1. Programming for Linguists An Introduction to Python

2. Contact Claudia Peersman claudia.peersman@ua.ac.be Lange Winkelstraat 40, room L202 (2 nd floor)

3. Literature “Think Python. How to Think Like a Computer Scientist?” by Allen B. Downey freely available at: http://greenteapress.com/thinkpython/thinkpython.html http://greenteapress.com/thinkpython/thinkpython.html “Natural Language Processing with Python. Analyzing Text with the Natural Language Toolkit” by Steven Bird, Ewan Klein, and Edward Loper freely available at: http://www.nltk.org/book http://www.nltk.org/book

4. The Python programming language Part 1 Formal vs. natural languages The way of the program Programming for linguists? What is a program? Debugging Your first program

5. Formal vs. natural languages Natural Languages: spoken languages, e.g. English, Dutch, French… not designed by people evolved naturally Formal Languages: designed by people for specific applications, e.g.: in mathematics: notation which denotes relationships among numbers and symbols in chemistry: represent the chemical structure of molecules

6. Many features in common: tokens, structure, syntax and semantics A lot of differences: Natural LanguagesFormal Languages AmbiguityNearly unambiguous RedundancyCompact Idioms and metaphors Literal: they mean exactly what they say

7. Some Examples 5 + 5 = 10 H2O 5 + 5 = 1$0 ??? Zz ??? Illegal tokens $ and Zz 5 +: 5 = 10 ??? Legal tokens, but illegal structure +:

8. The way of the program Programming = the art of problem solving: formulate problems think creatively about possible solutions express a solution clearly and accurately trial and error

9. Low-level vs. high-level languages Low-level languages = “machine languages”: only language a computer can execute High-level languages like Python, Perl, Java, C++ need to be processed to a low- level language to be executed by: compilers interpreters

10. An interpreter: processes the program a little at a time alternates between reading lines and performing computations A compiler: translates the high-level language completely first once a program is compiled, it can be executed repeatedly without further translation

11. Programming for linguists? aim: handle large linguistic corpora automatic frequency counts distribution of linguistic features across different categories, corpora look up context existing tools are limited, cost money

12. About Python… high-level language open source executed by an interpreter in two ways: interactive mode script mode

13. interactive mode: open the interpreter >>> prompt = ready to begin type a command interpreter prints the results >>> 1 + 1 2

14. script mode: open a new window in the interpreter type a number of commands save the program as a python script: e.g. test.py the program is executed whenever you tell the interpreter to run it the results are printed when the script is run

15. Which mode to use? interactive mode: good for testing small parts of the program before you go on does not save the program! script mode: put together all small parts of code in a sequence of instructions for the computer to execute save your program use it again in the future

16. What is a program? a sequence of instructions that specifies how to perform a computation for linguists: the computation can also be e.g. looking up the context of words in a text, calculating average word lengths, sentence lengths, …

17. Some basic instructions input: data you type, a text you load output: display data on the screen, send data to a file math: perform basic mathematical operations like +, -, X, :

18. conditional execution: check for certain conditions and execute the appropriate instructions repetition: perform some action repeatedly, usually with some variation Programming = breaking a large, complex task into smaller and smaller subtasks until the subtasks are simple enough to be performed with one of these basic instructions

19. Debugging

20. Bugs = programming errors Debugging = process of tracking down programming errors Three kinds of bugs: syntax errors runtime errors semantic errors

21. Syntax errors refer to the structure of the program and the rules about that structure if there is even a single syntax error in your code: Python will display an error message the execution of your program will quit immediately

22. An example parentheses: (1 + 2) : correct syntax 2) : syntax error Syntax errors are very common in the beginning. The more you practice and gain experience, the fewer mistakes you will make and the faster you will find them.

23. Runtime errors also called exceptions do not appear until after the program has started to run Python will display an error message For example: you give the instruction to open a file, but you have typed in the wrong file name or wrong directory

24. Semantic errors The program will run perfectly, but it will not produce the results you wanted: the meaning of the program (semantics) is wrong Tricky errors, because: Python will not display an error message !! you need to work backward looking at the output of the program and try to figure out what it is doing exactly

25. An example Python function read( ) vs. readline( ) vs. readlines( )

26. Debugging is equally important to programming itself: not only learn how to write a program learn to write a program that works learn to write a program that does what you want it to do Always try out small pieces of code before you go on with writing your program Try out your code on short pieces of text, so that you can verify your results manually

27. Your first program open IDLE (desktop) The first program is usually called “Hello, world!” In Python: >>> print “Hello, world!”or >>> print ‘Hello, world!’ Mind the quotation marks!

28. This is the print statement The quotation marks mark the beginning and the end of the text to be displayed The quotation marks do not appear in the result

29. Why we teach Python: e.g. in Java: public class Hello { public static void main( String[] args ) { System.out.println( "Hello, World!" ); } }

30. Make some mistakes What happens if you: leave out one of the quotation marks replace “ by ‘ or vice versa in one case spell “print” wrong double the quotation marks double the quotation marks, but change the order

31. By making mistakes on purpose you will: learn which details are important in writing program code learn to debug more efficiently, because you get to know what the error messages mean

32. Try it yourselves We will make time to try out new things as we proceed Programming is a new way of thinking for linguists If there is a problem or you have a question, do not hesitate to mention it immediately

33. Values and Types values = basic elements of a program e.g. print “Hello, world!” each value has a type: integer string float

34. Integer: all non-decimal numbers e.g. 105 String: a string of letters e.g. “Hello, World!” Float: numbers with a decimal point e.g. 10.5 The interpreter can tell you the type of a value: >>> type(105)

35. Try to find out what the type is of the following values: “Hello!” 3.1415 Dag Jan “123” “123.456”

36. Try this: >>> print 123,456 Float types always have a dot, never a comma To which kind of error could this lead? runtime error syntax error semantic error

37. Variables A name that refers to a value An assignment statement creates new variables and assigns values to them You can choose the name yourself e.g. >>> text = “Everything except ‘Hello, world!’” >>> age = 26 >>> pi = 3.1415

38. The variables now carry the values we assigned to them: >>> print text >>> print age >>> print pi The interpreter can again tell you the type: >>> type(text)

39. Variable names: can be arbitrarily long can contain both letters and numbers have to begin with a letter can contain uppercase letters are case sensitive ! If you use an illegal character in your name, you will get a syntax error message: e.g. my name, live@

40. You cannot choose a name that is a keyword in Python: and del from as elif global assert else if break except import class exec in continue finally is def for lambda not while or with pass yield print raise return try Tip: try to choose names which describe what the variable is used for

41. Statements Units of code that the Python interpreter can execute So far we have seen the print statement and the assignment statement A program usually contains a series of statements that are executed in an order predetermined by the programmer

42. e.g. >>> age1 = 20 >>> age2 = 40 >>> print age2 40 >>> average_age = (age1 + age2)/2 >>> print average_age 30

43. You always have to assign a value to a variable before you can work with it Variables have to be spelled in the same way throughout the program If you assign a new value to an existing variable, the old value is deleted e.g. >>>age = 20 >>>age = age + 20 >>>print age

44. Operators and Operands Operators = special symbols that represent computations e.g. +, -, *, /, ** Operands = the values the operator is applied to e.g. 2 + 2 Try 2/3

45. When both operands are integers, the result is again an integer If you want a floating-point result, you have to make one of the operands a floating-point number: >>> 2/3.0 0.66666666666666663 you can also give a command at the beginning of your script: from __future__ import division

46. Expressions A combination of values, variables, and operators Try: >>>x = 5 >>>x + 1 Now make a script of it (File  New window) and run it (Run  Run module)

47. In a script an expression all by itself does not print a result !!! How can you modify the script so that it does produce a result ?

48. Order of Operations The order of evaluation depends on the rules of precedence For mathematical operators, Python follows mathematical conventions: Parentheses Exponentiation Multiplication and division Addition and subtraction

49. String Operations In general: no mathematical operations on strings e.g. “hello”/ “hi”  TypeError: unsupported operand type(s) for /: 'str' and 'str’ Except: the + and * operators

50. Try: “hello” + “hi” “hello”*2 String + string = concatenation string * int = repetition

51. An expression that is either True or False e.g. the operator == >>>5 == 5 True >>>5 == 6 False True & False: not string Boolean Expressions

52. x == yx is equal to y x != y x is not equal to y x > y x is greater than y x < y x is smaller than y x >= y x is greater than or equal to y x <= y x is smaller than or equal to y Relational Operators

53. Remember that: = is an assignment operator  used to assign a value to a variable == is a relational operator  used to express equality order is again important =, =!  do not work!

54. and or not Return a boolean expression: True or False Logical Operators

55. Which would return True? x > 0 and x < 5 x == 3 or x == 4 not(x > 5)

56. Conditional statements check conditions and change the behaviour of the program accordingly if statement: e.g. >>>if x > 0 : print “x is positive” #body Conditional Execution

57. Only if the condition is True, the print statement will be executed There is no limit on the number of statements that can appear in the body There has to be at least one statement in the body You can use pass as a temporary substitute for code you have not written yet: if x > 0: pass

58. There are more than 2 possibilities if, elif (else if) & else e.g. if x > y : print “x is greater than y” elif x < y : print “x is smaller than y” else : print “x is equal to y” Chained Conditionals

59. There is no limit on the number of elif statements Every elif statement has to contain at least one statement The else statement has to come at the end, but is not necessary Each condition is checked in order

60. What would the result be if x = 8? if x == 0: print “x is 0” elif x > 0: print “x is greater than 0” elif x > 0 and x <10: print “x is between 0 and 10” else: print “x is smaller than 0”

61. If one of the conditions is True, the corresponding branch executes and the statement ends Even if more than one condition is True, only the first True branch executes !

62. Some Comments As programs grow and become more complicated, they get more difficult to read You can add notes which explain (for yourself and for others who read your code) what the program is doing: start a piece of code with “#” and add your comment everything from the # to the end of the line is ignored by the program

63. Put 2 numbers in different variables Print the results for the operands +, -, *, /, ** when they are applied to these 2 variables (with floating-point numbers as a result for division) Exercises

64. x = 2 y = 3 print "x =", x print "y =", y print "x + y =", x + y print "x - y =", x - y print "x * y =", x * y print "x / y =", x / float(y) print "x**y =", x**y

65. For Next Week… Write a script called yourname_ex1.py that calculates the average weight of 5 variables: 36.5 kg 47.8 kg 33 kg 68.3 kg 72 kg

66. Write a script called yourname_ex2.py that assigns an integer value to a variable “age” and prints “you are a minor” if the value is under 18, that prints “you are over 18” if the value is 18 or more and prints “you are kidding” if the value is less than 0.

67. Python install (please use these links): for windows: http://www.python.org/ftp/python/2.6.5/py thon-2.6.5.msi http://www.python.org/ftp/python/2.6.5/py thon-2.6.5.msi for mac: http://www.python.org/ftp/python/2.6.5/py thon-2.6.5-macosx10.3-2010-03-24.dmg http://www.python.org/ftp/python/2.6.5/py thon-2.6.5-macosx10.3-2010-03-24.dmg

68. Please mail by Tuesday next week: the scripts from the exercises the subject of your dissertation claudia.peersman@ua.ac.be Thank you