ENGR/CS 101 CS Session
Lecture 2


If possible, each student should have access
to a computer with Python 2.7, either on a
laptop or on one of the lab computers.
If there are not enough computers, two
students may share a lab computer. Each
should take turns typing.
Lecture 2
ENGR/CS 101 Computer Science Session
1
Outline


Software life cycle
Introduction to Python
Lecture 2
ENGR/CS 101 Computer Science Session
2
Software Life Cycle






Specification of the problem/task
Analysis and design of a solution
Implementation of the solution
Testing and debugging
Maintenance and evolution of the system
Obsolescence
Lecture 2
ENGR/CS 101 Computer Science Session
3
Specifications


How does the user interact with the program?
What are the results of the computation?
Lecture 2
ENGR/CS 101 Computer Science Session
4
Specification Example

Write a program that computes the volume of
a right cylinder. It should allow the user to
input the base radius and the height of the
cylinder, then compute and display the area
of the base and the volume of the cylinder.
The formula for computing the volume of a
right cylinder is:
𝑣𝑜𝑙𝑢𝑚𝑒 = 𝜋 × 𝑟𝑎𝑑𝑖𝑢𝑠 2 × ℎ𝑒𝑖𝑔ℎ𝑡
Lecture 2
ENGR/CS 101 Computer Science Session
5
Analysis and Design


Main work of creating applications
How will the program accomplish the
specifications?


Lecture 2
Identify the data being used
Identify the algorithm that will compute the result
ENGR/CS 101 Computer Science Session
6
Analysis and Design
Program = Data + Algorithm


Data is the information needed to do the
computation. Usually the nouns in a
specification.
What data do we need for the example
specification?




Lecture 2
constant 𝜋
radius of base
height of cylinder
volume of cylinder
ENGR/CS 101 Computer Science Session
7
Algorithm


Algorithm: a series of well-defined steps that
can be followed as a procedure.
Example:
1.
2.
3.
4.
Lecture 2
Ask the user for a radius and a height
Compute the base area of the cylinder
Compute the volume of the cylinder
Display the base area and volume of cylinder
ENGR/CS 101 Computer Science Session
8
Implementation




Use a programming language to implement
the program.
Syntax: What are the legal "sentences" in the
language? Semantics: What do the
"sentences" mean?
Compilers and interpreters enforce syntax.
Semantics determine whether the
computation is correct.
Lecture 2
ENGR/CS 101 Computer Science Session
9
Compiling vs. Interpreting

Some languages are compiled with a
program called a compiler. Source code file
is translated into a machine code file.


Examples: C/C++, Java, Pascal, COBOL, Fortran
Other languages are interpreted. An
interpreter is a program that receives
programming language statements and
executes them directly.

Lecture 2
Examples: (original) BASIC, LISP, Prolog, LOGO
ENGR/CS 101 Computer Science Session
10
Introduction to Python

On the lab computers, log into Windows
(reboot if in Linux). Press the 'Windows' key,
start typing 'python', click on 'IDLE (Python
GUI) 2.7'


IDLE is named for Eric Idle, of Monty Python fame
Python is an interpreted language, and IDLE
runs it as a shell in a window.
Lecture 2
ENGR/CS 101 Computer Science Session
11
Data types



Any kind of data item is considered an object
in Python.
Objects are classified into different kinds
called data types. Every object has some
specific type.
Primitive types are those implemented
directly in the language. E.g. integers,
floating point numbers
Lecture 2
ENGR/CS 101 Computer Science Session
12
Python as a Calculator

We can compute numeric expressions
directly into the shell at the >>> prompt.
>>> 2+2
4
>>> 100-75
25
>>> 7*9
63

An expression is a combination of operators
and operands
Lecture 2
ENGR/CS 101 Computer Science Session
13
Operators
+
*
/
//
%
**
Lecture 2
: addition
: subtraction, negation
: multiplication
: floating point division
: integer division
: integer remainder
: exponentiation
ENGR/CS 101 Computer Science Session
14
Assignment


Assignment means to give an object a
name.
These names are called variables


Syntax is: <var> = <expression>
The expression is evaluated and the variable
refers to the result.
>>> pi = 3.14159
>>> radius = 8.0
>>> height = 16
Lecture 2
ENGR/CS 101 Computer Science Session
15
Variable names

Must start with a letter or underscore (_)




Only start with underscore for system variables
Uses only letters, digits
Cannot be a reserved word of the language
Examples:



Lecture 2
pi
baseArea
cylinderVolume
ENGR/CS 101 Computer Science Session
16
Using Variables


A variable can be used as an operand. Its
value is whatever was assigned to it.
Example: compute volume of a cylinder with
radius 8 and height 16
>>> baseArea = pi * radius **2
>>> cylinderVolume = baseArea * height
>>> baseArea
201.06176
>>> cylinderVolume
3216.98816
Lecture 2
ENGR/CS 101 Computer Science Session
17
Python Programs



Computing things directly in the shell works,
but we don't really want to type things over
and over.
To make a Python program, use File -> New
File to get an editor window. We type the
program statements in this window and save
it to a file (File -> Save As) before running it.
Note: on a lab machine save your program to
a folder on your network drive.
Lecture 2
ENGR/CS 101 Computer Science Session
18
Python Programs

To run the program, use Run -> Run Module
The program will run in the shell window
To see the results, type the variable names.

Comments start with # to the end of a line.

Change the values assigned to radius and
height, save the program (Ctrl-S), and run the
program again.


Lecture 2
ENGR/CS 101 Computer Science Session
19
Keyboard Input


Recall in the specifications, we need to ask
the user to enter the values of the radius and
height instead of assigning them.
The input function allows us to do that



Syntax: <var> = input (<prompt>)
A prompt is a string that tells the user what is
expected. Strings are given using quote marks.
Example:
radius = input ('Enter the radius')
Lecture 2
ENGR/CS 101 Computer Science Session
20
Displaying Output


The specifications also say that we need to
display the results. After all, the user of the
program shouldn't have to know the names of
our variables to get the answer.
The print statement does this.


Syntax: print object1, object2, …
Example:
print 'The base area is', baseArea
Lecture 2
ENGR/CS 101 Computer Science Session
21
Running Your Program Later




After exiting IDLE, your program will be
saved.
To run the program later, browse to your
program file.
Right-click on the file, select 'Edit with IDLE'.
Use Run->Run Module to run the program. A
shell window will be created.
Lecture 2
ENGR/CS 101 Computer Science Session
22
Testing and Debugging




Testing should try to cover all possible ways
a user might interact with a program
A bug is a logic error that causes an incorrect
result.
Debugging is the process of finding and
correcting bugs.
A debugger is a program that displays the
internals of a program as it runs to aid in
debugging.
Lecture 2
ENGR/CS 101 Computer Science Session
23
Maintenance/Obsolescence



It is estimated that 80% of the cost of a
software product is expended after the first
release.
Maintenance includes fixing bugs, adapting to
changing environments, increasing
functionality
Obsolescence is handling the transition when
a software product will no longer be available
Lecture 2
ENGR/CS 101 Computer Science Session
24