1 1 00:00:00,650 --> 00:00:04,089 Welcome to the second lesson. We're going over three topics in this video: 2 2 00:00:04,089 --> 00:00:07,770 expressions, datatypes, and variables. Go ahead and start 3 3 00:00:07,770 --> 00:00:11,179 IDLE, which is the editor that comes installed with IDLE. 4 4 00:00:11,179 --> 00:00:14,660 On Windows click on the Start button, type IDLE, 5 5 00:00:14,660 --> 00:00:18,619 and I have multiple versions of Python installed on my computer, 6 6 00:00:18,619 --> 00:00:23,289 but I'm just going to use IDLE that comes with Python 3.5. The course notes 7 7 00:00:23,289 --> 00:00:25,859 describe how to start IDLE if you have Mac or Linux. 8 8 00:00:25,859 --> 00:00:29,869 IDLE has two parts: the interactive shell 9 9 00:00:29,869 --> 00:00:36,020 and, if you click on File, New File, the file editor 10 10 00:00:36,020 --> 00:00:40,270 will appear. The interactive shell runs Python instructions one at a time and shows 11 11 00:00:40,270 --> 00:00:41,649 you results immediately. 12 12 00:00:41,649 --> 00:00:45,079 It's great for just experimenting and seeing what instructions do. 13 13 00:00:45,079 --> 00:00:48,989 The file editor lets you enter Python code for complete programs. 14 14 00:00:48,989 --> 00:00:52,690 There are other editor apps for typing in Python programs. 15 15 00:00:52,690 --> 00:01:01,700 I use one called Sublime Text and there's another one called PyCharm. 16 16 00:01:01,710 --> 00:01:06,100 But this course will use IDLE because it comes with Python and there's no 17 17 00:01:06,100 --> 00:01:08,780 additional set up to do, and because it's the same across 18 18 00:01:08,780 --> 00:01:12,870 Mac, Windows, and Linux. The interactive shell and file editor windows look 19 19 00:01:12,870 --> 00:01:16,500 really similar so it's easy to get them confused. We can always tell the difference 20 20 00:01:16,580 --> 00:01:20,720 because the interactive shell will have the triple angle bracket prompt. 21 21 00:01:20,720 --> 00:01:24,860 In this lesson, we'll be entirely focused on the interactive show so close the 22 22 00:01:24,860 --> 00:01:30,580 file editor for now. Let's begin by typing a simple Python instruction. 23 23 00:01:30,580 --> 00:01:34,470 Type 2 + 2 and then press Enter. 24 24 00:01:34,470 --> 00:01:38,540 In Python 2 + 2 is called an "expression". 25 25 00:01:38,540 --> 00:01:42,100 It's the most basic kind of programming instruction in the Python language. 26 26 00:01:42,100 --> 00:01:45,840 Expressions consist of values, such as the 2s, 27 27 00:01:45,840 --> 00:01:49,080 and operators, such as the +. 28 28 00:01:49,080 --> 00:01:52,150 Expressions always evaluate, that is 29 29 00:01:52,150 --> 00:01:55,460 they always reduce down to a single value. 30 30 00:01:55,460 --> 00:01:59,750 So in this example 2 + 2 is evaluated down to a single value 31 31 00:01:59,750 --> 00:02:03,990 4. You can even think of the single value by itself as an expression: 32 32 00:02:03,990 --> 00:02:07,610 the expression 2 evaluates to itself, 33 33 00:02:07,610 --> 00:02:10,950 2. There's several other types of operators we can try out. 34 34 00:02:10,950 --> 00:02:15,200 Type the following: 5 - 3, which does subtraction 35 35 00:02:15,200 --> 00:02:19,050 and evaluates 2. 3 * 7, 36 36 00:02:19,050 --> 00:02:23,850 which does multiplication and evaluates to 21. Note that multiplication is done with 37 37 00:02:23,850 --> 00:02:24,120 the 38 38 00:02:24,120 --> 00:02:29,540 asterisk and not the X. If you try to type something like 39 39 00:02:29,540 --> 00:02:32,739 3 x 7, Python will just give you an error. 40 40 00:02:32,739 --> 00:02:35,750 For division we use the forward slash. 41 41 00:02:35,750 --> 00:02:39,690 So 22 / 7 evaluates to 42 42 00:02:39,690 --> 00:02:43,630 3.14 and there are few other less common math 43 43 00:02:43,630 --> 00:02:48,400 operators so check the course notes for them. Now, order of operation rules apply here. 44 44 00:02:48,420 --> 00:02:52,890 Multiplication and division are done first, then addition and subtraction. 45 45 00:02:52,890 --> 00:02:56,799 but you can use parentheses to override the usual precedence if you need to. 46 46 00:02:56,799 --> 00:03:02,000 So let's look at two similar expressions: 2 + 3 * 6, 47 47 00:03:02,000 --> 00:03:03,780 which evaluates to 20. 48 48 00:03:03,780 --> 00:03:07,580 And then take (2 +3), in parentheses, 49 49 00:03:07,580 --> 00:03:11,010 * 6, which evaluates to 30. 50 50 00:03:11,010 --> 00:03:14,510 So I have a visualization tool so 51 51 00:03:14,510 --> 00:03:18,030 that we can see how Python evaluates expressions step-by-step. In 52 52 00:03:18,030 --> 00:03:23,159 2 + 3 * 6, the multiplication is done 53 53 00:03:23,159 --> 00:03:26,739 first. So 3 * 6 evaluates to 18, and 54 54 00:03:26,739 --> 00:03:31,640 2 + 18 evaluates to 20. While the parentheses 55 55 00:03:31,640 --> 00:03:35,420 overrides the multiplication, so 2 +3 is done 56 56 00:03:35,420 --> 00:03:39,860 first. And a value just in parentheses by itself just 57 57 00:03:39,860 --> 00:03:43,859 evaluates to the value, and then 5 time 6 evaluates to 30. 58 58 00:03:43,859 --> 00:03:47,030 No matter how big the expression is Python evaluates it 59 59 00:03:47,030 --> 00:03:51,890 until it becomes a single value. Let's use this expression as an example: 60 60 00:03:51,890 --> 00:04:06,100 (5 - 1) * ((7 + 1) / (3 - 1)) 61 61 00:04:06,129 --> 00:04:09,239 No matter how complicated the expression is, 62 62 00:04:09,239 --> 00:04:12,359 all expressions are just values and operators, and it 63 63 00:04:12,359 --> 00:04:16,639 always evaluates down to a single value. You can see this 64 64 00:04:16,639 --> 00:04:22,990 step-by-step in the visualization tool: 7 +1 65 65 00:04:22,990 --> 00:04:26,570 evaluates to 8. 3 -1 then evaluates 66 66 00:04:26,570 --> 00:04:31,639 and then the operation inside the parnetheses evaluate 67 67 00:04:31,639 --> 00:04:35,280 and then finally it evaluates down to 16. 68 68 00:04:35,280 --> 00:04:39,870 All of these expressions evaluate to the same thing you can copy and paste 69 69 00:04:39,870 --> 00:04:43,610 any of these and they evaluate 70 70 00:04:43,610 --> 00:04:51,199 to the same value. 71 71 00:04:51,199 --> 00:04:56,010 If you type something that Python can't understand it will display an error 72 72 00:04:56,010 --> 00:04:58,349 message so let's trigger one of those in IDLE right now. 73 73 00:04:58,349 --> 00:05:01,360 Type 5 + and press Enter. 74 74 00:05:01,360 --> 00:05:05,440 You can always test to see whether instruction works by typing it in the 75 75 00:05:05,440 --> 00:05:06,419 interactive shell. 76 76 00:05:06,419 --> 00:05:10,700 These errors don't damage the computer. Don't worry. Professionals software developers get 77 77 00:05:10,740 --> 00:05:14,870 error message while writing code all the time. So don't think that you 78 78 00:05:14,870 --> 00:05:17,690 messed anything up. 79 79 00:05:17,690 --> 00:05:20,130 All values have a data type. A data type is 80 80 00:05:20,130 --> 00:05:23,700 a category of values, and every value belongs to exactly 81 81 00:05:23,700 --> 00:05:27,280 one data type. So whole number values like 82 82 00:05:27,280 --> 00:05:30,630 -2 and 30, for example, are integers. 83 83 00:05:30,630 --> 00:05:34,490 Numbers with a decimal points such as 3.14 are called 84 84 00:05:34,490 --> 00:05:38,700 floating point numbers. We also call integer and floating point values 85 85 00:05:38,700 --> 00:05:41,820 ints and floats for short. 86 86 00:05:41,820 --> 00:05:44,940 Note that even though 42 is an integer, 87 87 00:05:44,940 --> 00:05:51,430 42.0 is a float. So there's also data type for text values. These are called strings. 88 88 00:05:51,430 --> 00:05:55,980 When typing strings begin and end them with a single quote so Python knows 89 89 00:05:55,980 --> 00:05:57,590 where the text begins and ends. 90 90 00:05:57,590 --> 00:06:01,490 For example type 'Hello world'. 91 91 00:06:01,490 --> 00:06:06,300 When the plus operator is used on strings 92 92 00:06:06,300 --> 00:06:09,580 it's the "string concatenation" operator. Concatenation 93 93 00:06:09,580 --> 00:06:12,680 means, "joins the strings together". For example let us type 94 94 00:06:12,680 --> 00:06:15,780 'Alice' + 'Bob' 95 95 00:06:15,780 --> 00:06:20,290 And this expression evaluates down to be single string 96 96 00:06:20,290 --> 00:06:23,990 'AliceBob'. Less common is the multiplication 97 97 00:06:23,990 --> 00:06:28,600 operator for the... for string replication which operates on an integer and a string. 98 98 00:06:28,610 --> 00:06:32,430 This is handy if you have to make a string repeat itself. You can type in 99 99 00:06:32,430 --> 00:06:36,760 a string 'Alice' * 3 to create 100 100 00:06:36,760 --> 00:06:40,890 the string 'AliceAliceAlice'. Or try this expression which 101 101 00:06:40,890 --> 00:06:44,700 uses string concatenation and replication. A string 'Hello' 102 102 00:06:44,700 --> 00:06:48,620 + '!' 103 103 00:06:48,620 --> 00:06:53,730 * 10. And that will evaluate to a string value that has 104 104 00:06:53,730 --> 00:06:59,660 'Hello' with 10 exclamation marks after it. Your programs can save values to the 105 105 00:06:59,660 --> 00:07:01,470 computer's memory with variables. 106 106 00:07:01,470 --> 00:07:06,700 A variable is kind of like a box that can store a single value. 107 107 00:07:06,700 --> 00:07:10,430 You can do this with an assignment statement. In the interactive shell 108 108 00:07:10,430 --> 00:07:14,090 enter spam = 42. 109 109 00:07:14,090 --> 00:07:17,670 This creates a new variable named spam 110 110 00:07:17,670 --> 00:07:21,950 and stores the value 42 inside of it. 111 111 00:07:21,950 --> 00:07:26,070 You can think of it as a box with the label "spam" 112 112 00:07:26,070 --> 00:07:30,500 and it has the value 42 in it. And normally you would give variables names that describe 113 113 00:07:30,500 --> 00:07:34,440 the data they contain. Imagine if you were moving and you labeled all of your boxes 114 114 00:07:34,440 --> 00:07:40,200 "stuff". That would be too helpful. But for these lessons we just need a generic name so 115 115 00:07:40,240 --> 00:07:44,520 the words spam, eggs, bacon, and ham are often used in Python tutorials 116 116 00:07:44,520 --> 00:07:48,100 because of the spam comedy sketch from Monty Python. 117 117 00:07:48,100 --> 00:07:52,360 "Egg, bacon, sausage, and spam. Spam, bacon, sausage, and spam. Spam, egg, spam, spam, bacon, and spam. Spam, spam, spam, spam, egg, & spam. Spam-" 118 118 00:07:52,360 --> 00:08:02,250 You can use variables and expressions 119 119 00:08:02,250 --> 00:08:05,790 anywhere you would normally use values. A variable just evaluates 120 120 00:08:05,790 --> 00:08:09,130 to the value it contains. So when I type spam, 121 121 00:08:09,130 --> 00:08:12,290 that evaluates to 42. So in the interactive shell 122 122 00:08:12,290 --> 00:08:18,700 type spam = 'Hello'. That will store the string Hello' in the variable spam. 123 123 00:08:18,710 --> 00:08:22,140 Now when we have the expression spam + 'world'... 124 124 00:08:22,140 --> 00:08:26,820 oh, whoops, well Python does exactly what we tell it to 125 125 00:08:26,820 --> 00:08:30,410 which is why it doesn't have a space in between these words. Let's try that again. 126 126 00:08:30,410 --> 00:08:34,680 spam + ' world' with space in front. 127 127 00:08:34,680 --> 00:08:38,390 There we go. So this is just 128 128 00:08:38,390 --> 00:08:43,140 regular string concatenation because spam evaluates down to the 129 129 00:08:43,140 --> 00:08:46,300 string inside of it, and then that gets 130 130 00:08:46,300 --> 00:08:49,610 concatenated with the ' world' string. Additional assignment 131 131 00:08:49,610 --> 00:08:53,700 statements can change the value that's inside the variable. The old value is forgotten. 132 132 00:08:53,700 --> 00:08:58,760 This is called "overwriting" the variable. So I could set span to a new vari- 133 133 00:08:58,760 --> 00:09:02,510 uh, value such as 'Goodbye'. 134 134 00:09:02,510 --> 00:09:06,570 This same spam + ' world' expression 135 135 00:09:06,570 --> 00:09:10,780 evaluates to a different value. So you can think of 136 136 00:09:10,780 --> 00:09:14,430 the old value as just being tossed out and forgotten and the new value 137 137 00:09:14,430 --> 00:09:17,620 being assigned every time we have an assignment statement. 138 138 00:09:17,620 --> 00:09:22,070 Expressions are one kind of instruction in Python. The other kind are statements. 139 139 00:09:22,070 --> 00:09:25,810 Unlike expressions, statements don't evaluate to single value. 140 140 00:09:25,810 --> 00:09:29,850 Although an assignment statement can have an expression as part of itself. 141 141 00:09:29,850 --> 00:09:33,450 Let's try typing 142 142 00:09:33,450 --> 00:09:38,070 spam = 2 + 2. The 2 + 2 expression 143 143 00:09:38,070 --> 00:09:41,640 evaluates to 4 and so 4 is the value that gets 144 144 00:09:41,640 --> 00:09:45,030 assigned to spam. We can even use the variable itself 145 145 00:09:45,030 --> 00:09:48,260 to set the variable's new value. Try typing 146 146 00:09:48,260 --> 00:09:54,000 spam = 10 and then spam = spam + 1. 147 147 00:09:54,000 --> 00:09:56,260 This assigns spam a new value. 148 148 00:09:56,260 --> 00:09:59,570 The new value for spam is equal to the current value of spam, 149 149 00:09:59,570 --> 00:10:03,170 plus 1. Spam, spam, spam, variable, spam. 150 150 00:10:03,170 --> 00:10:07,930 So if a Python instruction evaluates to a single value, 151 151 00:10:07,930 --> 00:10:12,510 it's an expression. Otherwise it's a statement. But mostly we just generically 152 152 00:10:12,510 --> 00:10:14,270 called them instructions or code. 153 153 00:10:14,270 --> 00:10:17,860 To recap: IDLE is the editor program that you type code into. 154 154 00:10:17,860 --> 00:10:21,300 It has two parts: the interactive shell which runs code 155 155 00:10:21,300 --> 00:10:24,510 one instruction at a time, and the file editor 156 156 00:10:24,510 --> 00:10:30,220 where you'll take complete programs into. Expressions are made up of values and 157 157 00:10:30,220 --> 00:10:33,280 operators and they always evaluate down to one value. 158 158 00:10:33,280 --> 00:10:36,580 And every value belongs to exactly one data type. 159 159 00:10:36,580 --> 00:10:41,000 The data types we've seen so far are integers, floating point numbers, and strings. 160 160 00:10:41,090 --> 00:10:45,740 And strings contain text, and they start and end with a single quote character. 161 161 00:10:45,740 --> 00:10:49,890 And values can be stored in variables, which can then be used in expressions 162 162 00:10:49,890 --> 00:10:51,440 anywhere values are used.