Programming for Linguists An Introduction to Python 24/11/2011

From Last Week E x 1) def name( ): name = raw_input("what is your first name? ") length = len(name) last_letter = name[-1] print name," contains ", lenght, " letter(s) and ends with a(n) ”,last_letter name( )

Ex 2) def play( ): sentence = raw_input(“Sentence? ”) print sentence.upper( ) print sentence.lower( ) print sentence.title( ) print "The lowest index of the letter 'a' is”,sentence.index("a") if sentence.index("a")>3: print sentence.replace ("a","e") play( )

Ex 3) def tkofschip( ): verb=raw_input ("Please enter the root of a Dutch verb\n") if verb.endswith ("t") or verb.endswith("k") or verb.endswith ("f") or verb.endswith("s") or verb.endswith("c") or verb.endswith("h") or verb.endswith("p"): print verb+"te" else: print verb+"de" tkofschip( )

Fruitful Functions Functions which produce a result: calling the function generates a value >>>len(“python”) 6 Often contain a return statement: return immediately from the function and use the following expression as a return value

Try: def word_length1(word): return len(word) def word_length2(word): print len(word) a = word_length1(“hello”) b = word_length2(“hello”) type(a) type(b)

The return statement gives you a value which you can use in the rest of your script The print statement does not give you a value You can use multiple return statements, e.g.: def absolute_value(x): if x >= 0: return x else: return -x

You can return a value, a variable, a function, a boolean expression As soon as a return statement executes, the function terminates Code that appears after a return statement = dead code

Write a compare function that returns ‘1’ if x > y, ‘0’ if x == y, and ‘-1’ if x < y

def compare(x, y): if x == y: return 0 elif x > y: return 1 else: return -1

As we saw: one function can call another A function can also call itself A function that calls itself = recursive The process = recursion Recursion

Try this: def countdown(n): if n<=0: print ‘Happy Newyear!’ else: print n n = n - 1 countdown(n) countdown(10)

If a recursion never reaches a base case, it goes on making recursive calls forever  the program never terminates Generally not a good idea Python reports an error message when the maximum recursion depth is reached Infinite Recursion

e.g. def recurse( ): recurse( ) recurse( )

The while statement: used to perform identical or similar tasks def countdown(n): while n > 0: print n n = n – 1 print “Happy Newyear!”

3 steps: evaluate the condition, yielding True or False if the condition is True, execute the statements inside the body and return to step 1 ( = loop) if the condition is False, exit the while statement and continue with the execution of the next statement Mind the difference in indentation between the statements inside and outside the while statement !

The statements inside the body should change the value of one or more variables so that the condition becomes False at a certain point If a loop goes on forever = infinite loop You can use the break statement to jump out of the loop

This program will echo the keyboard input until the user types “done” while True: line = raw_input (“> ”) if line == “done”: break print line

Write a function that takes a string as an argument and prints the letters one by one using a while statement while index < len(fruit): letter = fruit[index] print letter index = index + 1

def print_letters(word): index = 0 while index < len(word): letter = word[index] print letter index = index + 1

Write a similar function that takes a string as an argument and prints the letters backward using a while statement

def print_backward(word): index = len(word) – 1 while index >= 0: letter = word[index] print letter index = index - 1

Lists A list is a sequence of values The values can be of any type The values = elements/items A list is always in between [ ] To create a new list: list = [10, “cheese”, 5.6, “this is a sentence”]

A list can contain another list (nested list): [‘hello’, 15, [‘my name is’, ‘Franky’]] to access them: index method list[:20] you can change existing lists numbers = [17, 21] numbers[1] = 10 print numbers [17, 10]

A list index works the same way as a string index: any integer expression can be used as an index if you try to read or write an element that does not exist, you get an IndexError if an index has a negative value, it counts backward from the end of the list the in operator also works on lists

Traversing a List For loop words = [‘work’, ‘run’, ‘play’, ‘jump’] for word in words: print word

if you need to update all elements: range function numbers = [1, 3, 5, 10] for elem in range(len(numbers)): numbers[elem] = numbers[elem] * 2 print numbers This loop traverses the list and updates each element

List Operations The + operator concatenates lists the * operator repeats a list a given number of times The slice operator [n:m] gives you a slice of the list

Try this: a = [1, 2, 3] b = [4, 5] print a + b print a*2 print a[1:2]

List Methods Python provides methods that operate on lists append method: a = [‘a’, ‘b’, ‘c’] a.append(‘d’) print a [‘a’, ‘b’, ‘c’, ‘d’]

deleting elements: using the index in the list: pop method  modifies the list and returns the element that was removed del method  modifies the list without returning the removed element remove method  if you do not know the index of the element

t = ['a', 'b', 'c’, ‘c’, ‘d’] x = t.pop(1) print t print x del t[0] print t t.remove(‘c’) print t

s = [2,1,4,3] s.count( )  4 s.sort( )  [1, 2, 3, 4] s.extend([5,6,7])  [1,2,3,4,5,6,7] s.insert(0,8)  [8,1,2,3,4,5,6,7] s.reverse( )  [7, 6, 5, 4, 3, 2, 1, 8]

From String to List From a word to a list of letters: list( ) s = “spam” print list(s) [‘s’, ‘p’, ‘a’, ‘m’] From a sentence to a list of words:.split( ) s = “This is a sentence” print s.split( ) [‘This’, ‘is’, ‘a’, ‘sentence’]

The split( ) function can also be used to split a string on other characters besides spaces s = “spam-spam-spam” print s.split(“-”) [‘spam’, ‘spam’, ‘spam’] “-” is called a delimiter in this case

From List to String Join( ) is the inverse of split( ) l = [‘this’, ‘is’, ‘a’, ‘sentence’] delimiter = “ ” delimiter.join(l) “this is a sentence”

List Arguments You can also pass a list into a function as argument def del_first(list1): del list1[0] return list1 del_first([1,2,3])

Many linguistic processing tasks involve pattern matching, e.g..startswith( ).endswith( ) To use regular expressions in Python we need to import the re library import re Regular Expressions for Detecting Word Patterns

.Wildcard, matches any character ^abcMatches some pattern abc at the start of a string abc$Matches some pattern abc at the end of a string [abc]Matches one of a set of characters [A-Z0-9] Matches one of a range of characters Some Basic Regular Expression Meta-characters

a|b|cMatches one of the specified strings (disjunction) *Zero or more of the previous item(s) +One or more of the previous item(s) ?Zero or one of the previous item(s) (i.e. optional) {n}Exactly n repeats where n is a non-negative integer {n,}At least n repeats

{,n}No more than n repeats {m,n}At least m and no more than n repeats a(b|c)+Parentheses that indicate the scope of the operators e.g. w(i|e|ai|oo)t matches wit, wet, wait and woot Matches any token In general, when using regular expressions it is best to use r'...' before the regular expressions

Counting all vowels in a given word: word='supercalifragilisticexpialidocious' vowels = re.findall(r'[aeiou]', word) nr_vowels = len(vowels) The re.findall( ) function finds all (non- overlapping) matches of the given regular expression

You can find a list of all regular expressions operations in Python on:

For Next Week Ex 1) Write a script that reads 5 words that are typed in by a user and tells the user which word is shortest and longest Ex 2) Write a function that takes a sentence as an argument and calculates the average word length of the words in that sentence

Ex 3) Take a short text of about 5 sentences. Write a script that will split up the text into sentences (tip: use the punctuation as boundaries) and calculates the average sentence length, the average word length and the standard deviation for both values How to calculate the standard deviation: tion

No lecture next week Some extra exercises will be posted on Blackboard instead.

Thank you!