Programming for Linguists An Introduction to Python 17/11/2011

From Last 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

w1 = 36.5 w2 = 47.8 w3 = 33 w4 = 68.3 w5 = 72 average_w = (w1 + w2 + w3 + w4 + w5)/5 print “The average weight of”, w1, “,”, w2, “,”, w3, “,”, w4, “and”, w5 “kg is:”, average_w, “kg.”

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.

age = 0 if age>=0 and age<18 : print "you are a minor” elif age>=18 : print "you are over 18” else: print "you are kidding"

Nested Conditionals One or more conditionals are nested within another if age >= 0: if age <18: print “you are a minor” else: print “you are over 18” else: print “you are kidding”

Nested conditionals are more difficult to read They can often be simplified by logical operators

if age>=0: if age =0 and age<18: print “you are a minor” if 0 <= age < 18: print “you are a minor”

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)

Try this: convert into an integer convert into a string convert 37 into a floating-point number make the division of 10/3 a floating-point division using a type conversion function

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

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)

See rary/math.html for a full overview of all math functions in Python rary/math.html

Composition So far: variables, statements, expressions in isolation 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

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.”

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)

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

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

for letter in fruit: print letter for elem in fruit: print elem for f in fruit: print f print f

Each time through the loop, the next character in the string is assigned to the variable after the for statement: letter, elem, f The loop continues until no characters are left If you do something with each element in 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

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

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

What is the result of the following? fruit = “bananas” fruit[3:3] And fruit[:] ?

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

greet = “Hello, world!” new_greet = “J” + greet[1:] print new_greet This example concatenates a new first letter onto a slice of greet It has no effect on the original string

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

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

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, count contains the result

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

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

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

Function = a named series of statements that performs a computation named: you define a function  again give it a name that describes what the function is doing sequence of statements: e.g. assign statements and/or print statements that are to be executed in the order predetermined by you

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( )

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

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( )

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

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

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

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

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

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

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

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

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

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

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

Fruitful Functions vs. Void Functions Fruitful functions: return a value e.g. >>>result = len(“python”) >>>type(result) Void functions: do not return a value e.g. >>>result = print_twice(“python”) >>>type(result)

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

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

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 “”.

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”)

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

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

For Next Week Tuesday 1) Define a function that asks the user’s name and gives an answer: What is your first name? >>>Claudia Claudia contains 7 letters and ends with an a.

2) Play with some string methods. Write a function that will print a sentence of your choice: in capitals in lower case like a title Find the lowest and the highest index of a letter in your sentence. If the index of the letter is higher than 3, replace the letter with an other letter.

3) Write a script that implements the Dutch grammar rule of 't kofschip. If you do not know this rule, please check Wikipedia! The program should start with asking the user to type in a verb root such as "werk"(work) or "speel" (play). The program checks the last letter of the lemma and returns the past tense of the lemma by adding ‘+te’ or ‘+de’. werk -> werkte speel -> speelde gil -> gildelach ->lachte (Note: the program only handles regular cases. Do not worry about implementing the irregular forms such as 'leven' ('lev' -> 'leefde’))

Please mail them to by next Tuesday: Thank you