1 00:00:05,300 --> 00:00:06,180 So in the previous video, 2 00:00:06,180 --> 00:00:10,020 we stored some strings in variables but we didn't really explain what was going on. 3 00:00:10,020 --> 00:00:10,930 So we wanna go through and 4 00:00:10,930 --> 00:00:14,680 actually explain what is happening a little bit more now behind the scenes. 5 00:00:14,680 --> 00:00:16,500 So when the Python programming is running, 6 00:00:16,500 --> 00:00:20,340 everything the program needs ends up being stored somewhere in the computer's memory. 7 00:00:20,340 --> 00:00:23,010 You don't actually need to know where that is because the computer 8 00:00:23,010 --> 00:00:23,870 actually tracks that. 9 00:00:23,870 --> 00:00:27,750 For the program code itself, in other words the instructions that you type in, 10 00:00:27,750 --> 00:00:31,640 will be stored in one area of memory, but also the data that it's gonna work on, 11 00:00:31,640 --> 00:00:36,140 the variables that you've typed in etc., will be stored somewhere else in memory. 12 00:00:36,140 --> 00:00:37,130 So the variable, 13 00:00:37,130 --> 00:00:40,900 the name that we assigned is just really a way to give a meaningful name to 14 00:00:40,900 --> 00:00:45,410 a area of memory into which we can place certain values like strings and so forth. 15 00:00:45,410 --> 00:00:50,050 So when we create a variable called greeting like greeting = Bruce. 16 00:00:51,340 --> 00:00:55,220 Python's actually allocating an area of computer memory for us, and 17 00:00:55,220 --> 00:00:58,900 it knows to refer to that area whenever we talk about the variable greeting. 18 00:00:58,900 --> 00:01:03,230 So think of it like a box in memory that's storing the values, Bruce in this case, 19 00:01:03,230 --> 00:01:06,470 and the box has got a name on, a sticker on it that says greeting. 20 00:01:06,470 --> 00:01:09,440 And when we type in greeting, the computer knows to go and 21 00:01:09,440 --> 00:01:11,760 find the value that's in that particular box. 22 00:01:11,760 --> 00:01:14,400 So that all happens automatically behind the scenes. 23 00:01:14,400 --> 00:01:17,610 Just in terms of creating these variable names, 24 00:01:17,610 --> 00:01:20,750 there are a few rules to keep in mind when you actually created them. 25 00:01:20,750 --> 00:01:24,880 Firstly, the actual name has to start with either a letter, it could be upper or 26 00:01:24,880 --> 00:01:27,160 lower case, or an underscore. 27 00:01:27,160 --> 00:01:33,270 So in other words, we can type _myName = Tim, that's valid. 28 00:01:33,270 --> 00:01:36,150 So a variable name can start with either the letter or an underscore but 29 00:01:36,150 --> 00:01:37,430 we can't start with a number. 30 00:01:40,530 --> 00:01:41,700 Actually get an error. 31 00:01:41,700 --> 00:01:44,870 But we can actually also use letters. 32 00:01:44,870 --> 00:01:47,810 We can also use numbers in our variable names, just not as the first letter. 33 00:01:47,810 --> 00:01:50,510 So, we can put something like Tim45 = Good. 34 00:01:50,510 --> 00:01:53,650 That's quite valid and we could put a combination. 35 00:01:53,650 --> 00:01:59,110 So we could put Tim_Was_57 = Hello and 36 00:01:59,110 --> 00:02:02,910 that's quite valid as well. 37 00:02:02,910 --> 00:02:05,780 Now the other important thing to consider here is that I could also 38 00:02:05,780 --> 00:02:08,750 type a variable name called Greeting, with uppercase G. 39 00:02:08,750 --> 00:02:13,560 And that's different and distinct to this variable name. 40 00:02:13,560 --> 00:02:16,870 So that one's all in lowercase, this one's got an uppercase first letter. 41 00:02:16,870 --> 00:02:18,540 Even though the actual word is the same, 42 00:02:18,540 --> 00:02:23,630 they are treated as different variable names as far as Python is concerned. 43 00:02:23,630 --> 00:02:25,500 Now in terms of the allocation of memory, 44 00:02:25,500 --> 00:02:28,270 it actually happens at the point that you've initialized it. 45 00:02:28,270 --> 00:02:31,850 So in other words once you've talked in the variable name and put an equal and 46 00:02:31,850 --> 00:02:34,880 then the value, at that point python will allocate the space for 47 00:02:34,880 --> 00:02:36,790 that in the computers memory. 48 00:02:36,790 --> 00:02:39,670 In terms of using variables we saw that we could 49 00:02:39,670 --> 00:02:41,210 use the prints statement to print some out. 50 00:02:41,210 --> 00:02:46,450 We could do something like TIm was here, or 51 00:02:46,450 --> 00:02:53,040 Tim was 57 + a single-quote space plus greeting. 52 00:02:53,040 --> 00:02:56,560 We could run thatm, Hello Bruce. 53 00:02:56,560 --> 00:02:59,870 Obviously it's taking the value of Hello from Tim_Was_57 and 54 00:02:59,870 --> 00:03:01,770 Bruce from the variable greeting and 55 00:03:01,770 --> 00:03:06,110 actually printing that on the screen with a space between the two there. 56 00:03:06,110 --> 00:03:09,720 We could do something like age equals 24, print age. 57 00:03:11,620 --> 00:03:16,440 We can also do something like greeting plus age, since we're on this program. 58 00:03:17,780 --> 00:03:19,640 Now this is where we actually get an error. 59 00:03:19,640 --> 00:03:22,410 So you can see it should the first Hello Bruce which we saw before, 60 00:03:22,410 --> 00:03:23,820 which was this first line. 61 00:03:23,820 --> 00:03:27,490 We then assigned a value of 24 to the variable age, and we printed the age, 62 00:03:27,490 --> 00:03:27,990 which 24 worked. 63 00:03:27,990 --> 00:03:32,340 We're going to try to do this next line, print greeting plus age. 64 00:03:32,340 --> 00:03:33,760 And we actually got an error in red. 65 00:03:33,760 --> 00:03:37,930 And you can see that Python's actually trying to help you here by actually sort 66 00:03:37,930 --> 00:03:39,990 of saying, look I've got a problem with what you've typed. 67 00:03:39,990 --> 00:03:44,340 It's actually come up and actually said, I can't convert int object to a string. 68 00:03:44,340 --> 00:03:47,610 Str is the abbreviation of string implicitly. 69 00:03:47,610 --> 00:03:50,730 So what we've tried to do is Get it to do something that Python can't work with, and 70 00:03:50,730 --> 00:03:53,300 this is what happens when you get an error. 71 00:03:53,300 --> 00:03:57,000 So Python will hopefully show you some information which hopefully leads to you 72 00:03:57,000 --> 00:04:01,010 figuring out what the problem was, and it actually shows also the actual lines. 73 00:04:01,010 --> 00:04:05,140 So if we move over a little bit you can see it's saying the name of the Python 74 00:04:05,140 --> 00:04:09,680 file which was one that I created for this video varibles.py py. 75 00:04:09,680 --> 00:04:13,260 And it says that line 12, which equates to line 12 over here. 76 00:04:13,260 --> 00:04:16,350 So it's very handy to look at these when you trying to figure out whether there's 77 00:04:16,350 --> 00:04:20,370 a problem, and you can go back to line 12, and know this was where the problem was. 78 00:04:21,500 --> 00:04:24,450 Now the other thing is that IntelliJ actually does a lot of this for 79 00:04:24,450 --> 00:04:28,090 you automatically as well, it tries to help you out with hints as well. 80 00:04:28,090 --> 00:04:30,770 So, over here we can see these little dashes. 81 00:04:30,770 --> 00:04:32,380 If I actually hover over there with my mouse, 82 00:04:32,380 --> 00:04:34,310 we actually get a helpful error message. 83 00:04:34,310 --> 00:04:37,600 Expected type string str got int instead. 84 00:04:37,600 --> 00:04:40,430 So that's IntelliJ, trying to give you some information to say, 85 00:04:40,430 --> 00:04:41,990 you've got this wrong, basically. 86 00:04:41,990 --> 00:04:46,250 And so it can be really useful when you're creating Python programs to keep 87 00:04:46,250 --> 00:04:49,170 checking these hints out to see whether there actually is an issue. 88 00:04:49,170 --> 00:04:50,860 Notice also there's a keyboard shortcut, 89 00:04:50,860 --> 00:04:53,160 if you want to expand this to get some more information. 90 00:04:53,160 --> 00:04:58,390 So on a Mac, I can actually type Cmd + F1, I get more information like so. 91 00:04:58,390 --> 00:05:01,920 And press Ctrl + F1 on Windows machine or a Linux machine and 92 00:05:01,920 --> 00:05:02,950 you can get the full message. 93 00:05:02,950 --> 00:05:06,050 So that's very handy to refer to that all the time, and 94 00:05:06,050 --> 00:05:08,940 later in the course we'll be looking at handling these errors. 95 00:05:08,940 --> 00:05:11,760 In other words, if you're program crashes or goes to crash, 96 00:05:11,760 --> 00:05:13,880 we can handle that more gracefully. 97 00:05:13,880 --> 00:05:17,360 But for now, think of those error messages that come up as your friend because what 98 00:05:17,360 --> 00:05:20,460 it's really doing, it's telling you that your code is not gonna work and 99 00:05:20,460 --> 00:05:23,180 providing as much information as possible to you to help you try and 100 00:05:23,180 --> 00:05:24,520 figure out what the solution is. 101 00:05:24,520 --> 00:05:27,550 So what can be frustrating, you really did learn a lot from errors like these, 102 00:05:27,550 --> 00:05:29,960 because you can figure out something that Python doesn't like, and 103 00:05:29,960 --> 00:05:33,050 how to work around that to come up with actually a solution. 104 00:05:33,050 --> 00:05:34,160 Okay, so moving on, 105 00:05:34,160 --> 00:05:37,380 and later in the course we'll be talking about these conversions. 106 00:05:37,380 --> 00:05:40,630 So, in terms of this particular error message, what's actually happening was 107 00:05:40,630 --> 00:05:45,950 again we determined that Python wasn't happy because we're trying to add the age, 108 00:05:45,950 --> 00:05:49,820 the numeric variable age to a number, and you can't actually do that. 109 00:05:49,820 --> 00:05:51,450 It doesn't actually work like that. 110 00:05:51,450 --> 00:05:53,420 So Python doesn't know what to do in that scenario. 111 00:05:53,420 --> 00:05:56,030 It knows what to do in the situation like this, 112 00:05:56,030 --> 00:05:57,400 we're actually adding strings together. 113 00:05:57,400 --> 00:05:59,830 Because Tim_Was_57 is a string, and 114 00:05:59,830 --> 00:06:03,370 greeting was a string and it knows to concatenate those. 115 00:06:03,370 --> 00:06:06,240 Likewise, if we tried it to do something with two numbers, 116 00:06:06,240 --> 00:06:08,460 it knows to add to some of those two numbers. 117 00:06:08,460 --> 00:06:10,480 But in this scenario where we've got a string and 118 00:06:10,480 --> 00:06:12,720 a number, Python doesn't actually know how to do that. 119 00:06:12,720 --> 00:06:15,510 Not know how to handle that and that's why we actually got the message. 120 00:06:15,510 --> 00:06:18,020 So what we dealing more with actually how to get around that. 121 00:06:18,020 --> 00:06:21,450 And do note that some other programming languages, Java being one of them, 122 00:06:21,450 --> 00:06:24,040 does actually allow you to actually do something like that. 123 00:06:24,040 --> 00:06:27,930 And in that scenario what would happen is it would actually automatically convert 124 00:06:27,930 --> 00:06:28,740 what you're trying to do. 125 00:06:28,740 --> 00:06:32,620 In this case, it would append the value of 24 to the string age. 126 00:06:32,620 --> 00:06:35,700 But Python doesn't do that, it makes you get it right at the start. 127 00:06:35,700 --> 00:06:38,390 And that's actually a good thing in many ways because it means you're not 128 00:06:38,390 --> 00:06:41,100 going to be getting any weird little bugs that crept about 129 00:06:41,100 --> 00:06:43,100 because of automatic type conversion. 130 00:06:43,100 --> 00:06:44,710 That you may get in other programming languages. 131 00:06:44,710 --> 00:06:47,290 So the bottom line is, you need to get that right first, and 132 00:06:47,290 --> 00:06:50,580 really it's just a matter of checking out the error message and fixing it. 133 00:06:50,580 --> 00:06:53,610 And we're talking more about this type of thing as the course progresses anyway. 134 00:06:53,610 --> 00:06:56,530 So moving on, let's talk now a little bit about variable types. 135 00:06:56,530 --> 00:06:59,080 So Python has got several built-in data types. 136 00:06:59,080 --> 00:07:01,580 We've got a heap of them and we'll be dealing with those in detail. 137 00:07:01,580 --> 00:07:06,270 Data types can be classified as numerics, sequences, mappings, files, 138 00:07:06,270 --> 00:07:08,590 classes, instances, and exceptions. 139 00:07:08,590 --> 00:07:11,810 And I [LAUGH] know that sounds like a lot and it probably is but 140 00:07:11,810 --> 00:07:14,890 you'll be introduced to all of these as the course progresses. 141 00:07:14,890 --> 00:07:17,650 But for now we're gonna look at numeric data types and 142 00:07:17,650 --> 00:07:19,620 one of the sequence types which you've already seen. 143 00:07:19,620 --> 00:07:22,450 String being a sequence type, which we've actually looked at before. 144 00:07:22,450 --> 00:07:26,960 Now Python 2, the older version of python which as I mention is no longer 145 00:07:26,960 --> 00:07:28,860 being used and we're not using it in this course. 146 00:07:28,860 --> 00:07:33,860 That actually had various data types such as int, long, float, and complex, 147 00:07:33,860 --> 00:07:37,980 but in Python 3 the change they've made is there's no longer a long type, and 148 00:07:37,980 --> 00:07:40,140 the int now behaves like a long used to. 149 00:07:40,140 --> 00:07:43,160 So they've really consolidated the amount of data types. 150 00:07:43,160 --> 00:07:45,810 Now we're not going to discuss complex numbers 151 00:07:45,810 --> 00:07:47,180 without going into too much detail. 152 00:07:47,180 --> 00:07:49,990 A complex number contains a real and 153 00:07:49,990 --> 00:07:52,600 an imaginary part based on the square root of minus one. 154 00:07:52,600 --> 00:07:56,170 Now, if you understand a complex number and wanna use Pyton to manipulate them, 155 00:07:56,170 --> 00:08:00,000 then by the end of the course you'll understand Python well enough to do so. 156 00:08:00,000 --> 00:08:04,320 But really the complex number theories in advance branch of mathematics slash 157 00:08:04,320 --> 00:08:07,640 engineering, it's not really appropriate to go into too much detail with that. 158 00:08:07,640 --> 00:08:10,240 But let's look at the basic data types, and 159 00:08:10,240 --> 00:08:12,800 a basic data type we're going to start with an integer. 160 00:08:12,800 --> 00:08:16,670 An integer being a whole number so a number that hasn't got any decimal points. 161 00:08:16,670 --> 00:08:19,070 First, you can see on the screen I have typed in an integer. 162 00:08:19,070 --> 00:08:20,620 So an integer is a just whole number. 163 00:08:20,620 --> 00:08:24,150 It's a number having no fractional part where as a float 164 00:08:24,150 --> 00:08:27,330 in the other example is another name for a real number. 165 00:08:27,330 --> 00:08:30,950 That is a number having a fractional part after the decimal point. 166 00:08:30,950 --> 00:08:35,150 There is a very small number of computer languages that make no distinction between 167 00:08:35,150 --> 00:08:36,860 real numbers and integers. 168 00:08:36,860 --> 00:08:39,630 I guess you could say that integers can be considered 169 00:08:39,630 --> 00:08:41,350 just special cases of real numbers. 170 00:08:41,350 --> 00:08:44,520 But when represented in a computer, computations using integers, 171 00:08:44,520 --> 00:08:48,390 these whole numbers, are significantly faster than using floating point numbers. 172 00:08:48,390 --> 00:08:51,420 So that's one of the main reasons that we actually distinguish in programming 173 00:08:51,420 --> 00:08:53,850 an integer as a whole number and a real number, 174 00:08:53,850 --> 00:08:56,960 a floating point number and that's because it's a lot faster to process. 175 00:08:56,960 --> 00:09:00,700 So anything that doesn't need, as a general rule, a floating point number for 176 00:09:00,700 --> 00:09:03,390 calculation purposes, you shouldn't actually be using it. 177 00:09:03,390 --> 00:09:06,770 Because you'll find that your programs will operate a lot faster because of it. 178 00:09:06,770 --> 00:09:10,810 There are pros and cons, because of the way that integers are stored in 179 00:09:10,810 --> 00:09:14,010 the computer's memory, there's actually a limit to the size of an int. 180 00:09:14,010 --> 00:09:17,040 It's about 9 trillion in European terms. 181 00:09:17,040 --> 00:09:19,990 I think it's nine quintillion, in American format. 182 00:09:19,990 --> 00:09:23,390 But effectively the Python 2 long data type the Python 3, 183 00:09:23,390 --> 00:09:26,000 effectively have no real maximum size. 184 00:09:26,000 --> 00:09:27,680 Now floating point numbers on the other hand, 185 00:09:27,680 --> 00:09:31,280 that's floats, they're used to represent numbers having a fractional part. 186 00:09:31,280 --> 00:09:35,750 So the maximum float value on a 64 bit computer is, well that's, it's just huge, 187 00:09:35,750 --> 00:09:37,390 we'll go through it, but it's quite large. 188 00:09:37,390 --> 00:09:41,730 Basically a number that actually moves the decimal point 308 places to the right. 189 00:09:41,730 --> 00:09:45,940 And the smallest float is a negative number which has 307 zeros before 190 00:09:45,940 --> 00:09:47,390 the decimal float. 191 00:09:47,390 --> 00:09:51,120 So, it's obviously a huge variance and 192 00:09:51,120 --> 00:09:53,830 a huge number can be stored in that floating point number. 193 00:09:53,830 --> 00:09:57,280 Floating points have 52 digits of precision, which should be adequate for 194 00:09:57,280 --> 00:10:00,600 most purposes from financial calculations where you're needing decimals. 195 00:10:00,600 --> 00:10:04,080 And because Python doesn't really have variable declarations, 196 00:10:04,080 --> 00:10:07,950 that's where you specify the type of a variable before you can use it, and 197 00:10:07,950 --> 00:10:10,170 lots of other computer languages do that. 198 00:10:10,170 --> 00:10:13,970 It's probably tempting to think that you don't need to understand the difference 199 00:10:13,970 --> 00:10:15,010 between an int, a long, and a float type, but 200 00:10:15,010 --> 00:10:18,120 really it is something you really need to consider when writing your programs. 201 00:10:18,120 --> 00:10:20,380 This makes more sense when we look at some of the operators that can be 202 00:10:20,380 --> 00:10:21,240 performed on numbers. 203 00:10:21,240 --> 00:10:23,040 So let's just go through the first few scenarios. 204 00:10:23,040 --> 00:10:26,480 So I typed a variable called a with an integer value of 12. 205 00:10:26,480 --> 00:10:29,450 Let's do another one, b = 3. 206 00:10:29,450 --> 00:10:31,670 And let's perform some calculations. 207 00:10:31,670 --> 00:10:39,450 We'll do print, we'll do a plus b, we should get the value of 15, 208 00:10:39,450 --> 00:10:44,920 a take b, we should get the value of nine. 209 00:10:44,920 --> 00:10:50,390 Print, a times b, the value of 36, we should get and we'll test this shortly. 210 00:10:50,390 --> 00:10:53,530 a divided by b, we should get the answer four, but 211 00:10:53,530 --> 00:10:56,120 there's a special rule there about that which I will talk about shortly. 212 00:10:56,120 --> 00:11:01,640 And we're going to introduce another operator, //b and 213 00:11:01,640 --> 00:11:04,680 I'll explain that shortly, and last one we're going to use is the remainder, 214 00:11:04,680 --> 00:11:09,330 remainder of a divided by b effectively. 215 00:11:09,330 --> 00:11:13,760 Otherwise dividing 12 by 3, how much remainder is left after that. 216 00:11:13,760 --> 00:11:16,320 So let's just run this, and then we'll just go through it. 217 00:11:16,320 --> 00:11:17,750 So the first one is 15. 218 00:11:17,750 --> 00:11:18,440 The second one is 9. 219 00:11:18,440 --> 00:11:20,080 And the third value is 36. 220 00:11:20,080 --> 00:11:22,610 And there's the fourth one, a divided by b. 221 00:11:22,610 --> 00:11:27,360 The actual answer is returned as a float, with a decimal point, .0. 222 00:11:27,360 --> 00:11:30,490 And that's the thing to keep in mind, with a divide, 223 00:11:30,490 --> 00:11:35,180 a division operation, in Python, by default it returns as a float. 224 00:11:35,180 --> 00:11:37,880 If you want the result to return as an integer, 225 00:11:37,880 --> 00:11:40,990 a whole number, you need to use the two slashes in this case. 226 00:11:40,990 --> 00:11:44,360 So a, slash slash b, returns four as a whole number. 227 00:11:44,360 --> 00:11:45,120 And lastly, 228 00:11:45,120 --> 00:11:50,300 the remainder of a divided by b was zero because b goes into a four times. 229 00:11:50,300 --> 00:11:52,200 Four times three is twelve and there would be no remainder. 230 00:11:52,200 --> 00:11:55,520 So that's how you do some basic calculations in Python. 231 00:11:55,520 --> 00:11:58,830 Now this slash and slash slash can really trip you up, and 232 00:11:58,830 --> 00:12:00,980 we're gonna be talking about loops later. 233 00:12:00,980 --> 00:12:04,030 But I just want to give you an example of typing in a loop where you could get into 234 00:12:04,030 --> 00:12:06,090 difficulty without using that slash or 235 00:12:06,090 --> 00:12:09,600 slash slash because it can be a common source of problems. 236 00:12:09,600 --> 00:12:10,700 So let's look at an example here. 237 00:12:10,700 --> 00:12:13,770 We're gonna type in some code, and again we'll talk more about this for, and 238 00:12:13,770 --> 00:12:14,900 the concept of loops later. 239 00:12:14,900 --> 00:12:19,930 But for now we'll just type in for 240 00:12:19,930 --> 00:12:24,980 i in range, 1, 4, : print i. 241 00:12:24,980 --> 00:12:28,390 So if we run this, You get the values 1, 2, 3. 242 00:12:28,390 --> 00:12:33,950 We've actually changed this here and put a divided by b, cuz instead 243 00:12:33,950 --> 00:12:37,260 of putting a constant number, a literal, we can actually put an expression like so. 244 00:12:37,260 --> 00:12:40,750 If we run that, we actually get an error. 245 00:12:40,750 --> 00:12:43,750 And that's because float object cannot be interpreted as an integer, 246 00:12:43,750 --> 00:12:45,020 and that's because the answer, 247 00:12:45,020 --> 00:12:47,990 if you remember, with division by default is a floating point number. 248 00:12:47,990 --> 00:12:49,540 The range actually needs an integer, so 249 00:12:49,540 --> 00:12:51,920 we'd actually put two slashes in that scenario. 250 00:12:51,920 --> 00:12:55,900 Run the code again, it actually correctly returns the results one, two, and three. 251 00:12:55,900 --> 00:12:58,500 As we progress through the course, you'll see other ways of actually handling this, 252 00:12:58,500 --> 00:12:59,820 but I just wanted to point that out. 253 00:12:59,820 --> 00:13:02,640 At this point in time, that's a common source of problems when you're 254 00:13:02,640 --> 00:13:03,960 looking at Python for the first time. 255 00:13:03,960 --> 00:13:06,710 And again, don't be too worried about the concepts of what this is doing, 256 00:13:06,710 --> 00:13:10,110 we'll be covering loops, which is what this actually is In future videos. 257 00:13:10,110 --> 00:13:13,640 So the code something like a plus b, that's actually an expression and 258 00:13:13,640 --> 00:13:14,840 I mentioned expressions. 259 00:13:14,840 --> 00:13:19,030 In Python an expression is anything that can be calculated to return a value. 260 00:13:19,030 --> 00:13:22,230 So we've really only used simple expressions to date like a plus b or 261 00:13:22,230 --> 00:13:24,550 a take b but we can get a lot more complex. 262 00:13:24,550 --> 00:13:26,650 We could do something, just let me clear some space here. 263 00:13:27,730 --> 00:13:31,290 So a lot more complex if we want to, 264 00:13:31,290 --> 00:13:37,320 we can do something like print (a + b / 3- 4 * 12), 265 00:13:37,320 --> 00:13:41,810 and that's quite a valid expression. 266 00:13:41,810 --> 00:13:45,600 So if we actually run this, you might be surprised to look at that and think, 267 00:13:45,600 --> 00:13:47,770 why is that come up with a value of 35? 268 00:13:47,770 --> 00:13:51,100 You'd expect looking at that, that the answer should actually be 12. 269 00:13:51,100 --> 00:13:57,980 It should be a plus b is 15 divided by 3 is 5, take 4 is 1, times 12 is 12. 270 00:13:57,980 --> 00:13:59,780 So why have we actually got 35 here instead of 12? 271 00:13:59,780 --> 00:14:03,170 [LAUGH] Now don't worry, your computer's not broken. 272 00:14:03,170 --> 00:14:05,930 And you can reliably perform arithmetic in Python. 273 00:14:05,930 --> 00:14:08,360 But, as is the case in all programming languages, 274 00:14:08,360 --> 00:14:12,280 you have to understand some of the basic rules of the arithmetic operators. 275 00:14:12,280 --> 00:14:14,560 Out problem here is operator precedence. 276 00:14:14,560 --> 00:14:17,260 Now operator precedence is a fancy term for 277 00:14:17,260 --> 00:14:19,890 the relative importance given to each of the operators. 278 00:14:19,890 --> 00:14:23,810 If all operators were equal, we'll get the expression how I read it earlier. 279 00:14:23,810 --> 00:14:31,180 By calculating a plus b is 15 divided by three is 5 take 4 is 1 time 12 is 12. 280 00:14:31,180 --> 00:14:33,570 But the thing to remember is multiplication and 281 00:14:33,570 --> 00:14:37,400 division have higher operator precedence than addition and subtraction. 282 00:14:37,400 --> 00:14:39,620 So those operators are performed first. 283 00:14:39,620 --> 00:14:41,580 And most languages you'll find working the same way so 284 00:14:41,580 --> 00:14:42,970 this is not just peculiar to Python. 285 00:14:42,970 --> 00:14:47,440 So what's actually happening is b divided by 3, this is actually happening first. 286 00:14:47,440 --> 00:14:51,790 So, b divided by 3 is happening first, which is 1. 287 00:14:51,790 --> 00:14:53,720 Then 4 times 12 is added, which is 48. 288 00:14:53,720 --> 00:14:57,950 So the expression is actually evaluated as 12 plus one is 13, take 48, 289 00:14:57,950 --> 00:14:59,260 which is negative 35. 290 00:14:59,260 --> 00:15:04,380 That's weird, but that's the way it actually happens by default 291 00:15:04,380 --> 00:15:08,340 because multiplication and division, as I mentioned, have got a higher precedence. 292 00:15:08,340 --> 00:15:11,760 So the case where you've actually got expressions that use just those operators, 293 00:15:11,760 --> 00:15:13,200 they will actually work from left to right. 294 00:15:13,200 --> 00:15:16,060 So we can type a divided by two times three, 295 00:15:16,060 --> 00:15:19,280 and that should give us the answer of four times three is 12. 296 00:15:19,280 --> 00:15:22,840 So we run that, we actually get the answer of twelve and 297 00:15:22,840 --> 00:15:26,410 even something like eight times three divided by two. 298 00:15:26,410 --> 00:15:30,340 24 divided by two is 12, we get the answer correct. 299 00:15:30,340 --> 00:15:32,510 Then the best way to actually overcome this, 300 00:15:32,510 --> 00:15:37,460 if you want your expressions to be clear and unambiguous to use parenthesis freely, 301 00:15:37,460 --> 00:15:39,300 even when Python doesn't specifically require them. 302 00:15:39,300 --> 00:15:42,690 So, in our earlier example, the one that didn't work, this first line here. 303 00:15:42,690 --> 00:15:47,710 So we could do something like, print, 304 00:15:47,710 --> 00:15:54,730 a plus b, divide it by three, take four, times 12. 305 00:15:54,730 --> 00:15:57,880 Now that's a lot of brackets there but what we're doing is we're actually putting 306 00:15:57,880 --> 00:16:00,900 the expressions in parentheses to make it clear what we're trying to perform. 307 00:16:00,900 --> 00:16:04,270 And when we actually run it we're actually correct, you now have the answer 12. 308 00:16:04,270 --> 00:16:08,480 So you can see we've got a plus b in its own set of brackets, that's gonna be 15. 309 00:16:08,480 --> 00:16:11,260 But then outside to that there's this bracket here, which covers that. 310 00:16:11,260 --> 00:16:15,830 So therefore, that calculation is now 15 divided by three, which is five. 311 00:16:15,830 --> 00:16:19,070 Then take four from that is one, then multiply by 12, is 12, 312 00:16:19,070 --> 00:16:20,810 and that's why we're getting the answer of 12. 313 00:16:20,810 --> 00:16:22,080 Okay, so I'm gonna stop the video here. 314 00:16:22,080 --> 00:16:25,710 In the next video we're gonna continue on talking about variables and 315 00:16:25,710 --> 00:16:26,910 operator precedence etc. 316 00:16:26,910 --> 00:16:28,320 So I'll see you in the next video.