1 1 00:00:01,590 --> 00:00:03,720 Hi guys and welcome to your first video 2 2 00:00:03,720 --> 00:00:05,740 in this Python refresher. 3 3 00:00:05,740 --> 00:00:08,113 In this video, let's talk about variables. 4 4 00:00:09,180 --> 00:00:13,140 A variable in Python is a name for a value. 5 5 00:00:13,140 --> 00:00:16,200 You can think of them as similar to those in algebra, 6 6 00:00:16,200 --> 00:00:18,580 but there are slight minor differences, 7 7 00:00:18,580 --> 00:00:21,890 that you'll understand as you programme more in Python. 8 8 00:00:21,890 --> 00:00:24,150 Here's how we define a variable. 9 9 00:00:24,150 --> 00:00:27,720 You can define a variable name, for example x, 10 10 00:00:27,720 --> 00:00:31,340 and then you type the equal sign 11 11 00:00:31,340 --> 00:00:35,560 and finally you type the value, for example, 15. 12 12 00:00:35,560 --> 00:00:36,970 And what Python is doing here 13 13 00:00:36,970 --> 00:00:38,450 and this is very important, 14 14 00:00:38,450 --> 00:00:43,420 is it's creating this value that's after the equal first. 15 15 00:00:43,420 --> 00:00:45,260 So here Python sees the value 15 16 16 00:00:45,260 --> 00:00:47,380 and says okay, you're gonna use the value 15, 17 17 00:00:47,380 --> 00:00:50,160 I will create that or store that somewhere 18 18 00:00:50,160 --> 00:00:53,300 and then it's gonna say, and what am I going to 19 19 00:00:53,300 --> 00:00:57,170 refer to that value as, and that's gonna be x. 20 20 00:00:57,170 --> 00:01:00,090 So the right side of this assignment, 21 21 00:01:00,090 --> 00:01:02,080 this equal sign happens first, 22 22 00:01:02,080 --> 00:01:03,963 and the left side happens later. 23 23 00:01:05,210 --> 00:01:08,100 Outside this point, there are few values in Python 24 24 00:01:08,100 --> 00:01:10,400 where Python has already created them 25 25 00:01:10,400 --> 00:01:12,410 before it runs your code. 26 26 00:01:12,410 --> 00:01:15,720 So Python doesn't create these values when you use them, 27 27 00:01:15,720 --> 00:01:18,720 but if you have something like 1,500 28 28 00:01:18,720 --> 00:01:20,460 then Python will do what I just said, 29 29 00:01:20,460 --> 00:01:23,780 it will create this value and store it in its memory 30 30 00:01:23,780 --> 00:01:28,190 and then actually become a name for this value just created. 31 31 00:01:28,190 --> 00:01:29,850 As well as integers, 32 32 00:01:29,850 --> 00:01:31,700 which are whole numbers like we've got here, 33 33 00:01:31,700 --> 00:01:35,790 you can define variables such as 9.99. 34 34 00:01:35,790 --> 00:01:39,340 And here what I've done is I've created the value 9.99 35 35 00:01:39,340 --> 00:01:43,400 which is a float value, or a value with a floating point 36 36 00:01:43,400 --> 00:01:45,950 and then told Python to give that value 37 37 00:01:45,950 --> 00:01:48,130 the name of price. 38 38 00:01:48,130 --> 00:01:50,170 Let's say we've got a variable called discount 39 39 00:01:50,170 --> 00:01:51,660 that equal to 0.2. 40 40 00:01:51,660 --> 00:01:55,130 We can then say that the final result of our price 41 41 00:01:55,130 --> 00:01:56,790 when the discount is applied 42 42 00:01:56,790 --> 00:02:01,310 would be the price multiplied by one minus the discount. 43 43 00:02:01,310 --> 00:02:03,430 And here's what happens when you execute this. 44 44 00:02:03,430 --> 00:02:05,650 Of course following the rules of mathematics, 45 45 00:02:05,650 --> 00:02:08,080 which Python does, this gets evaluated first, 46 46 00:02:08,080 --> 00:02:12,833 since it's between brackets and you get 0.8, one minus 0.2. 47 47 00:02:13,762 --> 00:02:17,820 Then you're gonna multiply 9.99 by 0.8 48 48 00:02:17,820 --> 00:02:19,720 and that value will get calculated 49 49 00:02:19,720 --> 00:02:22,040 and stored in Python's memory. 50 50 00:02:22,040 --> 00:02:25,133 Whatever that is, result will be the name for that value 51 51 00:02:25,133 --> 00:02:28,280 that it will refer to from here. 52 52 00:02:28,280 --> 00:02:30,130 If you wanted to print a value out 53 53 00:02:30,130 --> 00:02:31,340 and show it to the user, 54 54 00:02:31,340 --> 00:02:34,050 you can very easily do that with the print function. 55 55 00:02:34,050 --> 00:02:35,930 So here we will type print 56 56 00:02:35,930 --> 00:02:39,103 and then inside brackets we will type result. 57 57 00:02:40,240 --> 00:02:44,800 The brackets in here are not the same as they are out here. 58 58 00:02:44,800 --> 00:02:47,180 Here they were used for order of preference 59 59 00:02:47,180 --> 00:02:48,520 in mathematical expressions 60 60 00:02:48,520 --> 00:02:52,580 and here they're used to signal to the print function 61 61 00:02:52,580 --> 00:02:53,910 what we want to print. 62 62 00:02:53,910 --> 00:02:56,670 And this is fairly common in Python. 63 63 00:02:56,670 --> 00:03:00,780 Whenever you see a name, such as print, 64 64 00:03:00,780 --> 00:03:03,320 followed by these brackets, 65 65 00:03:03,320 --> 00:03:04,790 what you've got is a function. 66 66 00:03:04,790 --> 00:03:06,930 And a function in Python is something that 67 67 00:03:06,930 --> 00:03:10,430 performs an action such as showing something to the user, 68 68 00:03:10,430 --> 00:03:14,830 or it calculates an output, based on some inputs. 69 69 00:03:14,830 --> 00:03:16,070 Or it can do both. 70 70 00:03:16,070 --> 00:03:18,890 We're gonna look more at functions in a coming video. 71 71 00:03:18,890 --> 00:03:20,850 But this is how you'd show the user 72 72 00:03:20,850 --> 00:03:23,370 something in the console. 73 73 00:03:23,370 --> 00:03:25,060 So at this point, I'm gonna save this file 74 74 00:03:25,060 --> 00:03:26,293 and I'm going to run it. 75 75 00:03:27,550 --> 00:03:31,680 And you can see that the output is 7.992. 76 76 00:03:31,680 --> 00:03:34,600 So this output here is what this line eight, 77 77 00:03:34,600 --> 00:03:36,780 this print function will calculate. 78 78 00:03:36,780 --> 00:03:39,060 Notice that we've got a number of other variables up here 79 79 00:03:39,060 --> 00:03:40,780 but we are not printing them out, 80 80 00:03:40,780 --> 00:03:42,593 so they will never be shown. 81 81 00:03:44,070 --> 00:03:46,280 As well as integers and floats, 82 82 00:03:46,280 --> 00:03:49,682 Python has another data type called the string. 83 83 00:03:49,682 --> 00:03:52,590 Integers and floats are useful in mathematics 84 84 00:03:52,590 --> 00:03:53,990 or for calculating things, 85 85 00:03:53,990 --> 00:03:56,780 whereas strings are used to store characters, 86 86 00:03:56,780 --> 00:04:00,120 for example a person's name, a person's data of birth, 87 87 00:04:00,120 --> 00:04:01,960 a person's phone number, so on. 88 88 00:04:01,960 --> 00:04:04,430 Even though some of them do contain numbers, 89 89 00:04:04,430 --> 00:04:06,693 they're not gonna be used for mathematics, 90 90 00:04:06,693 --> 00:04:09,930 so we can store them in each string, each set. 91 91 00:04:09,930 --> 00:04:13,490 Let's say that we wanted the person's name to be stored. 92 92 00:04:13,490 --> 00:04:15,470 What we will do is we will same name equal, 93 93 00:04:15,470 --> 00:04:18,670 and then inside quotation marks we will type Rolf. 94 94 00:04:18,670 --> 00:04:20,840 What Python does is it creates this string first 95 95 00:04:20,840 --> 00:04:25,263 and then this name here because a name for that string. 96 96 00:04:26,430 --> 00:04:28,820 Notice that we can reassign a name, 97 97 00:04:28,820 --> 00:04:31,890 so that we can do something like this if you'd like. 98 98 00:04:31,890 --> 00:04:35,270 And now name, instead of being a name for Rolf, 99 99 00:04:35,270 --> 00:04:37,960 will be a name for Bob. 100 100 00:04:37,960 --> 00:04:40,370 So no two variables are created here, 101 101 00:04:40,370 --> 00:04:41,660 it's just the same variable, 102 102 00:04:41,660 --> 00:04:43,580 but we have essentially moved it. 103 103 00:04:43,580 --> 00:04:45,830 You can of course print this out if you want. 104 104 00:04:46,710 --> 00:04:48,560 And I will save this file and run it. 105 105 00:04:49,710 --> 00:04:52,350 And you'll see down here that we get Rolf printed out 106 106 00:04:52,350 --> 00:04:55,080 because that is the contents of our string. 107 107 00:04:55,080 --> 00:04:57,160 Note that Python uses the quotation marks 108 108 00:04:57,160 --> 00:04:58,810 to signal a string, 109 109 00:04:58,810 --> 00:05:00,690 but these quotations marks themselves 110 110 00:05:00,690 --> 00:05:02,460 are not part of the contents, 111 111 00:05:02,460 --> 00:05:04,163 they're just used as delimiters. 112 112 00:05:05,140 --> 00:05:06,530 Instead of double quotation marks, 113 113 00:05:06,530 --> 00:05:08,750 you can use single quotation marks if you like 114 114 00:05:08,750 --> 00:05:10,580 and that is totally fine as long as 115 115 00:05:10,580 --> 00:05:12,390 you don't mix and match them. 116 116 00:05:12,390 --> 00:05:13,550 If you mix and match them, 117 117 00:05:13,550 --> 00:05:16,400 then that won't work and Python will complain. 118 118 00:05:16,400 --> 00:05:17,630 I recommend you always stick to 119 119 00:05:17,630 --> 00:05:19,800 one same type of quotation marks, 120 120 00:05:19,800 --> 00:05:22,053 and I generally use double quotation marks. 121 121 00:05:23,640 --> 00:05:27,560 What happens if we print the name multiplied by two, 122 122 00:05:27,560 --> 00:05:30,630 which is how you would multiply any number? 123 123 00:05:30,630 --> 00:05:33,440 When you run this you'll See that the output is now Rolf, 124 124 00:05:33,440 --> 00:05:35,090 because that's line three, 125 125 00:05:35,090 --> 00:05:37,730 and then you get RolfRolf for line four. 126 126 00:05:37,730 --> 00:05:40,160 What's happened here is this has essentially done 127 127 00:05:40,160 --> 00:05:44,220 name plus name, it has added two names together. 128 128 00:05:44,220 --> 00:05:46,550 That's the same meaning as the multiplication. 129 129 00:05:46,550 --> 00:05:48,370 And when you add two strings together 130 130 00:05:48,370 --> 00:05:50,500 what Python does is it will join them 131 131 00:05:50,500 --> 00:05:52,870 and it will add one after another 132 132 00:05:52,870 --> 00:05:55,010 so you can end up with a longer string. 133 133 00:05:55,010 --> 00:05:56,520 This can be very useful at times, 134 134 00:05:56,520 --> 00:05:58,020 but normally is not what you want 135 135 00:05:58,020 --> 00:06:00,813 when you multiply a string by a number. 136 136 00:06:01,740 --> 00:06:03,260 Let's say you have two variables, 137 137 00:06:03,260 --> 00:06:06,620 a equals 25 and b equals eight. 138 138 00:06:06,620 --> 00:06:09,050 I mentioned earlier that 25 is special value, 139 139 00:06:09,050 --> 00:06:11,940 because it is under 255, 140 140 00:06:11,940 --> 00:06:15,390 so Python creates it as soon as Python starts up. 141 141 00:06:15,390 --> 00:06:17,710 If it was a larger value, then at this point, 142 142 00:06:17,710 --> 00:06:19,710 Python would create it. 143 143 00:06:19,710 --> 00:06:22,200 So we get the value 25 and we say that 144 144 00:06:22,200 --> 00:06:25,380 a is a name for that value. 145 145 00:06:25,380 --> 00:06:27,280 From here on we can refer to a, 146 146 00:06:27,280 --> 00:06:30,280 and really we'll be referring to the value 25. 147 147 00:06:30,280 --> 00:06:33,370 So what happens when b equals a? 148 148 00:06:33,370 --> 00:06:35,390 Well Python is gonna evaluate a 149 149 00:06:35,390 --> 00:06:38,120 and is gonna say okay, a is a name for 25, 150 150 00:06:38,120 --> 00:06:40,610 so we're gonna use 25 here instead. 151 151 00:06:40,610 --> 00:06:42,793 So this means the same thing as this. 152 152 00:06:44,660 --> 00:06:47,760 And then b is now a name for a, 153 153 00:06:47,760 --> 00:06:51,000 which really means that b is a name for 25. 154 154 00:06:51,000 --> 00:06:55,360 So both a and b are names for the number 25. 155 155 00:06:55,360 --> 00:06:56,990 So you can verify that that's correct 156 156 00:06:56,990 --> 00:06:58,480 by printing them both out. 157 157 00:06:58,480 --> 00:07:00,940 So I'm gonna print a and then print b, 158 158 00:07:00,940 --> 00:07:03,370 and you'll see that we get 25 and then 25, 159 159 00:07:03,370 --> 00:07:04,630 which is perfect. 160 160 00:07:04,630 --> 00:07:07,763 But what happens if we do b equal 17? 161 161 00:07:08,670 --> 00:07:11,243 Should a b 17 as well? 162 162 00:07:12,260 --> 00:07:13,470 Well let's run that. 163 163 00:07:13,470 --> 00:07:18,470 And now down here you get 25 and 25 for lines four and five 164 164 00:07:19,400 --> 00:07:24,170 and then you get 25 and 17 for lines nine and 10. 165 165 00:07:24,170 --> 00:07:25,770 What we've done here in line seven 166 166 00:07:25,770 --> 00:07:27,670 when we said b equals 17, 167 167 00:07:27,670 --> 00:07:30,960 is we've told Python to evaluate the value 17, 168 168 00:07:30,960 --> 00:07:34,920 and then make sure that b is a name for 17. 169 169 00:07:34,920 --> 00:07:36,930 Python completely disregards that b 170 170 00:07:36,930 --> 00:07:38,870 was a name for something else before 171 171 00:07:38,870 --> 00:07:43,160 and it just says okay, this b thing is a name for 17. 172 172 00:07:43,160 --> 00:07:46,060 A is completely unchanged. 173 173 00:07:46,060 --> 00:07:48,690 The assignment expression when used like this, 174 174 00:07:48,690 --> 00:07:50,780 does not change other variables, 175 175 00:07:50,780 --> 00:07:54,170 it just changes the variable that you refer to. 176 176 00:07:54,170 --> 00:07:59,050 There are ways for us to change a by using b, 177 177 00:07:59,050 --> 00:08:01,730 but not when we're talking about integers. 178 178 00:08:01,730 --> 00:08:04,090 We will talk about that later on in the videos 179 179 00:08:04,090 --> 00:08:06,740 when we learn about mutability in the manage. 180 180 00:08:06,740 --> 00:08:09,950 All right, that's it for this Python refresher on variables. 181 181 00:08:09,950 --> 00:08:13,340 Do remember that this is not a Python introduction course, 182 182 00:08:13,340 --> 00:08:16,410 because we've talked about some quite advanced things here, 183 183 00:08:16,410 --> 00:08:19,130 but it's just to get you back up to speed with Python 184 184 00:08:19,130 --> 00:08:21,010 in case you have experience from before, 185 185 00:08:21,010 --> 00:08:23,030 or with other programming languages. 186 186 00:08:23,030 --> 00:08:24,380 Thank you for joining me in this video 187 187 00:08:24,380 --> 00:08:26,030 and I'll see you in the next one.