 06:38.850 And after that I will do I is equal to I plus one. 06:38.850 --> 06:43.560 So while I is lower than ten strictly lower than ten we print hello with the I. 06:43.560 --> 06:45.600 And then we add one to I. 06:45.600 --> 06:48.000 And this is going to be run ten times. 06:48.000 --> 06:50.320 Let's actually see Okay. 06:50.320 --> 06:52.090 We have the exact same, you see. 06:52.120 --> 06:52.360 Hello. 06:52.390 --> 06:53.140 Zero until. 06:53.170 --> 06:53.980 Hello, nine. 06:53.980 --> 06:57.460 So this block of code is doing the same thing as that one. 06:57.460 --> 07:00.820 Now let's see in detail how this is executed. 07:00.850 --> 07:01.090 Okay. 07:01.120 --> 07:04.510 So the while loop is actually going to test here. 07:04.510 --> 07:07.420 You can see this is a condition okay. 07:07.420 --> 07:11.380 So you can see the while actually has an if. 07:11.380 --> 07:13.150 So it's an if with a loop. 07:13.180 --> 07:17.170 Here you can read this as if I is strictly lower than ten. 07:17.170 --> 07:20.050 So of course here we test a condition with a variable. 07:20.050 --> 07:22.660 So that's why we have created the variable before okay. 07:22.690 --> 07:26.710 In the for loop here you can directly create the variable inside the definition of the for. 07:26.710 --> 07:31.330 But for the while we're going to create the variable before are we going to use another variable from 07:31.330 --> 07:31.930 another place. 07:31.930 --> 07:34.120 But here we create this variable here okay. 07:34.150 --> 07:38.620 So if I is strictly lower than ten we're going to execute this block of code. 07:38.710 --> 07:44.020 Then the difference is that with the if you just execute this once or not and then you go on the next 07:44.020 --> 07:49.040 line here line number eight with a while after we have executed this. 07:49.040 --> 07:54.560 So if this condition is true, we go back to line five and we test the condition again. 07:54.560 --> 07:59.900 And as long as this condition is true, then we keep executing this block of code. 07:59.900 --> 08:02.990 If the condition is false, then we go out of the while. 08:03.020 --> 08:03.260 Okay. 08:03.290 --> 08:06.950 So what's happening exactly we have I is equal to zero. 08:06.980 --> 08:08.450 Then we test is I. 08:08.480 --> 08:10.880 So is zero strictly lower than ten. 08:10.910 --> 08:11.750 Yes. 08:11.750 --> 08:16.610 So we go in the while we print hello zero. 08:16.610 --> 08:21.800 And then we say that I is equal to its current value plus one okay. 08:21.830 --> 08:23.600 So I becomes one. 08:23.630 --> 08:24.650 Then we test again. 08:24.680 --> 08:25.130 Here we go. 08:25.130 --> 08:26.210 Back to line five. 08:26.240 --> 08:28.280 Is one strictly lower than ten. 08:28.310 --> 08:30.290 Yes we do the same thing. 08:30.290 --> 08:32.120 And then I is equal to two. 08:32.150 --> 08:34.430 We go all the way up to nine. 08:34.700 --> 08:37.040 Then we print with nine and then nine. 08:37.040 --> 08:38.570 So I becomes ten. 08:38.600 --> 08:41.720 We go back here is ten strictly lower than ten. 08:41.720 --> 08:42.290 No. 08:42.290 --> 08:46.490 So we go out of the loop and then we print end okay. 08:46.490 --> 08:47.640 So that's how it works. 08:47.640 --> 08:53.610 And a very important thing here is you can see we have to uh, in this case we have to increment the 08:53.610 --> 08:55.110 value ourself, okay. 08:55.140 --> 08:59.970 Because if we don't do this well we're going to have what we call an infinite loop. 08:59.970 --> 09:05.160 And that can be a problem because if you don't let's say comment this line, you see if you don't increment 09:05.190 --> 09:07.950 I then I is always going to be zero. 09:07.950 --> 09:10.350 And so this condition is always going to be true. 09:10.380 --> 09:14.280 And so we are going to execute this an infinite amount of time. 09:14.280 --> 09:16.770 And we are never going to go out of the while loop. 09:16.770 --> 09:21.720 So that's why we add one to I here and here to write this. 09:21.750 --> 09:24.690 Well there is an easier way to write that in Python. 09:24.900 --> 09:31.080 If you just want to add a number to a variable instead of doing variable is equal to variable plus number, 09:31.080 --> 09:35.640 you can just use plus equal like that okay. 09:35.640 --> 09:40.410 If you do I plus equal one just means that you add one to I. 09:40.440 --> 09:40.740 Okay. 09:40.740 --> 09:42.300 So it's a bit better like this. 09:42.330 --> 09:42.720 Great. 09:42.720 --> 09:48.270 And now you have seen the for loop and the while loop, and the question is when to use a while loop 09:48.270 --> 09:50.250 and when to use a for loop. 09:50.280 --> 09:50.430 Okay. 09:50.460 --> 09:55.770 Basically, you will use a while loop when you don't know how many times you need to execute the block 09:55.770 --> 09:56.490 of code. 09:56.520 --> 10:02.070 For example, if you read the temperature from a sensor and you need to keep executing action while 10:02.070 --> 10:07.050 the temperature is below ten degrees, then you will need a while loop because you don't know how long 10:07.050 --> 10:10.110 it will take for the temperature to rise above ten degrees. 10:10.110 --> 10:15.870 So you need to continue until the condition that the temperature is greater than ten is met. 10:15.900 --> 10:17.490 Now another example. 10:17.490 --> 10:23.100 If you need to read the temperature a thousand times and then to compute the average temperature, then 10:23.100 --> 10:27.870 you can use a for loop because you know exactly how many times you need to do the action. 10:27.900 --> 10:29.190 Here it's 1000 times. 10:29.190 --> 10:34.440 And so for the example we used here, because we know exactly how many times we need to print. 10:34.470 --> 10:34.710 Hello. 10:34.740 --> 10:36.180 We want to print hello ten times. 10:36.180 --> 10:39.720 Then you can see that a for loop is a more appropriate choice. 10:39.750 --> 10:44.400 And later in this course we will use both for and while loops for different purposes.