Overview of Programming and Python

Computers can be programmed
- Designed to do any job that a program tells them to.
Program: set of instructions that a computer follows to perform a task
- Commonly referred to as Software
Programmer: person who can design, create, and test computer programs
- Also known as software developer

Main Components of a computer

CPU - Central Processing Unit: The "brain" of the computer
- ISA - Instruction Set Architecture: the specific set of low-level instructions available to a CPU. Differs for various CPU types (Intel Pentium, Mac G4, etc)
ALU - Arithmetic and Logic Unit
- responsible for performing arithmetic calculations, as well as logical operations (comparisons for equality, inequality, for instance).
Main Memory (RAM - Random Access Memory)
- storage close to CPU
- Faster to access than hard disk
- stores executing programs and data being currently worked on
Secondary Memory
- hard disk, floppy disk, CD, DVD, etc.
Input devices
- mouse, keyboard, scanner, network card, etc.
Output devices
- screen/console, printer, network card, etc.
Operating System
- Examples: Mac OS, Windows XP, Linux
- Controls computer operations
- Manages allocation of resources for currently running applications

How Computers Store Data: Memory Concepts

bit: a binary digit
- Stores the value 0 or 1
- Smallest unit of storage in a computer
byte: 8 bits
- Smallest addressable unit of storage in a computer
- Storage units (variables) in a program are 1 or more bytes
- Each byte in memory has an address (a number that identifies the location)
Data stored in computer must be stored as binary number
Characters are converted to numeric code, numeric code stored in memory
- Most important coding scheme is ASCII
  - ASCII is limited: defines codes for only 128 characters
Unicode coding scheme becoming standard
- Compatible with ASCII
- Can represent characters for other languages

Programming, and Programming Languages

How a program works
- Program - a set of instructions for a computer to execute
- Program must be copied from secondary memory to RAM each time CPU executes it.
- CPU executes program in cycle:
Evolution of Programming languages
- Machine Language
  - CPU designed to perform simple operations on pieces of data
    - Examples: reading data, adding, subtracting, multiplying, and dividing numbers
  - Based on machine's core instruction set
  - Needed by computer, hard for humans to read (1's and 0's)
  - Example: 1110110101010110001101010
    - Say what?
  - To carry out meaningful calculation, CPU must perform many operations
- Assembly Language
  - translation of machine instructions to symbols, slightly easier for humans to read
  - Example: ADD $R1, $R2, $R3
    - Well, we know it has something to do with addition!
- High-level procedural languages
  - Abstraction of concepts into more human-readable terms
  - Closer to "natural language" (i.e. what we speak)
  - Easy to write and design, but must be translated for computer
  - Examples include C, Pascal, Fortran
- Object-oriented languages
  - Abstraction taken farther than procedural languages
  - Objects model real-world objects, not only storing data (attributes), but having inherent behaviors (operations, functions)
  - Easier to design and write good, portable, maintainable code
  - Examples include Smalltalk, C++, Java, Python
Bridging the gap between high-level code and machine code
- Interpreted languages
  - Translates and executes instructions in high-level language program
  - Interprets one instruction at a time
  - No separate machine language program
  - Python is an interpreted language
- Compiled Languages
  - A compiler program translates source code (what the programmer writes) to machine language (object code)
  - A linker program puts various object code files together into an executable program (or other target type, like a DLL)
  - C and C++ are compiled languages
- Java is a mix of both!
Key Words, Operators, and Syntax: an Overview
- Key words: predefined words used to write program in high-level language: has specific meaning
- Operators: perform operations on data: math operators for arithmetic operations
- Syntax: set of rules to be followed when writing program
  - Syntax error: prevents code from being translated
- Statement: individual instruction used in high-level language

Building Programs

Software development

Involves more than just writing code

Analysis and problem definition
Design - includes design of program or system structure, algorithms, user-interfaces, and more
Implementation (coding)
Testing - can be done during design, during implementation, and after implementation
Maintenance - usually the major cost of a software system. Not part of "development", but definitely part of the software life cycle

The Python Language

Basic Creation and Execution of a Python program

Python must be installed and configured prior to use.
- One of the items installed is the Python interpreter
Python interpreter can be used in two modes:
- Interactive mode: enter statements on keyboard
- Script mode: save statements in Python script

Interactive mode

When you start Python in interactive mode, you will see a prompt.
- Indicates the interpreter is waiting for a Python statement to be typed
- Prompt reappears after previous statement is executed
- Error message displayed If you incorrectly type a statement
Good way to learn new parts of Python

Script mode

Statements entered in interactive mode are not saved as a program
To have a program use script mode:
- Save a set of Python statements in a file.
- The filename should have the .py extension
- To run the file, or script, type
  python filename
  at the operating system command line

Integrated Development Environments

IDLE (Integrated Development Program): single program that provides tools to write, execute and test a program
- Automatically installed when Python language is installed
- Runs in interactive mode
- Has built-in text editor with features designed to help write Python programs
An Integrated Development Environment (IDE) is a software package that includes all necessary tools for creating a program. Typically includes:
- Text editor
- Compiler
- Loader
- Debugger
- ability to launch and run applications from within IDE environment
- Other useful tools

Programming is about Problem Solving

Algorithm - a finite sequence of steps to perform a specific task
- To solve a problem, you have to come up with the necessary step-by-step process before you can code it
- This is often the trickiest part of programming
Some useful tools and techniques for formulating an algorithm
- Top-down Refinement: Decomposing a task into smaller and simpler steps, then breaking down each step into smaller steps, etc
- Pseudocode: Writing algorithms informally in a mixture of natural language and general types of code statements
- Flowcharting: If you can visualize it, it's often easier to follow and understand!
Testing - algorithms must also be tested!
- Does it do what is required?
- Does it handle all possible situations?
Syntax vs. Semantics
- Syntax -- the grammar of a language.
  A syntax error: "I is a programmer."
- Semantics -- the meaning of language constructs
  Correct syntax, but a semantic error: "The car ate the lemur."

A sample Python program

def main():                                       # main function, start of program
    number = input('Enter a number: ')            # Read input from the user: password.
    print 'The number you entered is:', number    # Output the result with a message.
    print 'Thank you, have a good day!'           # Output a Good bye message.

main() # This line results in the def main(): line to be executed.

A link to this code file