Variables, Expressions and Statements Python – Part 2

Prepared by Department of Preparatory year Values and Types Values Basic things program works with e.g. letter, number 1, 2, ‘Hello World!’ Types Values belong to different types 2 is an interger ‘Hello World!’ is a string Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Numbers Integers: 12 0 -12987 0123 0X1A2 Type ‘int’ Can’t be larger than 2**31(2 31) Octal literals begin with 0 (0981 illegal!) Hex literals begin with 0X, contain 0-9 and A-F Floating point: 12.03 1E1 -1.54E-21 Type ‘float’ Same precision and magnitude as C double Long integers: 10294L Type ‘long’ Any magnitude Python usually handles conversions from int to long Complex numbers: 1+3J Type ‘complex’ Prepared by Department of Preparatory year

Prepared by Department of Preparatory year String >>> print "Per's lecture“ Per's lecture Single quotes or double quotes can be used for string literals Produces exactly the same value Special characters in string literals: \n newline, \t tab, others Triple quotes useful for large chunks of text in program code >>> print "One line.\nAnother line.“ One line. Another line. >>> print """One line, another line.""“ One line, another line. Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Values and Types Print statement for integers >>>print 4 4 Can check the value type >>> type(‘Hello world!’) <type ‘str’> >>> type (17) <type ‘int’> Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Values and Types Strings belog to the type str Integers belong to the type int Numbers with a decimal point belong to the type float. >>>type (3.2) <type ‘float’> Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Values and Types What about ’17’ and ‘3.2’? >>>type (’17’) <type ‘str’> >>>type (‘3.2’) Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Values and Types >>>print 1,000,000 Output ? 1 0 0 Semantic error Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Variables Variable is a name that refers to a value Assignment statement creates new variables and gives them values. >>>message=‘New message’ >>>n=17 >>>pi=3.1415926535897931 Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Variables Use print statement to display the value of a variable >>> print n 17 >>> print message New message Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Variables Type of variable is the type of value it refers to(The type of the variable is determined by Python) >>>type (pi) <type ‘float’> >>>type (n) <type ‘int’> >>>type (message) <type ‘str’> Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Variable Names Can contain both letters and numbers Begin with a letter Good idea to begin variable names with a lowercase letter Underscore character (_) can appear in a name (often in names with multiple words), e.g. my_name The variable name is case sensitive: ‘val’ is not the same as ‘Val’ Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Variable Names >>> 76trombones = 'big parade' SyntaxError: invalid syntax >>>more@=100000 SytaxError: invalid syntax >>>class =‘CS104’ Class -> one of Python’s keywords Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Python Keywords And del from not while As elif global or with Assert else if pass yield Break except import print Class exec in raise Continue finally is return Def for lambda try Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Statements Unit of code that Python interpreter can execute (print, assignment statement) print 1 x=2 print x Prepared by Department of Preparatory year

Operators and operands Operators – special symbols that represent computations (e.g. addition, division) Values the operator is applied to are called operands + addition - subtraction * multiplication / division ** exponentiation % modulus Prepared by Department of Preparatory year

Operators and operands 20+32 Hour-1 Hour*60+minute 5**2 (5+9)*(15-7) 7%3 Prepared by Department of Preparatory year

Operators and Operands >>>minute=59 >>>minute/60 0 ? Prepared by Department of Preparatory year

Operators and operands If both operands are integers, result is also an integer If either of the operands is a floating-point number Python performs floatin-point division; result is a float >>>minute/60.0 0.98333333333333328 In Python 3.0 or later the result is a float // operator performs integer division Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Expressions Combination of values, variables and operators 17 X X+17 Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Expressions >>>1+1 2 In a script, expression by itself doesn’t do anything. Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Order of operations Order of evaluation depends on rules of precedence. Python follows mathematical convention Parentheses – highest precedence Exponentiation –next highest precedence Multiplication, Division, Modulus (same precedence) Addition and Subtraction (same prec.) Same precedence operators – left to right Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Order of operations 2*(3-1) (1+1)**(5-2) 2**1+1 3*1**3 2*3-1 6+4/2*3 7%3+8/2 Prepared by Department of Preparatory year

Prepared by Department of Preparatory year String operations Concatenation operator + first =‘CS’ second=‘104’ print first+second Repitition operator * ‘spam’*3 ‘spamspamspam’ Prepared by Department of Preparatory year

Prepared by Department of Preparatory year Comments Notes that can be added to program to explain what the program is doing Start with the # symbol #compute the percentage of the hour that has elapsed percentage=(minute*100)/60 percentage=(minute*100)/60 #percentage of an hour Prepared by Department of Preparatory year

Reading Input from the Keyboard Programs commonly need to read input typed by the user on the keyboard. We will use the Python functions to do this. Python uses built-in functions to read input from the keyboard. A function is a piece of prewritten code that performs an operation and then returns a value back to the program. The input function can be used to read numeric data from the keyboard.

Reading Input from the Keyboard Reading Numbers with the input Function Use the input function in an assignment statement: variable = input (prompt) where, variable name of the variable that will reference the data = assignment operator input name of the function prompt string that is displayed on the screen For example: hours = input (‘How many hours did you work?’)

Reading Input from the Keyboard Reading Strings with the raw_input Function The raw_input function retrieves all keyboard input as a string. >>> name = raw_input(‘Enter your name:’) >>> print name Enter your name: Ahmad

Prepared by Department of Preparatory year Write scripts in IDLE Now we need to write proper scripts, saved in files In IDLE: 'File' 'New Window' Do immediately 'Save as…' Browse to directory 'Desktop' Create a directory 'Python course' Go down into it Enter the file name 't1.py' Save Work in the window called 't1.py' Enter the following code: Save the file: Ctrl-S, or menu 'File', 'Save' Run the script: F5, or menu 'Run', 'Run Module' "file Ex1.py" # this is a documentation string print "Hello world!" Prepared by Department of Preparatory year

End Part 2