PYTHON VARIABLES CHAPTER 2 FROM THINK PYTHON HOW

PYTHON VARIABLES : CHAPTER 2 FROM THINK PYTHON HOW TO THINK LIKE A COMPUTER SCIENTIST

VALUES AND TYPES A value is one of the basic data objects that a Python program works with. Two of the most used values are numbers and strings of letters. These values are categorized into types which include numbers(float, integer, string) Examples: 23 is an integer 3. 14159 is a floating point (float for short) ‘Hello World!’ is a string of characters >>> print 23, 3. 14, ’This is a cool class’ 23 3. 14 This is a cool class

TYPE() FUNCTION There is a function called type() that will give you (return) the type of a value. >>>type(23) <class ‘int’> >>>type(3. 456) <class ‘float’> >>>type(‘Hello’) <class ‘str’> >>>type(‘ 45’) <class ‘str’) Note this one!!!!

VARIABLES A variable is a name that refers to a value. A assignment statement is the usual way to create variables and assign values to them. Here a few examples N = 34 PI = 3. 1415926535 Name = ‘Richard Simpson’ The type of a variable is the type of the value it is assigned to. For example >>>type(PI) <class ‘float’) Note: if you change the value to a different type its type will change also!

VARIABLE NAMES • Variables names can be arbitrarily long. • The can contain numbers and letters and underscore. • Upper and lower case is acceptable. • A variable name cannot start with a number • Keywords are not allowed. These include • And, as, break, if, else, elif, for, from, import, in, not, or, return, while with, as well as some others.

STATEMENTS A statement (instruction) is a unit of code that the interpreter can execute. At this point we have only seen two statements, the print statement and the assignment statement. These are executed one at a time in the interactive mode. A script, ie as sequence of instructions, will also execute one at a time but much faster. Here is a script print 34 x=4 Print x There are many other statements that we shall learn

OPERATORS AND OPERANDS Operators are used to construction expressions The usual mathematical operators such as +, -, * and / should be quite familiar to you. Here are some example expressions 2+3 7+4*8 #what is the result here? 3. 24 + 5*(4. 0 -3. 2) 8 – 2**3 4. 0//3 # ** is the power (exponentiation) operator #divide and drop the decimal (floor division) 4/3 #also floor division. Result is 1 4. 0/3 # result here is 1. 33333333

EXPRESSION EVALUATION IN INTERACTIVE MODE >>>2+2 4 >>> 4//3 1 >>> 34 % 3 # divide and return remainder (modulus) 1 >>>2*2**3 16 # ** has higher precedence than *

ORDER OF OPERATIONS 1. Parentheses has the highest precedence 2. Exponentiation 3. Multiplication and Division 4. Addition and Subtraction 5. Operators of equal precedence are L to R PEMDAS >>> 2+4*3**2 -5 -10/2 gives what?

STRING OPERATORS There are two operators that work on strings (+ and *) The + operators performs concatenation >>> ‘Soft’+’ware’ Software The * operators duplicates a string. For example >>> ‘Soft’*4 Soft The others don’t work.

COMMENTS Comments can be anywhere, but must follow # The interpreter will not read anything past the # symbol #This is a comment X=4 # so is this Start every program that you turn in with the following comment collection # Project # : Topic name # Your name # Explain what the program does

EXERCISES Assume that the following statements are executed Val = 10 Len =15. 5 Spacer = ‘_’ What do the following valuate to? Val/3 Val/2. 0 int(Len)*2 Spacer*10 Do exercise 2. 4 page 16 (all three) and study glossary 2. 11