Programming for Linguists An Introduction to Python 29/11/2012.

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1 Programming for Linguists An Introduction to Python 29/11/2012

2 Today Exercises Conversion Functions Math Module String Methods For Loop In Operator Functions Keyboard Input

3 Ex. 1 Verzin een zin van minimum 5 woorden. Stop elk woord in een aparte variabele en print de zin op één regel d.m.v. een print statement.

4 word1 = “to” word2 = “be” word3 = “or” word4 = “not” print word1, word2, word3, word4, word1, word2

5 Ex. 2 Typ in interactieve modus “Mark + 4”. Waarom krijg je een foutmelding? Pas de code aan zodat je als resultaat van “Mark + 4” het cijfer 10.0 krijgt.

6 Mark = 6 print float(Mark + 4)

7 Ex. 3 Maak een nieuwe variabele aan met als value een integer getal. Schrijf een stukje code dat de computer laat checken of dit getal deelbaar is door twee. Zo ja, laat Python “(het getal) is een even getal” printen en in het andere geval “(het getal) is een oneven getal”.

8 nr = 10 quotient = nr/2 product = quotient*2 if product == nr: print nr,"is een even getal" else: print nr,"is een oneven getal"

9 OR with a simple google search “python + even number”: x = 10 if x % 2 == 0: print x,"is een even getal" else: print x,"is een oneven getal"

10 Ex. 4 In een lift kan maximum 150 kg. Maak 4 variabelen aan met een bepaald gewicht. Bereken het totale gewicht en als dit de maximum overschrijdt, print “De lift is te zwaar geladen.”. Als het gewicht precies 150 kg bedraagt, print “De lift heeft zijn maximum gewicht bereikt.”. Als het gewicht minder is dan 150 kg, print “De lift kan nog iemand meenemen.”

11 w1 = 35 w2 = 56 w3 = 84 w4 = 5 totaal = w1+w2+w3+w4 if totaal > 150: print "De lift is te zwaar geladen." elif totaal < 150: print "De lift kan nog iemand meenemen.” else: print "De lift heeft zijn maximum gewicht bereikt.”

12 Ex. 5 Maak 3 variabelen aan met als value een cijfer kleiner of gelijk aan 10. Schrijf een stukje code dat het gemiddelde berekent van die cijfers. Als dit cijfer niet kleiner is dan 5, print “Geslaagd.” Als het wel kleiner is, print “Niet geslaagd.”. Als het gelijk aan of groter is dan 7, print “Geslaagd met onderscheiding.” Als het gelijk aan of groter is dan 8, print “Geslaagd met grote onderscheiding.”.

13 from __future__ import division c1=6 c2=4 c3=12 gem = (c1+c2+c3)/3 if gem >= 8: print "Geslaagd met grote onderscheiding." elif 8 > gem >= 7 : print "Geslaagd met onderscheiding." elif 7 > gem >= 5: print "Geslaagd." else: print "Niet geslaagd."

14 Ex. 6 Met de functie len( ) kan je in Python de lengte van een argument weergeven. Om de lengte van bv. het woord “Nederlands” te bepalen, gebruik je de functie als volgt: len(“Nederlands”). Probeer deze functie eens uit op de zin “There’s no place like home” en probeer het resultaat te verklaren.

15 Ex. 7 Maak een nieuwe variabele aan met als value een naam. Laat Python met de len( ) functie uit de vorige oefening “ ‘(naam)’ heeft (aantal) letters.”.

16 naam = “Claudia” print “’”+naam+”’”, “heeft”, len(naam), “letters.”

17 Some General Remarks Always save your code in a script via the script mode in a new window after you tried it in interactive mode Please put all exercises in one script Do not give up  Google it!

18 Conversion Functions Python has a number of built-in functions To convert an argument into respectively an integer, a string and a floating-point number: int(20) str(20) float(20)

19 Python’s Math Module Module = a file that contains a collection of related functions If you want to use it, you have to import it first  import math  from __future__ import division Every import command is usually given at the beginning of a script

20 If you want to access one of the functions you have to use dot notation: e.g. use the square root function from the math module: import math math.sqrt(64) math.exp(8) math.pow(8, 2) # 8 raised to power 2 or import each function from the module: from math import sqrt, exp, pow sqrt(64)

21 Combinations are also possible  the argument of a function can also be a function e.g. math.sqrt(int(64.3)) note: the inside function is executed first See http://docs.python.org/release/2.6.2/lib rary/math.html for a full overview of all math functions in Python http://docs.python.org/release/2.6.2/lib rary/math.html

22 A string is a sequence of characters You can access the characters one at a time with the index in between square brackets  index always starts with zero! fruit = “bananas” first_letter = fruit[0] second_letter = fruit[1] last_letter = fruit[-1] Working with Strings

23 A segment of a string is called a slice Selecting a slice is similar to selecting a character e.g. s = “Monty Python” print s[0:5] print s[6:12] String Slices

24 The operator [n:m] returns the part of the string from the character with index n up to (but excludes) the character with index m Also possible: [:n]  from the beginning of the string up to the character with index n [n:]  from the character with index n till the end of the string

25 What is the result of the following? fruit = “bananas” fruit[3:3] fruit[:] fruit[1:] With which command can you replace fruit[1:2]

26 strings are immutable: you can’t change an existing string you can create a new string that is a variation on the original Starting from “Hello, world!”, make a new variable with the value “Jello, world!” using the bracket operator

27 Some String Methods in Python a = “this is a sentence.” a.capitalize( )  “This is a sentence.” a.lower( )  “this is a sentence.” a.upper( )  “THIS IS A SENTENCE.” a.title( )  “This Is A Sentence.” a.center(25)  “ this is a sentence. ” a.ljust(25)  “this is a sentence. ” a.rjust(25)  “ this is a sentence.”

28 a = “this is a sentence.” a.count(“this”)  1 a.startswith(“.”)  False a.endswith(“.”)  True a.replace(“a”, “no”)  “this is no sentence.” a.find(“i”)  2 (lowest index) a.index('i')  2 (lowest index) a.rfind('i')  5 (highest index) a.rindex('i')  5 (highest index)

29 greet = “Hello, world!” new_greet = “j” + greet[1:] print new_greet print new_greet.capitalize( ) This example concatenates a new first letter onto a slice of greet Which output would a new “print greet” give?

30 Relational operators work on strings ==, e.g. word1 == word2  is equal to word1 < word2  comes before (alphabetically) word1 > word2  comes after (alphabetically) String Comparison

31 In Python all the uppercase letters come before all the lowercase letters Tip: convert strings to a standard format, e.g. with the function.lower( ), before performing the comparison

32 a boolean operator that takes two elements and returns True if the first appears as a component of the second “a” in “banana” True “Python” in “Hello, hello” False The In Operator

33 When you need to process one character at a time: start at the beginning select each character in turn and do something with it continue to the end of the string This pattern of processing = traversal Traversal with a For Loop

34 fruit = “pear” for letter in fruit: print letter for elem in fruit: print elem for f in fruit: print f print f

35 Each time through the loop, the next character in the string is assigned to the variable after the for loop:  letter, elem, f The loop continues until no characters are left If you want to do something with each element in the body of the for loop, you have to use the same variable name as in the for loop Outside the for loop, the variable only has the last item as a value

36 word = “abracadabra” count = 0 for letter in word: if letter == “a”: count = count + 1 print count Looping and Counting

37 You have to initialize a variable (e.g. “count”) first and set it to 0 The for loop checks every character of the string and adds 1 to “count” if the character is an “a” When the for loop ends, the variable “count” contains the result

38 Some predefined functions: math.sqrt(64) a.upper(“hello”) len(“hello”) int( ) str( ) float( ) Functions

39 Creating New Functions You always have to define a new function: def name( ): #enter e.g. def print_address( ): print “Lange Winkelstraat 40” print “2000 Antwerpen” In the rest of the program you can call this function by typing print_address( )

40 Function = a named series of statements that performs a computation named: you define a function sequence of statements: e.g. assign (print) statements that are to be executed in the order predetermined by you

41 Rules for names = variables: first character cannot be a number you cannot use a Python keyword as a name case sensitive Some tips: try to choose a name that relates to what the function is doing avoid choosing the same name for a variable and a function

42 To end a function: in interactive mode  enter empty line in script mode  new line/empty line(s) Note: functions can take no, one or more argument(s) type(37) print_address( )

43 More arguments: >>>def print_word(token, number): print token*number >>>word = “Nevermore” >>>print_word(word, 3) Note: the names of the arguments are only temporary substitutes for other variables in the program: parameters

44 Composition Functions you created yourself can also be combined: def repeat_words( ): print_word(word, 3 ) print_word(word, 4 ) repeat_words ( )

45 Try it yourself: assign the word “python” to a variable, define a function that calculates and prints the word length of “python”, 1) using no argument 2) using the variable as an argument

46 1) word = “python” def word_length( ): print len(word) word_length( )

47 2) def word_length(word): print len(word) word_length(“hello”) text = “hi” word_length(text)

48 What would happen if you try to print the variable word outside the function if it is inside the function only? def word_length( ): word = “python” print len(word) print word

49 When you create a variable inside a function, it is local, i.e. it only exists inside the function !

50 Flow of Execution The order in which statements are executed Begins at the top of the program One at a time From top to bottom Functions do not change the order, but statements inside a function are only executed after the function is called

51 Functions are like a detour: statements are executed in order a function is called the program jumps to the body of the function and executes all statements inside the body (and does the same for functions within the function) the program continues with the statements under the function call

52 Arguments and Parameters def print_twice(franky): print franky the function assigns the argument to a parameter named franky whenever the function is called it will print the value of the parameter you pass into the function twice

53 Try: print_twice(‘spam’) print_twice(101) print_twice(123.45) What would happen if you try: print_twice(‘spam ’*5) print_twice(franky)

54 Why Use Functions? Less work for you: if you write a function once, you can use it again as much as you want during the program You can group a piece of your code, so that it is easy to read and debug If there is a bug, you only need to correct it once

55 Why Use Functions? (2) If you save them, you can import them in other scripts You can use the return values of fruitful functions in the rest of your program

56 Ex. Define a function that will count the letters of a word and prints the word + “has less than 5 letters” or “has more than 5 letters” or “no word given” if you put in an empty string “”.

57 Possible answer: def word_length(word): if len(word) < 5 and len(word) != 0: print word + “ has less than 5 letters” elif len(word) == 0: print "no word given” else: print word + " has more than 5 letters” word_length(“abracadabra”)

58 Keyboard Input In Python: raw_input( ) uses input from the keyboard When this function is called, the program stops and waits for the user to type something raw_input( ) always gives you a string input( ) assumes Python code

59 e.g. question = “What is your name? \n” print question name = raw_input(question) print “Welcome ” + name

60 For Next Week Tuesday Exercises on blackboard Deadline: 5/12

61 claudia.peersman@ua.ac.be Thank you