Understanding the Basics

1. Understanding the Basics

2. PRINTING

3. Some Python codes • 6+9 • Print 6+9 • Print “6+9” • aa=6+9 print aa • aa=6 bb=9 aa+bb

4. Print Python command print – Displays data, values, and expressions The single and double quotation mark string objects, which are collections of texts surrounded by quotes.

5. Be familiar with Python printing >>> print ("Hello, World! ") Hello, World! >>> "hello" 'hello' >>> "world" 'world' >>> "hello"+"world" 'helloworld'

6. Be familiar with Python printing >>> "hello" * 3 'hellohellohello' >>> "hello" * 300

7. Variables • Are not declared, just assigned • The variable is created the first time you assign it a value • Are references to objects • Type information is with the object, not the reference • Everything in Python is an object

8. Variables • variable: A named piece of memory that can store a value. • Usage: • Compute an expression's result, • store that result into a variable, • and use that variable later in the program. • assignment statement: Stores a value into a variable. • Syntax: name = value • Examples: x = 5 gpa = 3.14 x 5 gpa 3.14 • A variable that has been given a value can be used in expressions. x + 4 is 9 • Exercise: Evaluate the quadratic equation for a given a, b, and c.

9. Everything is an object • Everything means everything, including functions and classes (more on this later!) • Data type is a property of the object and not of the variable >>> x = 7 >>> x 7 >>> x = 'hello' >>> x 'hello' >>>

10. Look at a sample of code… assignments, arithmetics, if, print Look at a sample of code… x = 34 - 23 # A comment. y = “Hello”# Another one. z = 3.45 if z == 3.45 or y == “Hello”: x = x + 1 y = y + “ World”# String concat. print x print y

11. Declare a variable and assign its value Example: Area of a rectangle >>> width = 20 >>> height = 45 >>> area = width * height >>> area 900

12. Evaluation of expressions in Python >>> print ("Hello, World! ") Hello, World! >>> 10 + 25 35 >>> 124 – 125 -1 >>> 1/3 0.333333333333333

13. Print of long lines • print : Produces text output on the console. • Syntax: print "Message" print Expression • Prints the given text message or expression value on the console, and moves the cursor down to the next line. print Item1, Item2, ..., ItemN • Prints several messages and/or expressions on the same line. • Example: print "Hello, world!" age = 45 print "You have", 65 - age, "years until retirement" Output: Hello, world! You have 20 years until retirement

14. Enough to Understand the Code • Assignment uses = and comparison uses ==. • For numbers +, -, *, /, % are as expected. • Special use of + for string concatenation. • Special use of % for string formatting. • Logical operators are words (and, or, not) not symbols (&&, ||, !). • The basic printing command is “print.” • First assignment to a variable will create it. • Variable types don’t need to be declared. • Python figures out the variable types on its own.

15. Basic Datatypes • Integers (default for numbers) z = 5 / 2 # Answer is 2, integer division. • Floats x = 3.456 • Strings Can use “ ” or ‘ ’ to specify. “abc” ‘abc’ (Same thing.) Unmatched ones can occur within the string. “matt’s” Use triple double-quotes for multi-line strings or strings than contain both ‘ and “ inside of them: “““a‘b“c”””

16. Comments • Start comments with # – the rest of line is ignored. • Can include a “documentation string” as the first line of any new function or class that you define. • The development environment, debugger, and other tools use it: it’s good style to include one. defmy_function(x, y): “““This is the docstring. This function does blah blah blah.”””# The code would go here...

17. Python Build-in types • Numbers 3.1415 • Strings “Hello World” • Lists [1,2,3,4] • Dictionaries {‘test’: ‘yum’} • Files input=open(‘file.txt’, ‘r’)

18. TEST: What are the outputs? Why? >>> a=3 >>> b=4 >>> a+1 >>> b*3 >>> b/3 >>> b/3.0 >>> b**2 >>> 2+4.0 >>> 2.0**b

19. Hello World Program

20. Hello World Program • Implement by three different languages • In C • In JAVA • In Python

21. “Hello World” in C main() { printf("hello, world\n"); }

22. “Hello World” in C #include <iostream> using namespace std; main() { cout<<“Hello World” ; }

23. “Hello World” in JAVA class myfirstjavaprog { public static void main(String args[]) { System.out.println("Hello World!"); } }

24. “Hello World” in Python print "hello World!"

25. “Hello World” in Python print (“Hello World!!”)

26. Your first Python program- hello.py # This program says hello and asks for my name. hello = 'Hello world! ' print(hello) print('What is your name?') myName = input() print('It is good to meet you, ' + myName)

27. hello.py executed sequentially # This program says hello and asks for my name. Any text following a # sign is a comment. Comments are not for the computer, but for you, the programmer.  hello = 'Hello world! ' # assign the string to a name print(hello) # call the print( ) function print('What is your name?') # call the print( ) function myName = input() # call the input( ) function print('It is good to meet you, ' + myName)

28. Arithmetic Expressions • expression: A data value or set of operations to compute a value. Examples: 1 + 4 * 3 42 • Arithmetic operators we will use: • + - * / addition, subtraction/negation, multiplication, division • % modulus, a.k.a. remainder • ** exponentiation • precedence: Order in which operations are computed. • * / % ** have a higher precedence than + -1 + 3 * 4 is 13 • Parentheses can be used to force a certain order of evaluation.(1 + 3) * 4 is 16

29. Integer division • When we divide integers with / , the quotient is also an integer. 3 52 4 ) 14 27 ) 1425 12135 2 75 54 21 • More examples: • 35 / 5 is 7 • 84 / 10 is 8 • 156 / 100 is 1 • The % operator computes the remainder from a division of integers. 3 43 4 ) 14 5 ) 218 1220 2 18 15 3

30. Real numbers • Python can also manipulate real numbers. • Examples: 6.022-15.999742.02.143e17 • The operators +-*/% **() all work for real numbers. • The / produces an exact answer: 15.0 / 2.0 is 7.5 • The same rules of precedence also apply to real numbers:Evaluate () before */% before +- • When integers and reals are mixed, the result is a real number. • Example: 1 / 2.0 is 0.5 • The conversion occurs on a per-operator basis. • 7 / 3 * 1.2 + 3 / 2 • 2 * 1.2 + 3 / 2 • 2.4 + 3 / 2 • 2.4 + 1 • 3.4

31. Math commands • Python has useful commands for performing calculations. • To use many of these commands, you must write the following at the top of your Python program: from math import *

32. Numbers

33. Number in Action • Perhaps the best way to understand numerical objects is to see them in action. >>> a=3 #Name Created >>> b=4

34. Number in Action >>> a+1, a-1 # (3+1), (3-1) (4,2) >>> b*3, b/2 (12,2)

35. Number in Action >>> 1 / 2.0 0.5 >>> 1/2 0 >>> 1 % 2 1

36. Simple Data Types • Triple quotes useful for multi-line strings >>> s = """ a long... string with "quotes" or anything else""">>> s' a long\012string with "quotes" or anything else' >>> len(s)45

37. Numbers: Integers • Integer – the equivalent of a C long • Long Integer – an unbounded integer value. >>> 132224 132224 >>> 132323 ** 2 17509376329L >>>

38. Numbers: Floating Point • int(x) converts x to an integer • float(x) converts x to a floating point • The interpreter shows a lot of digits >>> 1.23232 1.2323200000000001 >>> print 1.23232 1.23232 >>> 1.3E7 13000000.0 >>> int(2.0) 2 >>> float(2) 2.0

39. exercises • Geometric calculations for robots • Speed and acceleration • Pythagoras Theorem • Braitenberg Vehicles. • Logic operators. • Defining functions. • If

40. Numbers: Complex • Built into Python • Same operations are supported as integer and float >>> x = 3 + 2j >>> y = -1j >>> x + y (3+1j) >>> x * y (2-3j) End of lecture 1

41. 4.5 4.5 7.5 x x y y Numbers are immutable >>> x = 4.5 >>> y = x >>> y += 3 >>> x 4.5 >>> y 7.5

42. Strings

43. Strings • The next major build-in type is the Python STRING --- an ordered collection of characters to store and represent text-based information • Python strings are categorized as immutable sequences --- meaning they have a left-to-right order(sequence) and cannot be changed in place (immutable) • This definition of immutable sequences is sufficient for now. You will learn more from examples.

44. Single- and Double-Quoted • Single- and Double-Quoted strings are the same >>> ‘Hello World’ , “Hello World” • The reason for including both is that it allows you to embed a quote character of the other inside a string >>> “knight’s” , ‘knight”s’

45. Strings in Action • Basic operations: 1. len() 2. + 3. * length concatenate repetition

46. len() • The len build-in function returns the length of strings >>> len(‘abc’) >>> a=‘abc’ >>> len(a) length

47. + • Adding two string objects creates a new string object >>> ‘abc’ + ‘def’ >>> a=‘Hello’ >>> b=‘World’ >>> a + b >>> a+ ‘ ’ +b Hello World with space Concatenation of strings

48. * • Repetition may seem a bit obscure at first, but it comes in handy in a surprising number of contexts • For example, to print a line of 80 dashes >>> print ‘-’ * 80

49. Slices and Indexes

50. Indexes in slices • Characters in a string are numbered with indexes starting at 0: • Example: name = "P. Diddy" • Accessing an individual character of a string: variableName[index] • Example: print name, "starts with", name[0] Output: P. Diddy starts with P