Python Programming, 1/e 1
Coming up: The Software
Development Process
1
Python Programming:
An Introduction to
Computer Science
Chapter 2
Dan Fleck
The Software Development Process
• The process of creating a program is often
broken down into stages according to the
information that is produced in each phase.
1. Analyze the problem
2. Determine requirements
3. Create a Design
4. Implementation
5. Testing
6. Maintenance
The Software Development Process
• Analyze the Problem
Figure out exactly the problem to be
solved. Try to understand it as much as
possible.
The Software Development Process
• Determine Requirements
Describe exactly what your program will
do.
– Don’t worry about how the program will
work, but what it will do.
– Includes describing the inputs, outputs, and
how they relate to one another.
Python Programming, 1/e 2
The Software Development Process
• Create a Design
– Formulate the overall structure of the
program.
– This is where the how of the program gets
worked out.
– You choose or develop your own algorithm
that meets the requirements.
The Software Development Process
• Implement the Design
– Translate the design into a computer
language.
– In this course we will use Python.
The Software Development Process
• Test/Debug the Program
– Try out your program to see if it worked.
– If there are any errors (bugs), they need to
be located and fixed. This process is called
debugging.
– Your goal is to find errors, so try everything
that might “break” your program! (Correct
and incorrect inputs)
Why is it called debugging?
The First "Computer Bug”
Moth found trapped between points
at Relay # 70, Panel F, of the Mark
II Aiken Relay Calculator while it
was being tested at Harvard
University, 9 September 1945. The
operators affixed the moth to the
computer log, with the entry: "First
actual case of bug being found".
They put out the word that they had
"debugged" the machine, thus
introducing the term "debugging a
computer program".
Courtesy of the Naval Surface
Warfare Center, Dahlgren, VA.,
1988.U.S. Naval Historical Center
Photograph.
Python Programming, 1/e 3
The Software Development Process
• Maintain the Program
– Continue developing the program in
response to the needs of your users.
– In the real world, most programs are never
completely finished – they evolve over
time.
Example : Temperature Converter
Analysis
• Analysis – the temperature is given in
Celsius, user wants it expressed in
degrees Fahrenheit.
• Requirements
– Input – temperature in Celsius
– Output – temperature in Fahrenheit
– Output = 9/5(input) + 32
Example : Temperature Converter
Design
• Design
– Input: Prompt the user for input (Celsius
temperature)
– Process: Process it to convert it to
Fahrenheit using F = 9/5(C) + 32
– Output: Output the result by displaying it on
the screen
Example : Temperature Converter
• Before we start coding, let’s write a rough
draft of the program in pseudocode
• Pseudocode is precise English that describes
what a program does, step by step. However,
There is no “official” syntax for pseudocode
• Using pseudocode, we can concentrate on
the algorithm rather than the programming
language.
Python Programming, 1/e 4
Temperature Converter Pseudocode
• Pseudocode:
– Input the temperature in degrees Celsius
(call it celsius)
– Calculate fahrenheit as (9/5)*celsius+32
– Output fahrenheit
• Now we need to convert this to Python!
Temperature Converter Python Code
#convert.py
# A program to convert Celsius temps to Fahrenheit
# by: Susan Computewell
def main():
celsiusString = raw_input("What is the Celsius temperature? ”)
celsius = int(celsiusString) # Convert from a string to an integer (number)
fahrenheit = (9.0/5.0) * celsius + 32
print "The temperature is ",fahrenheit," degrees Fahrenheit."
main()
Lets try it in IDLE after the next slide
Using IDLE a Python Development
Environment
• Open IDLE
• In the Python shell you can run dynamic
Python commands (this shell is the
window that opens)
• File New opens the window to write a
program
• Run Run Module runs your program
(or press F5)
How to run outside of IDLE
• If you have a python source file
(something.py) to run it outside of IDLE
on the command line:
• C:\python something.py
• C:\python c:\SomeDir\myPythonFiles\something.py
• WARNING: This Python is on your PATH. To add it
see this video:
http://showmedo.com/videos/video?name=960000
• On Mac this is typically done automatically.
Python Programming, 1/e 5
Question
• Does that mean I can create a Python
source file in anything, not just in IDLE?
Like Windows Notepad? Or something
else?
• Answer: Yes! IDLE is an integrated
development environment (IDE), so it
makes it EASIER, but you can use any
plain-text editor. (MS Word isn’t plain
text.)
Need more IDLE Help?
• Try reading this webpage on using
IDLE:
• http://hkn.eecs.berkeley.edu/~dyoo/
python/idle_intro/index.html
Temperature Converter Testing
• Once we write a program, we should
test it!
• What are some values with known
answers?
>>>
What is the Celsius temperature? 0
The temperature is 32.0 degrees Fahrenheit.
>>> main()
What is the Celsius temperature? 100
The temperature is 212.0 degrees Fahrenheit.
>>> main()
What is the Celsius temperature? -40
The temperature is -40.0 degrees Fahrenheit.
>>>
Program Revisited
#convert.py
# A program to convert Celsius temps to Fahrenheit
# by: Susan Computewell
def main():
celsiusString = raw_input("What is the Celsius temperature? ”)
celsius = int(celsiusString) # Convert from a string to an integer
(number)
fahrenheit = (9.0/5.0) * celsius + 32
print "The temperature is ",fahrenheit," degrees Fahrenheit."
main()
Comments
starts a function definition
Python Programming, 1/e 6
Elements of Programs : Identifiers
• Names of variables: celsius, fahrenheit
• Names of functions: range, main, input
• Names of modules: convert
These names are called identifiers
• Every identifier must begin with a letter or
underscore (“_”), followed by any sequence of
letters, digits, or underscores.
• Good programmers use meaningful names
• Identifiers are case sensitive.
Elements of Programs : Identifiers
Identifiers are case sensitive.
• In Python, identifiers:
– myVar
– MYVAR
– myvar
• Are all DIFFERENT because Python is case-
sensitive
Lets try it in IDLE
Reserved Words
Some identifiers are part of Python itself. These
identifiers are known as reserved words. This
means they are not available for you to use as a
name for a variable, etc. in your program.
Using identifiers in expressions
>>> x = 5
>>> x
5
>>> print x
5
>>> print spam
Traceback (most recent call last):
File "<pyshell#15>", line 1, in -toplevel-
print spam
NameError: name 'spam' is not defined
>>>
• NameError is the error when you try to use a
variable without a value assigned to it.
Python Programming, 1/e 7
Math Operators
– Simpler expressions can be
combined using operators.
– +, -, *, /, **, %
– Spaces are irrelevant within
an expression.
– The normal mathematical
precedence applies.
– ((x1 – x2) / 2*n) + (spam /
k**3)
Precedence is:
PEMDAS - (), **, *, /, +, -
Elements of Programs
• Output Statements
– A print statement can print any number of
expressions.
– Successive print statements will display on
separate lines.
– A bare print will print a blank line.
– If a print statement ends with a “,”, the
cursor is not advanced to the next line.
Elements of Programs
print 3+4
print 3, 4, 3+4
print
print 3, 4,
print 3+ 4
print “The answer is”,
3+4
7
3 4 7
3 4 7
The answer is 7
Assignment Statements
• <variable> = <expr>
variable is an identifier, expr is an
expression
• The expression on the RHS is
evaluated to produce a value which is
then associated with the variable
named on the LHS.
• x = 3.9 * x * (1-x)
• fahrenheit = 9.0/5.0 * celsius + 32
• x = 5
Python Programming, 1/e 8
Assignment Statements
• Variables can be reassigned as many
times as you want!
>>> myVar = 0
>>> myVar
0
>>> myVar = 7
>>> myVar
7
>>> myVar = myVar + 1
>>> myVar
8
>>>
Assigning Input
• Input: gets input from the user and
stores it into a variable.
• <variable> = raw_input(<prompt>)
• The raw_input function ALWAYS
returns a String (but you can convert it
to a number)
Converting Strings to Numbers
• someFloatString = ‘1.343’!
• someVar = float(someFloatString)!
• print someVar!
• More on this later…
Function Meaning
float(<expr>) Convert expr to a floating point value
int(<expr>) Convert expr to an integer value
long(<expr>) Convert expr to a long integer value
str(<expr>) Return a string representation of expr
eval(<string>) Evaluate string as an expression
Assigning Input
• First the prompt is evaluated
• The program waits for the user to enter a
value and press <enter>
• The expression that was entered is assigned
to the input variable as a string.
Python Programming, 1/e 9
How to find information yourself
• To the Python docs!
– http://docs.python.org/
• Library Reference – kinda hard to read
• http://docs.python.org/library/index.html
• Language Reference – very easy to
read
• http://docs.python.org/reference/index.html
Strings: Can there be more?
• Lets try and see what else we can find
about them in the Python
Documentation.
• What Methods are supported?
– How can find if a string is all numbers?
– How can I find the index of character “h” in
“Python”?
String Methods you should know
• upper
• lower
• replace
• count
• find
• isdigit
• split
Use the dot operator to call a
function that is part of a class:
firstName = “Dan”
yellName = firstName.upper()
Using non-built in modules
• The math module adds more functions
to use like cosine and sine, square root,
etc…
• Lets try it!
• Math is NOT built-in (like String) so we
need to tell Python we want to use it
Python Programming, 1/e 10
Using Python’s Modules
• Python has a lot of code available in
modules for you to use
• Using modules, you must “import” them.
Import Statement
• Importing a module tells the Python
interpreter you plan to use some or all
of the function definted in that module.
• To use those functions though, you
must prepend the name of the module:
• import math
• Then call function XYZ defined in the
math module as:
– someVar = math.XYZ()
What modules are available?
• Many! Find info in the module index
String Concatenation and
Multiplication
• pStr = “Python”
• rStr = “Rocks”
• prStr = pStr + rStr
• This is string concatenation – adding two
strings to get a new string
• pMulti = pStr * 5 # Use variable
• pMulti = “Hello” * 3 # Use a literal value
• This is string “multiplication”, says create a
new string by concatenating 5 in a row
Python Programming, 1/e 11
Another way to print – Fill in the blank
>>> numCats=5
>>> numDogs=7
>>> print "There were %d cats and %d dogs" %(numCats, numDogs)
There were 5 cats and 7 dogs
>>>
Printing is the SAME as creating a new string. You are really formatting the
String and then printing it. This also works:
>>> myString = "There were %d cats and %d dogs" %(numCats,
numDogs)
>>> print myString
There were 5 cats and 7 dogs
Inside the String you can put placeholders for other values. The placeholders
specify a type.
%d = Signed integer
%f = Floating point (decimal format)
%s = String
Placeholder for ints
String formats
You can also specify a minimum field
width like this: “%20d” . This will force
the number to take up 20 spaces.
>>> print “Num 1 = %10f” %(123.456)
Num 1 = 123.456000
To print in columns:
print “Col1 Col2”
print “%10f %10f” %(12.23, 222.45)
print “%10f %10f” %(444.55, 777)
Col1 Col2
12.230000 222.450000
444.550000 777.000000
String formats
Lots of other String formats are found here:
http://docs.python.org/library/stdtypes.html#string-formatting
A few final words on Math
• Shortcut operators are available:
– x = x + 1
– x += 1
– someVariable = 13 * someVariable
– someVariable *= 13
– *= /= += -= %=
– Do the operation on the current variable,
and save the result.
Python Programming, 1/e 12
Number Bases
• Decimal: Digits 0-9
• Binary: Digits 0-1
• Hexadecimal: Digit 0-9, A-F
23 = (2*10
1)
+(3*10
0)
# Decimal (base 10)
3001 = (3*10
3
)
+ (0*10
2
) + (0*10
1
)+ (1*10
0
)
In base 10 the red digits can be what?
Binary: Base 2
23 = 2*10
1
+3*10
0
# Decimal (base 10)
• 2
6
= 64
• 2
5
= 32
• 2
4
= 16
• 2
3
= 8
• 2
2
= 4
• 2
1
= 2
• 2
0
= 1
23 = 16 + 4 + 2 + 1
= (1*2
4
)
+
(0*2
3
) + (1*2
2
)+ (1*2
1
)+ (1*2
0
)
= 0b10111
48 = (1*2
5
) + (1*2
4
)
+
(0*2
3
) + (0*2
2
)+ (0*2
1
)+ (0*2
0
)
= 0b11000
0b101 = (1*2
2
)+ (0*2
1
)+ (1*2
0
) = 5
0b1101 = (1*2
3
) + (1*2
2
)+ (0*2
1
)+ (1*2
0
) = 13
Binary: Base 2
23 = 2*10
1
+3*10
0
# Decimal (base 10)
• 2
6
= 64
• 2
5
= 32
• 2
4
= 16
• 2
3
= 8
• 2
2
= 4
• 2
1
= 2
• 2
0
= 1
23 = 16 + 4 + 2 + 1
= (1*2
4
)
+
(0*2
3
) + (1*2
2
)+ (1*2
1
)+ (1*2
0
)
= 0b10111
48 = (1*2
5
) + (1*2
4
)
+
(0*2
3
) + (0*2
2
)+ (0*2
1
)+ (0*2
0
)
= 0b11000
0b101 = (1*2
2
)+ (0*2
1
)+ (1*2
0
) = 5
0b1101 = (1*2
3
) + (1*2
2
)+ (0*2
1
)+ (1*2
0
) = 13
Hexadecimal: Base 16
Digits: 0-9, A-F: A=10, B=11, C=12,.. F=15
• 16
3
= 4096
• 16
2
= 256
• 16
1
= 16
• 16
0
= 1
23 = 16 + 4
= (1*16
1
)
+
(4*16
0
) = 0x14
48 = (3*16
1
) + (0*16
0
) = 0x30
250 = (15*16
1
)+ (10*16
0
) = 0xFA
163 = (10*16
1
) + (3*16
0
) = 0xA3
Python Programming, 1/e 13
Hexadecimal: Base 16
Digits: 0-9, A-F: A=10, B=11, C=12,.. F=15
• 16
3
= 4096
• 16
2
= 256
• 16
1
= 16
• 16
0
= 1
23 = 16 + 4
= (1*16
1
)
+
(4*16
0
) = 0x14
48 = (3*16
1
) + (0*16
0
) = 0x30
250 = (15*16
1
)+ (10*16
0
) = 0xFA
163 = (10*16
1
) + (3*16
0
) = 0xA3
General Approach: Decimal to Hex
• From Base 10 to Base 16
1. Divide the decimal number by 16.
2. Treat the division as an integer division.
3. Write down the remainder (in hexadecimal).
4. Divide the result again by 16.
5. Treat the division as an integer division.
6. Repeat step 2 and 3 until result is 0.
7. The hex value is the digit sequence of the
remainders from the last to first.
From here: http://www.permadi.com/tutorial/numDecToHex/
General Approach: Decimal to Binary
• Same as last slide, but replace 16 with 2.
In Python: Output Strings
• You need to display numbers as binary
or hexadecimal strings:
• x = 32
• xHex = hex(32) #What data type is xHex?
• xBin = bin(32) # Only in Python 2.6+
Python Programming, 1/e 14
In Python: Input From User
• You need to input different bases.
Python knows how to make a number
from the string using “eval”.
• x = eval(‘0x10’) # What is x? What data type?
• x = eval(‘0b10’)
• x = eval(‘10’)
How do I ask a user for
a String?
It’s all about the jokes
• There are 10 kinds of people in this
world, those who know binary and those
who don’t.
• (I’ll be here all week.)
