WEBVTT 1 00:00:01.590 --> 00:00:03.720 Hi guys and welcome to your first video 2 00:00:03.720 --> 00:00:05.740 in this Python refresher. 3 00:00:05.740 --> 00:00:08.113 In this video, let's talk about variables. 4 00:00:09.180 --> 00:00:13.140 A variable in Python is a name for a value. 5 00:00:13.140 --> 00:00:16.200 You can think of them as similar to those in algebra, 6 00:00:16.200 --> 00:00:18.580 but there are slight minor differences, 7 00:00:18.580 --> 00:00:21.890 that you'll understand as you programme more in Python. 8 00:00:21.890 --> 00:00:24.150 Here's how we define a variable. 9 00:00:24.150 --> 00:00:27.720 You can define a variable name, for example x, 10 00:00:27.720 --> 00:00:31.340 and then you type the equal sign 11 00:00:31.340 --> 00:00:35.560 and finally you type the value, for example, 15. 12 00:00:35.560 --> 00:00:36.970 And what Python is doing here 13 00:00:36.970 --> 00:00:38.450 and this is very important, 14 00:00:38.450 --> 00:00:43.420 is it's creating this value that's after the equal first. 15 00:00:43.420 --> 00:00:45.260 So here Python sees the value 15 16 00:00:45.260 --> 00:00:47.380 and says okay, you're gonna use the value 15, 17 00:00:47.380 --> 00:00:50.160 I will create that or store that somewhere 18 00:00:50.160 --> 00:00:53.300 and then it's gonna say, and what am I going to 19 00:00:53.300 --> 00:00:57.170 refer to that value as, and that's gonna be x. 20 00:00:57.170 --> 00:01:00.090 So the right side of this assignment, 21 00:01:00.090 --> 00:01:02.080 this equal sign happens first, 22 00:01:02.080 --> 00:01:03.963 and the left side happens later. 23 00:01:05.210 --> 00:01:08.100 Outside this point, there are few values in Python 24 00:01:08.100 --> 00:01:10.400 where Python has already created them 25 00:01:10.400 --> 00:01:12.410 before it runs your code. 26 00:01:12.410 --> 00:01:15.720 So Python doesn't create these values when you use them, 27 00:01:15.720 --> 00:01:18.720 but if you have something like 1,500 28 00:01:18.720 --> 00:01:20.460 then Python will do what I just said, 29 00:01:20.460 --> 00:01:23.780 it will create this value and store it in its memory 30 00:01:23.780 --> 00:01:28.190 and then actually become a name for this value just created. 31 00:01:28.190 --> 00:01:29.850 As well as integers, 32 00:01:29.850 --> 00:01:31.700 which are whole numbers like we've got here, 33 00:01:31.700 --> 00:01:35.790 you can define variables such as 9.99. 34 00:01:35.790 --> 00:01:39.340 And here what I've done is I've created the value 9.99 35 00:01:39.340 --> 00:01:43.400 which is a float value, or a value with a floating point 36 00:01:43.400 --> 00:01:45.950 and then told Python to give that value 37 00:01:45.950 --> 00:01:48.130 the name of price. 38 00:01:48.130 --> 00:01:50.170 Let's say we've got a variable called discount 39 00:01:50.170 --> 00:01:51.660 that equal to 0.2. 40 00:01:51.660 --> 00:01:55.130 We can then say that the final result of our price 41 00:01:55.130 --> 00:01:56.790 when the discount is applied 42 00:01:56.790 --> 00:02:01.310 would be the price multiplied by one minus the discount. 43 00:02:01.310 --> 00:02:03.430 And here's what happens when you execute this. 44 00:02:03.430 --> 00:02:05.650 Of course following the rules of mathematics, 45 00:02:05.650 --> 00:02:08.080 which Python does, this gets evaluated first, 46 00:02:08.080 --> 00:02:12.833 since it's between brackets and you get 0.8, one minus 0.2. 47 00:02:13.762 --> 00:02:17.820 Then you're gonna multiply 9.99 by 0.8 48 00:02:17.820 --> 00:02:19.720 and that value will get calculated 49 00:02:19.720 --> 00:02:22.040 and stored in Python's memory. 50 00:02:22.040 --> 00:02:25.133 Whatever that is, result will be the name for that value 51 00:02:25.133 --> 00:02:28.280 that it will refer to from here. 52 00:02:28.280 --> 00:02:30.130 If you wanted to print a value out 53 00:02:30.130 --> 00:02:31.340 and show it to the user, 54 00:02:31.340 --> 00:02:34.050 you can very easily do that with the print function. 55 00:02:34.050 --> 00:02:35.930 So here we will type print 56 00:02:35.930 --> 00:02:39.103 and then inside brackets we will type result. 57 00:02:40.240 --> 00:02:44.800 The brackets in here are not the same as they are out here. 58 00:02:44.800 --> 00:02:47.180 Here they were used for order of preference 59 00:02:47.180 --> 00:02:48.520 in mathematical expressions 60 00:02:48.520 --> 00:02:52.580 and here they're used to signal to the print function 61 00:02:52.580 --> 00:02:53.910 what we want to print. 62 00:02:53.910 --> 00:02:56.670 And this is fairly common in Python. 63 00:02:56.670 --> 00:03:00.780 Whenever you see a name, such as print, 64 00:03:00.780 --> 00:03:03.320 followed by these brackets, 65 00:03:03.320 --> 00:03:04.790 what you've got is a function. 66 00:03:04.790 --> 00:03:06.930 And a function in Python is something that 67 00:03:06.930 --> 00:03:10.430 performs an action such as showing something to the user, 68 00:03:10.430 --> 00:03:14.830 or it calculates an output, based on some inputs. 69 00:03:14.830 --> 00:03:16.070 Or it can do both. 70 00:03:16.070 --> 00:03:18.890 We're gonna look more at functions in a coming video. 71 00:03:18.890 --> 00:03:20.850 But this is how you'd show the user 72 00:03:20.850 --> 00:03:23.370 something in the console. 73 00:03:23.370 --> 00:03:25.060 So at this point, I'm gonna save this file 74 00:03:25.060 --> 00:03:26.293 and I'm going to run it. 75 00:03:27.550 --> 00:03:31.680 And you can see that the output is 7.992. 76 00:03:31.680 --> 00:03:34.600 So this output here is what this line eight, 77 00:03:34.600 --> 00:03:36.780 this print function will calculate. 78 00:03:36.780 --> 00:03:39.060 Notice that we've got a number of other variables up here 79 00:03:39.060 --> 00:03:40.780 but we are not printing them out, 80 00:03:40.780 --> 00:03:42.593 so they will never be shown. 81 00:03:44.070 --> 00:03:46.280 As well as integers and floats, 82 00:03:46.280 --> 00:03:49.682 Python has another data type called the string. 83 00:03:49.682 --> 00:03:52.590 Integers and floats are useful in mathematics 84 00:03:52.590 --> 00:03:53.990 or for calculating things, 85 00:03:53.990 --> 00:03:56.780 whereas strings are used to store characters, 86 00:03:56.780 --> 00:04:00.120 for example a person's name, a person's data of birth, 87 00:04:00.120 --> 00:04:01.960 a person's phone number, so on. 88 00:04:01.960 --> 00:04:04.430 Even though some of them do contain numbers, 89 00:04:04.430 --> 00:04:06.693 they're not gonna be used for mathematics, 90 00:04:06.693 --> 00:04:09.930 so we can store them in each string, each set. 91 00:04:09.930 --> 00:04:13.490 Let's say that we wanted the person's name to be stored. 92 00:04:13.490 --> 00:04:15.470 What we will do is we will same name equal, 93 00:04:15.470 --> 00:04:18.670 and then inside quotation marks we will type Rolf. 94 00:04:18.670 --> 00:04:20.840 What Python does is it creates this string first 95 00:04:20.840 --> 00:04:25.263 and then this name here because a name for that string. 96 00:04:26.430 --> 00:04:28.820 Notice that we can reassign a name, 97 00:04:28.820 --> 00:04:31.890 so that we can do something like this if you'd like. 98 00:04:31.890 --> 00:04:35.270 And now name, instead of being a name for Rolf, 99 00:04:35.270 --> 00:04:37.960 will be a name for Bob. 100 00:04:37.960 --> 00:04:40.370 So no two variables are created here, 101 00:04:40.370 --> 00:04:41.660 it's just the same variable, 102 00:04:41.660 --> 00:04:43.580 but we have essentially moved it. 103 00:04:43.580 --> 00:04:45.830 You can of course print this out if you want. 104 00:04:46.710 --> 00:04:48.560 And I will save this file and run it. 105 00:04:49.710 --> 00:04:52.350 And you'll see down here that we get Rolf printed out 106 00:04:52.350 --> 00:04:55.080 because that is the contents of our string. 107 00:04:55.080 --> 00:04:57.160 Note that Python uses the quotation marks 108 00:04:57.160 --> 00:04:58.810 to signal a string, 109 00:04:58.810 --> 00:05:00.690 but these quotations marks themselves 110 00:05:00.690 --> 00:05:02.460 are not part of the contents, 111 00:05:02.460 --> 00:05:04.163 they're just used as delimiters. 112 00:05:05.140 --> 00:05:06.530 Instead of double quotation marks, 113 00:05:06.530 --> 00:05:08.750 you can use single quotation marks if you like 114 00:05:08.750 --> 00:05:10.580 and that is totally fine as long as 115 00:05:10.580 --> 00:05:12.390 you don't mix and match them. 116 00:05:12.390 --> 00:05:13.550 If you mix and match them, 117 00:05:13.550 --> 00:05:16.400 then that won't work and Python will complain. 118 00:05:16.400 --> 00:05:17.630 I recommend you always stick to 119 00:05:17.630 --> 00:05:19.800 one same type of quotation marks, 120 00:05:19.800 --> 00:05:22.053 and I generally use double quotation marks. 121 00:05:23.640 --> 00:05:27.560 What happens if we print the name multiplied by two, 122 00:05:27.560 --> 00:05:30.630 which is how you would multiply any number? 123 00:05:30.630 --> 00:05:33.440 When you run this you'll See that the output is now Rolf, 124 00:05:33.440 --> 00:05:35.090 because that's line three, 125 00:05:35.090 --> 00:05:37.730 and then you get RolfRolf for line four. 126 00:05:37.730 --> 00:05:40.160 What's happened here is this has essentially done 127 00:05:40.160 --> 00:05:44.220 name plus name, it has added two names together. 128 00:05:44.220 --> 00:05:46.550 That's the same meaning as the multiplication. 129 00:05:46.550 --> 00:05:48.370 And when you add two strings together 130 00:05:48.370 --> 00:05:50.500 what Python does is it will join them 131 00:05:50.500 --> 00:05:52.870 and it will add one after another 132 00:05:52.870 --> 00:05:55.010 so you can end up with a longer string. 133 00:05:55.010 --> 00:05:56.520 This can be very useful at times, 134 00:05:56.520 --> 00:05:58.020 but normally is not what you want 135 00:05:58.020 --> 00:06:00.813 when you multiply a string by a number. 136 00:06:01.740 --> 00:06:03.260 Let's say you have two variables, 137 00:06:03.260 --> 00:06:06.620 a equals 25 and b equals eight. 138 00:06:06.620 --> 00:06:09.050 I mentioned earlier that 25 is special value, 139 00:06:09.050 --> 00:06:11.940 because it is under 255, 140 00:06:11.940 --> 00:06:15.390 so Python creates it as soon as Python starts up. 141 00:06:15.390 --> 00:06:17.710 If it was a larger value, then at this point, 142 00:06:17.710 --> 00:06:19.710 Python would create it. 143 00:06:19.710 --> 00:06:22.200 So we get the value 25 and we say that 144 00:06:22.200 --> 00:06:25.380 a is a name for that value. 145 00:06:25.380 --> 00:06:27.280 From here on we can refer to a, 146 00:06:27.280 --> 00:06:30.280 and really we'll be referring to the value 25. 147 00:06:30.280 --> 00:06:33.370 So what happens when b equals a? 148 00:06:33.370 --> 00:06:35.390 Well Python is gonna evaluate a 149 00:06:35.390 --> 00:06:38.120 and is gonna say okay, a is a name for 25, 150 00:06:38.120 --> 00:06:40.610 so we're gonna use 25 here instead. 151 00:06:40.610 --> 00:06:42.793 So this means the same thing as this. 152 00:06:44.660 --> 00:06:47.760 And then b is now a name for a, 153 00:06:47.760 --> 00:06:51.000 which really means that b is a name for 25. 154 00:06:51.000 --> 00:06:55.360 So both a and b are names for the number 25. 155 00:06:55.360 --> 00:06:56.990 So you can verify that that's correct 156 00:06:56.990 --> 00:06:58.480 by printing them both out. 157 00:06:58.480 --> 00:07:00.940 So I'm gonna print a and then print b, 158 00:07:00.940 --> 00:07:03.370 and you'll see that we get 25 and then 25, 159 00:07:03.370 --> 00:07:04.630 which is perfect. 160 00:07:04.630 --> 00:07:07.763 But what happens if we do b equal 17? 161 00:07:08.670 --> 00:07:11.243 Should a b 17 as well? 162 00:07:12.260 --> 00:07:13.470 Well let's run that. 163 00:07:13.470 --> 00:07:18.470 And now down here you get 25 and 25 for lines four and five 164 00:07:19.400 --> 00:07:24.170 and then you get 25 and 17 for lines nine and 10. 165 00:07:24.170 --> 00:07:25.770 What we've done here in line seven 166 00:07:25.770 --> 00:07:27.670 when we said b equals 17, 167 00:07:27.670 --> 00:07:30.960 is we've told Python to evaluate the value 17, 168 00:07:30.960 --> 00:07:34.920 and then make sure that b is a name for 17. 169 00:07:34.920 --> 00:07:36.930 Python completely disregards that b 170 00:07:36.930 --> 00:07:38.870 was a name for something else before 171 00:07:38.870 --> 00:07:43.160 and it just says okay, this b thing is a name for 17. 172 00:07:43.160 --> 00:07:46.060 A is completely unchanged. 173 00:07:46.060 --> 00:07:48.690 The assignment expression when used like this, 174 00:07:48.690 --> 00:07:50.780 does not change other variables, 175 00:07:50.780 --> 00:07:54.170 it just changes the variable that you refer to. 176 00:07:54.170 --> 00:07:59.050 There are ways for us to change a by using b, 177 00:07:59.050 --> 00:08:01.730 but not when we're talking about integers. 178 00:08:01.730 --> 00:08:04.090 We will talk about that later on in the videos 179 00:08:04.090 --> 00:08:06.740 when we learn about mutability in the manage. 180 00:08:06.740 --> 00:08:09.950 All right, that's it for this Python refresher on variables. 181 00:08:09.950 --> 00:08:13.340 Do remember that this is not a Python introduction course, 182 00:08:13.340 --> 00:08:16.410 because we've talked about some quite advanced things here, 183 00:08:16.410 --> 00:08:19.130 but it's just to get you back up to speed with Python 184 00:08:19.130 --> 00:08:21.010 in case you have experience from before, 185 00:08:21.010 --> 00:08:23.030 or with other programming languages. 186 00:08:23.030 --> 00:08:24.380 Thank you for joining me in this video 187 00:08:24.380 --> 00:08:26.030 and I'll see you in the next one.