1 00:00:05,330 --> 00:00:09,100 So in the previous video, we started talking about variables for Python and 2 00:00:09,100 --> 00:00:11,900 thus, we ended talking about operator precedence. 3 00:00:11,900 --> 00:00:16,710 So you saw that by using brackets we can make sure that the calculations we think 4 00:00:16,710 --> 00:00:18,830 work, use the correct order or 5 00:00:18,830 --> 00:00:23,220 actually are processed in the right order to avoid getting us incorrect results. 6 00:00:23,220 --> 00:00:26,660 And that's mainly by putting brackets around each sort of section. 7 00:00:26,660 --> 00:00:29,580 Now there's another way of doing that, that would be easier, and 8 00:00:29,580 --> 00:00:32,140 in some cases you may wanna instead look at something 9 00:00:32,140 --> 00:00:35,330 that's relatively complex, like you can see with that print statement. 10 00:00:35,330 --> 00:00:38,240 You can actually use variables to hold the intermediate values. 11 00:00:38,240 --> 00:00:44,040 So for example, we could say, a variable with the name of c = a + b. 12 00:00:46,130 --> 00:00:49,510 And then we can also do d = c / 3. 13 00:00:49,510 --> 00:00:54,000 And then we could do e = d / 4. 14 00:00:54,000 --> 00:00:56,580 And then we could do print(e * 12). 15 00:00:56,580 --> 00:01:01,190 Now if we actually run that, we should get the same result in both cases. 16 00:01:02,310 --> 00:01:03,690 We get 12 for this line here. 17 00:01:03,690 --> 00:01:07,150 But we also get 12 for this line here. 18 00:01:07,150 --> 00:01:10,340 And that's because we've broken up those individual components, and 19 00:01:10,340 --> 00:01:13,080 assigned them to different variables, to get the same result. 20 00:01:13,080 --> 00:01:17,110 So definitely there's gonna be times where it makes sense to break up an expression 21 00:01:17,110 --> 00:01:18,860 like this into smaller parts. 22 00:01:18,860 --> 00:01:21,720 It's really gonna be up to you to compare the two examples to see which 23 00:01:21,720 --> 00:01:23,010 one works better for you. 24 00:01:23,010 --> 00:01:26,320 Once you start getting used to the concept of brackets, 25 00:01:26,320 --> 00:01:29,850 it's easy to see actually at a glance with that one, as to what's happening 26 00:01:29,850 --> 00:01:33,240 with this one because you're introducing different variables. 27 00:01:33,240 --> 00:01:36,710 It can take a bit of work to look at that because you've actually got more output 28 00:01:36,710 --> 00:01:37,290 to look at. 29 00:01:37,290 --> 00:01:39,840 But the bottom line is there's no right or wrong here. 30 00:01:39,840 --> 00:01:42,810 It's whatever's more appropriate and more readable for you. 31 00:01:42,810 --> 00:01:46,960 So, moving on now, let's actually start talking about another data type and 32 00:01:46,960 --> 00:01:48,930 that's the data type string. 33 00:01:48,930 --> 00:01:52,770 Now you've seen us use strings in earlier examples, to print messages on the screen. 34 00:01:52,770 --> 00:01:55,930 But I want to go through now and show you some of the other cool things you can do 35 00:01:55,930 --> 00:01:57,750 when you're actually working with strings. 36 00:01:57,750 --> 00:02:01,580 So strings are one of Python's what they call sequence data types, 37 00:02:01,580 --> 00:02:03,540 which makes sense because that's what it really is. 38 00:02:03,540 --> 00:02:06,480 It's really a sequence of characters that you can put together 39 00:02:06,480 --> 00:02:08,210 that form an actual string. 40 00:02:08,210 --> 00:02:10,960 So we've seen how to print them out and concatenate strings together. 41 00:02:10,960 --> 00:02:13,730 But we can also pick out individual characters or substrings. 42 00:02:13,730 --> 00:02:17,460 So I'm gonna delete some of this now, and let's just see how that works. 43 00:02:17,460 --> 00:02:21,180 I'm gonna type parrot = "Norwegian Blue". 44 00:02:21,180 --> 00:02:23,090 We can print(parrot). 45 00:02:23,090 --> 00:02:29,180 And no surprises there, if we run that to see what actually happens. 46 00:02:30,590 --> 00:02:37,270 Norwegian Blue, we should expect, we can actually do print(parrot[3]) like so. 47 00:02:39,560 --> 00:02:43,960 If you run that, you can see, what's happened is by doing that it's 48 00:02:43,960 --> 00:02:46,170 actually printed the letter w. 49 00:02:46,170 --> 00:02:49,190 Now you might be looking at that and thinking, "well why is that w?" 50 00:02:49,190 --> 00:02:53,530 because we specified the third character, so shouldn't that be counting from left to 51 00:02:53,530 --> 00:02:57,640 right, N is the first character, o is the second and r is the third. 52 00:02:57,640 --> 00:03:00,190 And in actual fact w is the fourth character. 53 00:03:00,190 --> 00:03:01,270 So why did that work? 54 00:03:01,270 --> 00:03:04,860 It's actually usual for programming languages to start counting at zero, 55 00:03:04,860 --> 00:03:06,260 and not at one. 56 00:03:06,260 --> 00:03:08,000 So that's parrot zero is N. 57 00:03:08,000 --> 00:03:10,290 So we can actually go to confirm that. 58 00:03:12,430 --> 00:03:15,720 Parrot[0], if we actually run that is an N. 59 00:03:15,720 --> 00:03:19,320 So you'll find that in most languages, as I said, they'll actually start counting 60 00:03:19,320 --> 00:03:22,870 from position zero and not from position one as we would normally do as humans. 61 00:03:22,870 --> 00:03:25,720 So there's 14 characters in Norwegian Blue, numbered from zero to 13. 62 00:03:25,720 --> 00:03:30,110 If we try to print(parrot[14]), we get an error, string index out of range. 63 00:03:30,110 --> 00:03:37,680 So if we actually try and do that So 14. 64 00:03:37,680 --> 00:03:38,530 We'll actually get an error. 65 00:03:38,530 --> 00:03:44,180 And that's because as far as it's concerned, there's no character 66 00:03:44,180 --> 00:03:47,640 at that position, because there's only characters in position 0 to 13. 67 00:03:47,640 --> 00:03:50,340 The other thing you can actually do is count backwards. 68 00:03:50,340 --> 00:03:53,070 So, you can type print(parrot[-1]). 69 00:03:53,070 --> 00:03:57,430 And if we actually run that, negative one gives us e. So 70 00:03:57,430 --> 00:04:04,130 what's happened is that it started here at position zero and went to negative one. 71 00:04:04,130 --> 00:04:06,120 Negative one goes all to the way to the end of the string and 72 00:04:06,120 --> 00:04:08,020 starts counting from the end. 73 00:04:08,020 --> 00:04:12,550 So we get the letter e coming up, which is the last character from the string. 74 00:04:12,550 --> 00:04:13,820 And we can work backwards like that. 75 00:04:13,820 --> 00:04:16,839 We can also print what Python calls a slice by, 76 00:04:16,839 --> 00:04:20,740 instead of providing one number, provide two numbers separated by a colon. 77 00:04:20,740 --> 00:04:22,320 So in other words, it's gonna be like a range. 78 00:04:22,320 --> 00:04:28,260 So, we can do print(parrot[0:6]), 79 00:04:28,260 --> 00:04:33,630 if you run that, you get Norweg, which is the six characters starting, 80 00:04:33,630 --> 00:04:36,720 which is the length of six characters starting at position zero. 81 00:04:36,720 --> 00:04:41,290 So the first number is the number in the string to start, printing out, and 82 00:04:41,290 --> 00:04:43,790 the sixth is how many characters to actually print out. 83 00:04:43,790 --> 00:04:45,880 So it's actually two ways at looking at this. 84 00:04:45,880 --> 00:04:51,100 First, you could say that printing parrot[0:6] starts from 85 00:04:51,100 --> 00:04:56,350 character's position index zero up to but not including index position six. 86 00:04:56,350 --> 00:04:59,420 But we can also say the six character starting from index zero. 87 00:04:59,420 --> 00:05:03,440 But in Python, the first is technically more correct even if it seems to amount 88 00:05:03,440 --> 00:05:07,090 where they both seem, well actually both do end up with showing the same amount. 89 00:05:07,090 --> 00:05:08,580 If the first number is omitted, 90 00:05:08,580 --> 00:05:11,130 the slice is gonna start from the beginning of the string. 91 00:05:11,130 --> 00:05:14,000 If the second number is omitted, it's gonna run to the end of the string. 92 00:05:14,000 --> 00:05:15,360 Just to show you what I mean by that. 93 00:05:15,360 --> 00:05:23,680 Type print(parrot[:6]. 94 00:05:23,680 --> 00:05:25,040 That's gonna start from automatically, 95 00:05:25,040 --> 00:05:26,540 because we haven't specified a number here. 96 00:05:26,540 --> 00:05:29,410 It's gonna start from the start of the string. 97 00:05:29,410 --> 00:05:31,680 That's gonna print the same as the previous line. 98 00:05:31,680 --> 00:05:34,930 But if we also did print(parrot[6:]) and 99 00:05:34,930 --> 00:05:39,710 didn't specify the second number, it's gonna run to the end of the string. 100 00:05:39,710 --> 00:05:43,800 So if you run that, you can see it's moved, started at the position, so 101 00:05:43,800 --> 00:05:47,080 it's actually went to effectively the seventh position, so it's counting, 102 00:05:47,080 --> 00:05:51,600 zero, one, two, three, four, five, six, ian Blue. 103 00:05:51,600 --> 00:05:54,880 So ian Blue and it's going along because we haven't specified where to end, 104 00:05:54,880 --> 00:05:58,180 so it's actually moved on to the end, continued and done the entire string. 105 00:05:58,180 --> 00:06:00,140 And if you start dealing with negative indices, 106 00:06:00,140 --> 00:06:03,000 it quickly becomes apparent why the second definition is not gonna work. 107 00:06:03,000 --> 00:06:09,430 So in other words, if we start trying to do this, print(parrot[-4:2]) 108 00:06:09,430 --> 00:06:16,370 that's not actually gonna print anything. 109 00:06:16,370 --> 00:06:18,750 That's because you can't go back from the starting position in this 110 00:06:18,750 --> 00:06:19,610 context. 111 00:06:19,610 --> 00:06:22,900 But if we actually go back and put a -2 there, and run that. 112 00:06:24,880 --> 00:06:28,360 You can see it's actually started and produced Bl and that's because it 113 00:06:28,360 --> 00:06:31,850 started from the start of the string, it's gone back four characters, up to and not 114 00:06:31,850 --> 00:06:36,030 including the second to last character in the string which was the u. 115 00:06:36,030 --> 00:06:39,220 Another thing you can actually do is you can actually do a print. 116 00:06:39,220 --> 00:06:42,540 You can actually do funny slices that skip characters, counting in steps. 117 00:06:42,540 --> 00:06:48,380 So I could do print(parrot[0:6:2]). 118 00:06:48,380 --> 00:06:50,340 And that should give us the answer Nre. 119 00:06:50,340 --> 00:06:53,270 So we run that, Nre as you can see. 120 00:06:53,270 --> 00:06:57,900 So starting from index position zero, extract all the characters up to, but 121 00:06:57,900 --> 00:07:01,210 not including index position six in steps of two. 122 00:07:01,210 --> 00:07:05,200 As you saw in the first example and that started on N. 123 00:07:05,200 --> 00:07:07,690 And skip to r and skip to e. 124 00:07:07,690 --> 00:07:14,520 And we can change that and do something like print(parrot[0:6:3]), 125 00:07:14,520 --> 00:07:19,400 so make the step increment three, and we should get N and w in that case, and 126 00:07:19,400 --> 00:07:21,740 w because it's skipping three characters at time. 127 00:07:21,740 --> 00:07:24,550 So slightly useless application, but 128 00:07:24,550 --> 00:07:28,910 I'm gonna show you an example. Type number = "9. 129 00:07:28,910 --> 00:07:31,820 And there's numbers here, 130 00:07:31,820 --> 00:07:37,900 223,372,036,854,775,807". 131 00:07:37,900 --> 00:07:42,410 And if I actually do a print(number[1::4]) with two colons, 132 00:07:42,410 --> 00:07:47,460 if I actually run that, you can see how it's actually skipped the numbers and 133 00:07:47,460 --> 00:07:52,150 actually just printed out the commas at each position automatically. 134 00:07:52,150 --> 00:07:55,210 So that's a good way to step through and extract portions out of the particular 135 00:07:55,210 --> 00:07:56,760 string that you wanna work with. 136 00:07:56,760 --> 00:08:01,810 But using a better example, let's just try numbers = "1, 137 00:08:01,810 --> 00:08:06,760 2, 3, and notice I'm putting one space after each, 4, 138 00:08:06,760 --> 00:08:09,500 5, 6, 7, 8, 9" like so. 139 00:08:09,500 --> 00:08:14,840 And if we actually do, print(number 140 00:08:14,840 --> 00:08:20,200 print(numbers[0::3]). 141 00:08:20,200 --> 00:08:21,170 We actually run that. 142 00:08:22,580 --> 00:08:25,280 You can see we've quickly extracted out the numbers, and 143 00:08:25,280 --> 00:08:27,270 we've avoided the commas and the spaces there. 144 00:08:27,270 --> 00:08:30,640 So you can see how it would be useful, if you wanted to extract information out in 145 00:08:30,640 --> 00:08:31,620 a particular format. 146 00:08:31,620 --> 00:08:34,760 So moving on, let's talk a little bit about string operators. 147 00:08:34,760 --> 00:08:36,130 So we've talked about operators before, and 148 00:08:36,130 --> 00:08:39,470 you've seen the plus operator when we use strings to concatenate. 149 00:08:39,470 --> 00:08:43,710 So if we had something like this. 150 00:08:43,710 --> 00:08:46,810 String1 = "he's " 151 00:08:46,810 --> 00:08:49,180 String2 = "probably ". 152 00:08:51,170 --> 00:08:55,930 We could do print(string1 + string2). 153 00:08:55,930 --> 00:08:57,490 We've seen that before. 154 00:08:57,490 --> 00:09:00,590 If we run that, the two strings get added together, he's probably. 155 00:09:00,590 --> 00:09:02,280 But if you're using a string literal, and 156 00:09:02,280 --> 00:09:05,710 that's when you're just typing the value in direct and you're not using a variable, 157 00:09:05,710 --> 00:09:07,560 you don't actually need to use the plus sign. 158 00:09:07,560 --> 00:09:12,440 So we could do something like this, we could do 159 00:09:12,440 --> 00:09:17,710 print("he's " "probably " "pining"). 160 00:09:17,710 --> 00:09:18,730 We can run that. 161 00:09:19,910 --> 00:09:23,710 We can see how Python quite happily allows you do actually do something like that. 162 00:09:23,710 --> 00:09:25,640 Now I'm mentioning that just to show you how it works. 163 00:09:25,640 --> 00:09:28,780 I wouldn't think you'll ever wanna do something like that because it's a little 164 00:09:28,780 --> 00:09:29,920 bit totally confusing. 165 00:09:29,920 --> 00:09:33,110 And probably not really useful because you can obviously just do this, 166 00:09:33,110 --> 00:09:37,310 you can actually delete that out anyway and obviously you get the same result. 167 00:09:37,310 --> 00:09:41,010 But it's just that, showing you that it can potentially be done. 168 00:09:41,010 --> 00:09:43,480 So next, you can actually multiply strings. 169 00:09:43,480 --> 00:09:46,490 It sounds a bit weird because multiplication generally is only on 170 00:09:46,490 --> 00:09:52,460 numbers, but if you do something like this: print("Hello " *5). 171 00:09:52,460 --> 00:09:54,850 Python will give you, Hello Hello Hello Hello Hello so 172 00:09:54,850 --> 00:09:57,290 it will give you whatever's in the string. 173 00:09:57,290 --> 00:09:58,340 And it will do that five times. 174 00:09:58,340 --> 00:10:00,190 It's sort of like appending to that five times. 175 00:10:00,190 --> 00:10:02,510 And operator precedence applies. 176 00:10:02,510 --> 00:10:07,780 So if we do something like print("Hello " *5 177 00:10:07,780 --> 00:10:12,340 +4) remembering what we know about operator precedence. 178 00:10:12,340 --> 00:10:13,040 We try and run that. 179 00:10:14,400 --> 00:10:18,210 We actually get an error, can't convert int object into a string implicitly. 180 00:10:18,210 --> 00:10:21,890 And that's because, hello, the times 5, this part here, 181 00:10:21,890 --> 00:10:26,700 that actually evaluates to a string, and the attempt to then add 4 to it fails, 182 00:10:26,700 --> 00:10:31,440 and that's really not the way to do it, so if you want to do something like that, 183 00:10:31,440 --> 00:10:34,510 you'd actually either put it brackets, like so. 184 00:10:34,510 --> 00:10:37,870 And if you run that you then get the correct result. 185 00:10:37,870 --> 00:10:42,530 Alternatively you would type print("Hello " * 5 + "4"). 186 00:10:42,530 --> 00:10:45,800 If you run that, that's 187 00:10:45,800 --> 00:10:50,470 actually if you wanted to append the number 188 00:10:50,470 --> 00:10:55,520 four onto the end of the strings that have been modified out five times. 189 00:10:55,520 --> 00:10:58,410 There's also an operator to check if one string 190 00:10:58,410 --> 00:11:01,850 is a substring of another, which can be useful for searching purposes. 191 00:11:01,850 --> 00:11:04,970 So we did something like today = "friday". 192 00:11:07,010 --> 00:11:10,630 We can do print("day" in today). 193 00:11:10,630 --> 00:11:13,310 So that's the in operator. 194 00:11:13,310 --> 00:11:15,070 So that should return True. 195 00:11:15,070 --> 00:11:18,340 Returns a true or false to indicate whether it thinks 196 00:11:18,340 --> 00:11:22,650 that whatever we've typed here, day in this case, is actually in that word. 197 00:11:22,650 --> 00:11:24,940 So if we run that, we should get True. 198 00:11:24,940 --> 00:11:26,330 And so you've got True there on the screen. 199 00:11:26,330 --> 00:11:30,510 If we did something like print("fri" in 200 00:11:30,510 --> 00:11:34,690 today) we should also get a True, which we'll test shortly. 201 00:11:34,690 --> 00:11:38,710 And if we do something different like print("thur" in 202 00:11:38,710 --> 00:11:41,750 today) we should get a False for that. 203 00:11:41,750 --> 00:11:44,430 And lastly, if we did something like, 204 00:11:44,430 --> 00:11:49,640 ("parrot" in "fjord") that should obviously be a False as well. 205 00:11:49,640 --> 00:11:54,960 So True, True, "day" in today, if "fri" in today. if Thursday or 206 00:11:54,960 --> 00:11:58,510 Thur is not in that variable called today. 207 00:11:58,510 --> 00:12:03,130 And, obviously the word, parrot, is not in fjord either. 208 00:12:03,130 --> 00:12:06,170 So that's you got some basic information about operators. 209 00:12:06,170 --> 00:12:09,430 And I hope you got a lot out of the two videos on variables in Python. 210 00:12:09,430 --> 00:12:11,240 In the next video, we're gonna take it to the next level. 211 00:12:11,240 --> 00:12:14,740 We're gonna look at various ways that we can format strings, and also some of 212 00:12:14,740 --> 00:12:18,510 the string methods that are built into Python, to allow us to manipulate them. 213 00:12:18,510 --> 00:12:19,900 So, I'll see you in that next video.