CSCE 110 — Programming I Basics of Python: Variables ...

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CSCE 110 — Programming I
Basics of Python:Variables,Expressions,and
Input/Output
Dr.Tiffani L.Williams
Department of Computer Science and Engineering
Texas A&M University
Fall 2011
Python
I
Python was developed in 1989 by Guido van Rossum in the
Netherlands.
I
Python was released for public distribution in early 1991.
I
How did Python begin?
I
van Rossum was having a hard time getting the job done with
the existing tools available.
I
He envisioned that there was an easier way to get things done.
I
While Python has been around for over 15 years,Python is still
relatively new to general software development.However,it
has a lot of a lot of support from the community and new
users are finding themselves programming in Python (and
loving it) everyday.
Be an Explorer
I
Since you are learning a new language,it is very important
that you experiment with the material.
I
Be curious about trying different things.
I
Don’t be afraid to make a mistake or crash your computer.
That’s how we learn best.
Python Distribution Used in this Course
I
In my opinion,the easiest way to get Python (and the most
popular modules) is to use the Enthought Python Distribution
(EPD).
I
It’s free for academic use!
I
It’s available for the major operating systems:Windows,Mac,
and Linux
I
It’s what’s installed on the lab machines.
I
Here’s the URL if you want to install it on your personal
computer.
http://www.enthought.com/products/edudownload.php
I
At the time of this writing,EPD uses version 2.7 of Python.
We will not cover any nuances associated with Python 3.x in
this course.
WingWare IDE 101
I
Now that we have a Python distribution,we need a way to
type in our code.
I
We will use WingWare IDE 101 in this course.
I
It’s free for academic use!
I
It’s available for the major operating systems:Windows,Mac,
and Linux
I
It’s what’s installed on the lab machines.
I
Here’s the URL if you want to install it on your personal
computer.
http://wingware.com/downloads/wingide-101/4.0.3-1/
binaries
Now,we are ready to start programming!
I
We will take it slow and easy in the beginning.I want you to
get comfortable getting acquainted with the basics.
I
Following along in class is not enough.
I
You MUST type in the programs on your computer and see
how they work.
I
You MUST type in the programs so that if you make a mistake
you can learn how to make the appropriate correction.
I
You MUST type in the programs because programming
promotes active (and not passive) learning.
Interactive Execution in the
Python Shell
“Hello,World!”
I
Python’s
print
statement is the tool for displaying program
output to your users.
I
Type the following in the command-line editor.
>‌>‌> print ’Hello,World!’
Using Python as a Calculator
>‌>‌> 3 + 4
>‌>‌> 15/3
>‌>‌> 12 * 10 + 4
Operators
I
Mathematical operators:
+,-,*,/,%,**
I
addition (
+
)
I
subtraction (
-
)
I
multiplication (
*
)
I
division (
/
)
I
modulus or remainder (
%
)
I
exponenentiation (
**
)
I
Comparision operators:
<,<=,>,>=,==,!=
I
strictly less than (
<
)
I
less than or equal to (
<=
)
I
strictly greater than (
>
)
I
greater than or equal to (
>=
)
I
equal to (
==
)
I
not equal to (
!=
)
I
Expression conjunctive operators:
and
,
or
,
not
Variables and Assignment
>‌>‌> counter = 0
>‌>‌> miles = 1000.0
>‌>‌> name = ’Bob’
>‌>‌> counter = counter + 1
>‌>‌> kilometers = 1.609 * miles
>‌>‌> print miles
1000.0
>‌>‌> print counter
1
>‌>‌> print kilometers
1609.0
Variables
I
Variables are the set of valid strings that are allowed as names
in a computer language such as Python.
I
The rules for forming Python variables are:
I
First character must be a letter or underscore (_)
I
Any additional characters can be alphanumeric or underscore
I
Case-sensitive
I
No variables can begin with a number.
I
No symbols other than alphanumerics or underscores are ever
allowed.
I
No variable can be the same as keywords,which form the
foundation of the language.
Keywords
and
as
assert
break
class
continue
def
del
elif
else
except
exec
finally
for
from
global
if
import
in
is
lambda
not
or
pass
print
raise
return
try
while
with
yield
Numbers
I
Python supports five basic numerical types.
I
int
(signed integers)
I
long
(long integers)
I
bool
(Boolean values)
I
float
(floating point real numbers)
I
complex
(complex numbers)
I
Here are some examples.
I
int:100 200 -437
I
long:-84140l 299556678883L
I
bool:True False
I
float:3.456 -33.55 34.1
I
complex:6.23+1.5j -1.23-875J 0+1j
Strings
I
Strings are a contiguous set of characters in between quotation
marks.
I
Strings are immutable sequences.
I
A program can refer to elements or subsequences of strings.
However,strings cannot be modified in place.
I
Python allows for either pairs of single or double quotes.Triple
quotes (three consecutive single or double quotes) can be used
to escape special characters.
I
Subsets of strings can be taken using the index (
[ ]
) and
slice (
[:]
) operators,which work with indexes starting
a t 0 i n t h e b e g i n n i n g o f t h e s t r i n g a n d w o r k i n g t h e i r w a y f r o m
-1 at the end.
I
Slice (
[]
) gives the character from the given index or location
I
Range slice (
[x:y]
) gives the characters starting at index x
and ending at index y -1
I
The plus (
+
) sign is the string concatenation operator.
I
The asterisk (
*
) is the repetition operator.
String Examples
>‌>‌> string ="Texas A&M University"
>‌>‌> string[0]
’T’
>‌>‌> string[2:5]
’xas’
>‌>‌> string[4:]
’s A&M University’
>‌>‌> string[:6]
’Texas ’
>‌>‌> string * 2
’Texas A&M UniversityTexas A&M University’
>‌>‌> string = ’It\’s a girl!’
>‌>‌> string
"It’s a girl!"
Writing Python Programs
User Input
1.
Type the following text directly into your editor.
>‌>‌> name = raw_input("Please enter your name:")
>‌>‌> print"Hello",name,"- good to see you!"
2.
Type the above text into a file called hello.py and save it as
hello.py.Afterward,run the program.
3.
Discuss the difference between the two different approaches for
entering Python programs.
Write the Following Python Programs
1.
Full name greeting.
Write a program that asks for a
person’s first name,then middle,and then last.Afterwards,it
should greet the person using their full name.
2.
Guessing game?
Write a program that asks a person to guess
a number.Add 1 to the guessed number,and then suggest
that their guess was close to being correct.
3.
Guessing game (version 2)?
Write a program that asks a
person to guess a number.Tell the user that their guess was
off by a random amount.