Title arial bold 28pt
Lecture 1
Introduction & Preliminaries
SMM283
Introduction to Python
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QUESTIONS AND ANSWERS
How do we communicate with the computer?
Programming languages
How do we get computers to perform complicated tasks?
Tasks are broken down into a sequence of instructions
Why Python?
Powerful, easy to download, write and read
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QUESTIONS AND ANSWERS
When was Python invented and by whom?
Early 1990s: Guido van Rossum
invented the Python programming language
Python is a high-level, general-purpose programming language for solving problems on modern computer systems
Useful resources at www.python.org
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How did the language Python get its name?
Named for the British comedy group Monty Python (really!)
We will be using the editor IDLE to create programs. How did IDLE get its name?
Stands for Integrated DeveLopment Environment
What is an interpreted language?
Uses an interpreter, translates high-level language one statement at a time into machine language and then runs
QUESTIONS AND ANSWERS
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QUESTIONS AND ANSWERS
What are the meanings of the terms “programmer” and “user”?
Programmer: a person who solves problems by writing programs on a computer
User: any person who runs a program
What is the meaning of the term “code”?
Python statements that the programmer writes
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QUESTIONS AND ANSWERS
Are there certain characteristics that all programs have in common?
Input, processing, output
What are the meanings of the terms “hardware” and “software”?
Hardware: physical components of the computer
Software: the programs
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QUESTIONS AND ANSWERS
How are problems solved with a program?
Step-by-step procedure devised to process given data and produce requested output.
What is a zero-based numbering system?
Numbering begins with zero instead of one
Prerequisites to learning Python?
Be familiar with how folders and files are managed
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QUESTIONS & ANSWERS
What conventions are used to show keystrokes?
key1+key2 means “hold down key1 and then press key2”
Ctrl+C places selected material into the Clipboard
key1/key2 means “Release key1 and then press key2”
Alt/F opens the FILE menu on a menu bar.
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QUESTIONS AND ANSWERS
How can the programs for the examples in the lectures be obtained?
They will be uploaded on Moodle after each class.
Where will new programs be saved?
Create a special folder to hold your programs
Where can I research questions I have about Python?
Documentation at https://www.python.org/doc/
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PROGRAM DEVELOPMENT CYCLE
FIGURE 1.0 The problem solving process.
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PROGRAM PLANNING
Analyze: Define the problem.
Design: Plan the solution to the problem.
Code: Translate the algorithm into a programming language.
Test and correct: Locate and remove any errors in the program.
Complete the documentation: Organize all the material that describes the program.
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PROGRAMMING TOOLS
Algorithms
Flowcharts
Pseudocode
Hierarchy charts
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FLOWCHART SYMBOLS
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PSEUDOCODE
Abbreviated plain English version of actual computer code
Symbols used in flowcharts replaced by English-like statements
Allows programmer to focus on steps required to solve problem
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HIERARCHY CHART
Shows the overall program structure
Depict organization of program, omit specific processing logic
Describe what each part, or module, of the program does
Each module subdivided into a succession of submodules
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DECISION STRUCTURE
FIGURE 1.1 Pseudocode and flowchart for a decision structure.
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REPETETIVE STRUCTURE
FIGURE 1.2 Pseudocode and flowchart for a loop.
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BEHIND THE SCENES: HOW DOES PYTHON WORK?
FIGURE 1.3 Steps in interpreting a Python program
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DETECTING AND CORRECTING SYNTAX ERRORS
Programmers inevitably make typographical errors when editing programs, called syntax errors
The Python interpreter will usually detect these
Syntax: rules for forming sentences in a language
When Python encounters a syntax error in a program, it halts execution with an error message
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THE IDLE – PYTHON PROGRAMMING INTERACTIVE SHELL
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STARTING THE IDLE
Windows:
Invoke with double click of
MAC:
Open Finder, select Applications, select the Utilities folder, select Terminal, and then enter IDLE at the prompt
LINUX and UNIX: usually be found at /usr/bin/idle3
FIGURE 1.0 IDLE tile from Windows
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STARTING THE IDLE
FIGURE 1.1 The Python shell.
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STARTING THE IDLE
FIGURE 1.1 The Python shell after the
expression 4 + 10 has been evaluated.
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FIGURE 1.2 The Python shell after the
statement print(“Hello World!”) has been executed.
STARTING THE IDLE
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A PYTHON CODE EDITOR WALKTHROUGH
FIGURE 1.3 The File drop-down list; new file and the editor.
Type Python code here
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FIGURE 1.4 The code editor window
containing a three-line Python program.
A PYTHON CODE EDITOR WALKTHROUGH
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A PYTHON CODE EDITOR WALKTHROUGH
FIGURE 1.4 A Save As dialog box.
Name it
Browse in a folder you wish to save it to.
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A PYTHON CODE EDITOR WALKTHROUGH
FIGURE 1.5 The code editor window containing
a three-line Python program.
DO NOT WORRY IF YOU DO NOT UNDERSTAND EVERYTHING IN THE CODE AT THIS POINT!
WE WILL COVER THEM IN DETAIL LATER.
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FIGURE 1.6 Press the F5 key to execute.
The outcome of the Python program in Fig. 1.3.
A PYTHON CODE EDITOR WALKTHROUGH
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A PYTHON CODE EDITOR WALKTHROUGH
FIGURE 1.7 A Save message box.
Notice the program is not previously saved
DO NOT WORRY IF YOU DO NOT UNDERSTAND EVERYTHING IN THE CODE AT THIS POINT!.
WE WILL COVER THEM IN DETAIL LATER.
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AN OPEN-A-PROGRAM WALKTHROUGH
FIGURE 1.8 An Open dialog box.
Click on desired program
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AN OPEN-A-PROGRAM WALKTHROUGH
FIGURE 1.9 Example
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AN OPEN-A-PROGRAM WALKTHROUGH
FIGURE 2.0 Example
DO NOT WORRY IF YOU DO NOT UNDERSTAND EVERYTHING IN THE CODE AT THIS POINT!
WE WILL COVER THEM IN DETAIL LATER.
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AN OPEN-A-PROGRAM WALKTHROUGH
FIGURE 2.1 Example
DO NOT WORRY IF YOU DO NOT UNDERSTAND EVERYTHING IN THE CODE AT THIS POINT.!
WE WILL COVER THEM IN DETAIL LATER.
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The File Drop-down Menu
FIGURE 2.3 The File drop-down menu.
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CORE OBJECTS, VARIABLES, INPUT, AND OUTPUT
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NUMBERS IN PYTHON
Numbers are referred to as numeric literals
Two broad types of numbers: ints and floats
Whole number written without a decimal point called an int
Number written with a decimal point called a float
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NUMBERS IN PYTHON
Arithmetic Operators
Addition, subtraction, multiplication, division, and exponentiation.
Result of a division is always a float
Result of the other operations is a float if …
Either of the numbers is a float
Otherwise is an int.
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THE print FUNCTION
Used to display numbers on the monitor
If n is a number, print(n) displays number n.
The print function can display the result of evaluated expressions
A single print function can display several values
Example: Program demonstrates operations
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VARIABLES IN PYTHON
In mathematics problems, quantities are referred to by names
Names given to the values are called variables
Example: Program uses revenue, costs … calculates profit
Example: Program uses stock spot price, outstanding shares … calculates the equity value
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VARIABLES IN PYTHON
Assignment statements
Expression on right evaluated
That value assigned to variable
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VARIABLES IN PYTHON
Variable names in Python
Begin with letter or underscore _
Can only consist of letters, numbers, underscores
Recommend using descriptive variable names
Convention will be
Begin with lowercase
Use cap for additional “word” in the name
Example: rateOfChange
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VARIABLES IN PYTHON
Names in Python are case-sensitive
There are thirty-three words, called reserved words (or keywords)
Have special meanings in Python
Cannot be used as variable names
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THE abs(),int(),round() FUNCTIONS
Example: Program evaluates functions, prints results
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AUGMENTED ASSIGNMENTS
Remember: expression on right side of assignment statement evaluated before assignment is made
var = var + 1
Python has special operator to accomplish same thing
var += 1
Often referred as short-cut operators
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AUGMENTED ASSIGNMENTS
Example:
Program illustrates different augmented assignment operators.
[Run]
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TWO OTHE INTEGER OPERATORS
Integer division operator
Written //
Modulus operator
Written %
Example:
[Run]
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PARENTHESES, ORDER OR PRECEDENCE
Parentheses used to clarify meaning of an expression
Order of precedence
Terms inside parentheses (inner to outer)
Exponentiation
Multiplication, division (ordinary and integer), modulus
Addition and subtraction
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SYNTAX ERRORS
Grammatical and punctuation errors are called syntax errors.
TABLE 1.0 Three Syntax Errors
FIGURE 1.1 Syntax error message boxes.
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Runtime Errors
Errors discovered while program is running called runtime errors or exceptions
Table 2.2 Three Runtime Errors
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RUNTIME ERRORS
When Python encounters exception
Terminates execution of program, displays message
FIGURE 1.2 An Error Message for an Exception Error
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LOGIC ERRORS
Occurs when a program does not perform the way it was intended
Example
average = firstNum + secondNum / 2
Syntax correct, logic wrong: should be
average = (firstNum + secondNum) / 2
Logic errors are most difficult type of error to locate
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NUMERIC OBJECTS IN MEMORY
Consider this code:
This is
what
happens:
FIGURE 1.3 Numeric objects in memory.
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STRINGS IN PYTHON
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STRING LITERALS
Sequence of characters that is treated as a single item
Written as a sequence of characters surrounded by either single quotes (‘) or double quotes (“).
Opening and closing quotation marks must be the same type
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STRING VARIABLES
Variables also can be assigned string values
Created (come into existence) the first time they appear in assignment statements
When an argument of a print statement
Quotation marks not included in display
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INDICES AND SLICES
Position or index of a character in a string
Identified with one of the numbers 0, 1, 2, 3, . . . .
FIGURE 1.0 Indices of the characters of
the string “spam & eggs”.
If str1 is a string, then str1[m:n] is the substring beginning at position m and ending at position n – 1
Example “spam & eggs”[2:7]
FIGURE 1.1 Aid to visualizing slices.
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INDICES AND SLICES
Example: Program shows use of indices
Run
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NEGATIVE INDECES
Python allows strings to be indexed by their position with regards to the right
Use negative numbers for indices.
FIGURE 1.2 Negative indices of the characters of
the string “spam & eggs”.
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NEGATIVE INDICES
Program illustrates negative indices.
Program illustrates default bounds
Run
Run
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STRING CONCATENATION & REPETITION
Two strings can be combined to form a new string
Consisting of the strings joined together
Represented by a plus sign
Combination of strings, plus signs, functions, and methods can be evaluated
Called a string expression
Asterisk operator can be used with strings to repeatedly concatenate a string with itself
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STRING FUNCTIONS AND METHODS
Table 1.3 String Operations (str1 = “Python”)
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CHAINED METHODS
Lines can be combined into a single line said to chain the two methods
Executed from left to right
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THE input FUNCTION
Prompts the user to enter data
User types response, presses ENTER key
Entry assigned to variable on left
Note:
In Python 2.7, raw_input() returns a string, and input() tries to run the input as a Python expression.
In Python 3 .4 input() returns a string. If you want the old behavior try, eval(input()).
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THE input FUNCTION
Example: Program parses a name
Run
Note: if you are using Python 3.4, then use the function input(), instead of raw_input().
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More String Functions
Example: Program shows use of int, float, and eval functions
Note: The eval() function lets a python program run python code within itself
Run
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STRING FUNCTIONS WITH NUMBERS
int and float also work with numbers
The str function converts a number to its string representation
–
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INTERNAL DOCUMENTATION
Benefits of documentation:
Other people easily understand program.
You can better understand program when you read it later.
Long programs are easier to read
Purposes of individual pieces can be determined at a glance.
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INTERNAL DOCUMENTATION
Example: Program shows use of documentation.
Note: if you are using Python 3.4, then use the function input(), instead of raw_input().
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LINE CONTINUATION
A long statement can be split across two or more lines
End each line with backslash character ( \ )
Alternatively any code enclosed in a pair of parentheses can span multiple lines.
This is preferred style for most Python programmers
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INDEXING AND SLICING OUT OF BOUNDS
Python does not allow out of bounds indexing for individual characters of strings
Does allow out of bounds indices for slices
Given: str1 = “Python”
Then print(str1[7]) print(str1[-7])
These are OK
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OUTPUT IN PYTHON
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OPTIONAL print ARGUMENT sep
Consider statement: print(value0, value1, …, valueN)
Print function uses string consisting of one space character as separator
Optionally change the separator to any string we like with the sep argument
Note: If you are using Python 2.7, unlike in Python 3.5, the sep argument is not supported because print is not a function and is treated as a keyword i.e. use
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OPTIONAL print ARGUMENT end
Print statement ends by executing a newline operation.
Optionally change the ending operation with the end argument
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ESCAPE SEQUENCES
Short sequences placed in strings
Instruct cursor or permit some special characters to be printed.
First character is always a backslash (\).
\t induces a horizontal tab
\n induces a newline operation
Example: Program demonstrates the use of the escape sequences \t and \n.
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ESCAPE SEQUENCES
Backslash also used to treat quotation marks as ordinary characters.
\” causes print function to display double quotation mark
\\ causes print function to display single backslash
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JUSTIFYING OUPUT IN A FIELD
Example: Program demonstrates methods ljust(n), rjust(n), and center(n)
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JUSTIFY OUTPUT WITH format
Given: str1 is a string and w is a field width
Given: num is a number and w is a field width
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JUSTIFY OUTPUT WITH format
Example: Program illustrates formatting
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JUSTIFY OUTPUT WITH format
Table 1.0 Demonstrate number formatting.
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Justify Output with format
Example: Program formatting with curly brackets
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READING THIS WEEK
Lecturer notes, which map to
SCHNEIDER, I. D., (2015), An Introduction to Programming Using Python, Global Edition
Chapter 1 & 2
LAMBERT, A. K., (2012), Fundamentals of Python: First Programs, 1st Edition
Chapter 1 & 2
DEITEL M., H., DEITEL & ASSOCIATES, (2002), Python How to Program
Chapter 1
Additional Resources
History and evolution of computer languages:
www.scriptol.org/history.php
Algorithm:
http://www.wisegeek.com/what-is-an-algorithm.htm
Software:
http://www.wordiq.com/definition/Software
Python Tutorial:
http://docs.python.org/tut/
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HOMEWORK THIS WEEK
See separate file on Moodle.
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