Transcript programming I

Introduction to
Computational Linguistics
Programming I
Resumé
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Algorithm: A well defined procedure
for the solution of a given problem in a
finite number of steps.
Program: A set of instructions,
written in a specific programming
language, which a computer follows in
solving a problem.
Control Structures specify how
instructions are followed
Examples of
Control Structures
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Sequence: a series of instructions that are
executed in order.
Branch: an instruction involving a condition
that affects the next instruction executed.
Loop: for handling repeated execution.
Module: a named collection of instructions
that we write once but use in different
contexts
Sequence
Branch
Loop
Module
Python
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Python is a programming language that
allows us to write programs.
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Python programs have to obey certain
syntactic conventions
Python is also an interpreter, i.e. a
machine which understands the
conventions of the language
Python is also an Interpreter that
Understands Python Programs
PROGRAM
WRITTEN
IN PYTHON
INPUT
PYTHON
INTERPRETER
OUTPUT
Here is the World’s Simplest
Python Program
print “Hello World”
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When this program is run (interpreted)
it causes “Hello World” to be printed on
the screen.
Demo
The print statement
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The print statement consists of the
word print followed by one or more
arguments separated by commas:
>>> print "a", "b", "c"
abc
In this case the arguments are strings.
Strings
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In Python strings are enclosed in either
single quotes or double quotes.
'abc'
"abc"
"my name is"
"Mike's name is"
Multi-line programs
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Here is a program consisting of a
sequence of three instructions
print "My name is "
print "Mike"
print "!"
Data
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Data is the raw material that programs
manipulate.
Data comes in different flavours called types
Examples of data types are
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integer
strings
Each type of data is associated with
characteristic operations.
Examples of operators
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Integers are associated with operators
such as +, -, * etc.
Using these operators we can write
down complex expressions
e.g. 2+2; 2-2; 2*2
Other Arithmetic Operators
Operation
Symbol
Example
Exponentiation
**
5 ** 2 == 25
Multiplication
*
2 * 3 == 6
Division
/
14 / 3 == 4
Remainder
%
14 % 3 == 2
Addition
+
1 + 2 == 3
Subtraction
-
4 - 3 == 1
Arithmetic expressions
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a number: 1
a variable: b
an expression involving an operator
a+1
a complex expression
a + 3*2 / 3
to eliminate ambiguity complex expressions
can use parentheses
(a + (3*2)) /3
Exercise 1.1
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Discover the value of the following
arithmetic expressions using the python
interpreter
1 + 2/3 + 4
(1 + 2) / (3 + 4)
100 % 10
115 % 10
Multi-line program using
arithmetic expressions
print "2 + 2 is", 2+2
print "3 * 4 is", 3 * 4
print 100 - 1, " = 100 – 1"
print "(33 + 2) / 5 + 11.5 =
",(33 + 2) / 5 + 11.5
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What is the output of this program?
Python Editor
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Python allows us to edit, save, check and run
programs.
In python shell, go to File menu.
Select New
Editor window appears
Type program into editor window.
To check or run program – go to Run menu
and select check or run. You will be prompted
to save file. Any filename is OK, but
filename.py is best.
Exercise 1.2
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Use editor to write programs that
print your name and date of birth
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on same line
on separate lines
print the numbers from 0 to 4
Save the work in different files
Run the programs