programming for linguists an introduction to python 17/11/2011
TRANSCRIPT
Programming for Linguists
An Introduction to Python17/11/2011
From Last Week…Write a script called yourname_ex1.py
that calculates the average weight of 5 variables:36.5 kg47.8 kg33 kg68.3 kg72 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 ConditionalsOne 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<18:
print “you are a minor”
if age>=0 and age<18:print “you are a minor”
if 0 <= age < 18: print “you are a minor”
Conversion FunctionsPython 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 104.89 into an integerconvert 104.89 into a stringconvert 37 into a floating-point
numbermake the division of 10/3 a
floating-point division using a type conversion function
Python’s Math ModuleModule = 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 mathmath.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, powsqrt(64)
See http://docs.python.org/release/2.6.2/library/math.html for a full overview of all math functions in Python
CompositionSo far: variables, statements,
expressions in isolation
Combinations are also possible:the argument of a function can
also be a functione.g. math.sqrt(int(64.3))
note: the inside function is executed first
Some String Methods in Pythona = “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 beginningselect each character in turn and
do something with itcontinue 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 charactere.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 computationnamed: 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 FunctionsYou 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 numberyou cannot use a Python keyword
as a namecase sensitive
Some tips:try to choose a name that relates
to what the function is doingavoid choosing the same name for
a variable and a function
To end a function:in interactive mode enter empty
linein 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
CompositionFunctions 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 ExecutionThe 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 ordera function is calledthe 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
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 valuee.g.
>>>result = len(“python”)>>>type(result)<type ‘int’>
Void functions: do not return a valuee.g.
>>>result = print_twice(“python”)>>>type(result)<type ‘NoneType’>
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 InputIn 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 questionname = 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?>>>ClaudiaClaudia 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 capitalsin lower caselike 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 -> speeldegil -> gilde lach ->lachte
(Note: the program only handles regular cases. Do not worry about implementing the irregular forms such as 'leven' ('lev' -> 'leefde’))
