Thanks to: Dr. John S. MallozziDepartment of Computer Science

1. Introduction2. Overview of programming in

Python

Software

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• Program – a set of instructions that allows a computer to perform a task

• When a program is used to make the computer perform a task, the program is executed

• Gives a computer its flexibility Change the program; change the task that can be

performed• Programs can be used by anyone• Programs must be written by specially trained

and educated people – programmers

Binary Code

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• A computer is an electronic device – it doesn’t “understand” any language

• Communication is by electronic switches (on-off)

• Switch positions can be represented by (encoded using) the digits 1 (on) and 0 (off) Two positions, so called binary code Recall: bit sequences can be written using

hexadecimal• Binary code is also called machine

language This is the only language “understood” by a

computer

Language Levels

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• Working in machine language is extremely tedious and error-prone for humans

• Easier-to-use languages were developed Low-level languages are like machine

language, but allow the use of labels instead of binary codes

High-level languages are closer to spoken languages, although without ambiguity and much more limited

• A program written in such a language must be translated into machine language

• Only machine-language can be executed

High-level Language

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• More like ordinary spoken language, but inflexible and unambiguous

• Much easier for programmers to use than low-level languages (assembly) or machine language

• One statement generally corresponds to many internal computer instructions

Python

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• A popular high-level, interpreted language, which is reasonably easy to learn

• Has form close enough to English to be much easier to use than low-level languages

• Is simpler in form than many other high-level languages, such as Java or C++

• Is available as a free download• Comes with many online resources

Help Tutorials and References Modules – libraries of pre-written solutions

Obtaining and Starting Python

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• Python is a free download, and is easy to install and set up if you want to. You can always use Lab computers.

• Instructions are available in textbooks (optional) and online.

• If you are going to install Python you can use the instructions available at the following link:http://

www.iona.edu/faculty/mgnoutcheff/InstallingPython3.htm

IDLE - Immediate Mode

A programmer’s editor and execution shell that comes with Python (be sure to use “Idle Basic”

in the labs.)

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For many applications,You must be sure thatYou see this before youstart

IDLE - The “Super Calculator”

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• Python’s immediate mode, as seen in Idle, can be used like a calculatorYou enter an expression at the prompt: >>>After you press enter/return, the expression

you entered is evaluated, and the result is printed (in a distinct color) on the next line

• Unlike a calculator, Python’s immediate mode allows much more, such as use of names for results and program steps, manipulation of text, etc.

• Example of use:

IDLE - Prototyping

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• Immediate mode allows you to do anything of which the computer is capable

• This allows you to “try out” different ideas about solving a problem

• Once you have the idea, you can write a program, so that the method you have come up with can be saved to be used and reused

Example

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• Here is a mathematical formula:

In words: to add the integers from 1 up to some positive integer, multiply that integer by the next integer, then take half the result

• You cannot use a computer to prove this, but you can use it to help you believe it

• We will use some Python features in this example, but don’t worry about them (we’ll get to all eventually)—just focus on how we are using the computer

2)1(...321 nnn

Example (continued)

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• Features of Python we will use: range, followed by two integers in parentheses, gives

you a list of the integers from the first integer to one less than the second one Example: range(1,10) is [1,2,3,4,5,6,7,8,9]

To add all the numbers on a list, use sum To multiply, use * and to divide use /

Overview of Programming in Python

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• Creating a file• Saving the file• Entering a program• Translating and executing the program• Testing and error correction• Three Rules Of Programming:

1) THINK before you program 2) A program is a human-readable essay on

problem solving that also executes on a computer. 3) The best way to improve programming and

problem-solving skills is to Practice, Practice, Practice.

4) THINK before and WHILE you program

Creating a File

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• Once you have a series of commands that perform a desired task, you can save them as a program

• From the Idle menu, choose New Window

Saving the File

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• Before doing anything else, add your full heading at the top of the file in a comment, and save the file

• Use your U drive!• Must add the .py

extension to the file name. Not automatic! If you don’t, text color

will no longer show

Saving the File (filename.py)

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Entering a Program

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• Once the file has been saved, you can enter Python commands or statements

• Learning the commands available will take some effort, but not nearly as much as for a spoken language

• For now, enter the traditional first program:print (“Hello World”)

• Add another output statement with a custom greeting of your own choosing.

Executing the Program

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• Save the file to preserve the changes• Use the Run menu, and choose Run Module• (If you have not saved, will prompt to save)

You will be returned to the Shell, and your program will be executed

Result

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• If you entered the program incorrectly, you will get an error message instead

• In that case, return to the editing window, fix the program, and repeat the execution steps

Testing and Error Correction

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• There are three main kinds of errors Typos (Precision!) IDLE editor can help spot and fix

these Incorrect Python

Solution: find out how to do it correctly, then change the program and try again

Correct Python, but incorrect results Solution: carefully think again about what you

wanted, and be sure you used the right Python commands, in the right order

• The second kind of error is frustrating at first, but eventually becomes easy to fix

• The third kind is called a bug, and may cause a lot of trouble – best is to avoid these by being careful about your planning and logic from the start

Algorithmic Problem Solving – Basic Strategies

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• Designing with algorithms• Idea of an algorithm• Example• Computational example• Another way – Turtle Graphics

Designing with Algorithms

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• The best language in which to plan is your own native, spoken language – result is an algorithm

• Once the plan is done, you can convert it to a program – implementation of the algorithm

• Another way to approach a problem is the use of prototyping

• Combining the two ideas, designing an algorithm and prototyping, can lead to a successful program to solve a problem

• Key to all this is knowing what a computer can do and how to express the commands to make it do so

Idea of an Algorithm

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• An algorithm is a step-by-step description of how to complete a taskIdea is to carefully specify all steps –

do not expect listener/reader/computer to fill in details Example: This is not an algorithm

To get to my house turn left twice, then slight right, then right, and then into driveway

Humans are good at leaving out stepsComputers are terrible at filling in

missing steps• A program is an algorithm (or

several algorithms) that has been implemented – translated into a computer language

Example

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• Problem – Shopping: drive to bank, get money, then drive to a store to buy bread, then drive home

• Algorithm: Drive to bank Go into bank Get money out of bank Drive to store Go into store Buy bread Drive home

• Too much detail for a human, still too little for a computer

Computational Example—Algorithm

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• Problem: Draw the letter ‘E’ on the screen by using stars (asterisks):

Solution: (keeping in mind that printing is done left-to-right, top-to-bottom) Print top horizontal segment Print first short vertical segment Print middle horizontal segment Print second short vertical segment Print bottom horizontal segment

Computational Example:Program Skeleton

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• Translation is easy using English!• Do this, but in comments• Fill in details later• Result is the “skeleton” of a program, which

records the algorithm in comments

Computational Example—Program

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• To draw a horizontal segment, print a number of stars ( Use 7 for now) on a single line

• To draw a vertical segment, print a single star on each of a number (we’ll use 3) of lines

Another Way – Turtle Graphics

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• Computers are capable of graphics, allowing a more “natural” way to make a letter E

• Python comes with Turtle Graphics• Idea is to use angles and distances to move

a “turtle” which draws as it moves• Lab is the place for detail; for now we do just

enough to draw letter E• The algorithm is the same, but now we think

of drawing the letter without lifting the pen from the paper (there are other ways, of course)

• We need only a few turtle commands to do this

The Program

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• We must get the Turtle Graphics library—this is done by importing it We ask for everything

in the library using * for the name of what to import:from turtle import *

• We need the commands forward, back, right, and left

• We also use the hideturtle() command so that all we see is the drawing

Result

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