Python – Part 2

1. Python – Part 2 Variables, Expressions and Statements

2. 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

3. 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’ Numbers Prepared by Department of Preparatory year

4. 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

5. 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

6. 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

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

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

9. 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

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

11. 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

12. 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

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

14. 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

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

16. 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

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

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

19. 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

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

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

22. 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

23. 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

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

25. 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

26. 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.

27. 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?’)

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

29. 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

30. Part 2 End