Ling
431/631: Corpus Linguistics
Ben
Bergen
Meeting 12: The most useful thing you will ever
learn, ever
November 5, 2007
Why
program
Often you
want to get something done that would be arduous to perform by hand, and for
which there doesn't exist any easily accessible program. For example:
- Find the mean length of sentences ending with
a question mark in a document.
- Calculate if words early in a sentence tend
to be shorter than words late in a sentence.
- Replace all I.P.A. characters in a document
with UNICODE equivalents.
- Run through a list pairing lemmas with
inflected forms, search for frequencies of all inflected forms, and sum
the results for each lemma.
What
programming is
Computers
follow very explicit instructions; a program is a set of such instructions that
tells the computer what operations to perform. A program is structured like a
flow chart.
Conditional
paths: If X then Y else Z : Loops:
Do X until or while Y:
A program
takes input and
produces output of
various types - these can be input from the keyboard or a file, and output to a
file, to the screen, etc.
- E.g., a simple Python function print prints text to the screen
>> print "Hello, world!"
Hello,
world!
>>
greeting = "Howzit."
>>
print greeting
Howzit.
- You can also collect input from the user
through the keyboard using the input function
>> greeting = input("What greeting should I use?")
>> print greeting
[prints out whatever greeting you type in]
A couple of
things have been implicit in these examples.
First, we've
used data, namely
strings of characters. Other types of data are numbers, arrays, etc.
- An example with numbers
>> print 2 + 2
4
Second, we've
been using a variable, greeting, and assigning values to it using the equal sign.
- In this next example, variables are assigned
numerical values from input and on the basis of calculations using other
variables and numbers. Also, print is used with multiple arguments.
>> radius = input("What is the radius of the
circle?")
>> circumf = radius * 2 * 3.14
>> print "The circumference
is", circumf
[prints
out circumference]
Third, we've
been implicitly using operations like * and +. There's a huge number of useful
predefined operations and functions available in modules that you can include in your program,
like splitting a string at the white spaces, removing carriage returns, etc.
- Import modules like so:
import sys #
will import the module called 'sys'
Operations
can be performed conditionally, using if.
>> age = input("How old are you?")
>> if age > 29:
>> print "You are not to be
trusted."
>> else:
>> print
"You need to buy these expensive trendy jeans!"
[prints
out appropriate string]
A key
mechanism in programming is the use of loops. A loop defines a criterion or set of
criteria under which an operation or set of operations should be performed
repeatedly. Python has while and for loops.
- for does something to every member of a list
>> for form in "sneer", "rot",
"yodel":
>> print
form, "is a regular verb"
sneer is a regular verb
rot is a regular verb
yodel is a regular verb
This means: For every element n in the list, print "n is a regular verb". The block
inside the loop is executed once for each element, and each time, the current
element is assigned to the variable form [in this case]. Another example:
>> for number in range(1,10):
>> print
"Today's number:", number
>> print "Ta-da!"
- while does something as long as specified
conditions hold
>> number = 0
>> high = input("How high should I count? ")
>> while number <= high:
>> print
number
>> number
= number + 1
>> print "Done."
