In this module we will be covering a basic introduction to python.

Computer programs are written in a particular computer language.  The rules of how a programming language is written is called its SYNTAX.  Each programming language has its own SYNTAX rules. You MUST follow the SYNTAX rules to properly write the computer program using "statements" that will tell the computer what we want it to do for us.

The computer program once written in the programming language must be either "Compiled" or "Interpreted".  “Compiled” means that each line of written programming code ( SOURCE CODE)  is translated into the binary equivalent (everything in 0s and 1s  - MACHINE CODE ). Machine code is the ONLY code that the computer understands.

If the compiler finds errors in reading the 0 and 1 patterns, then it returns a message of  SYNTAX errors in the source code.  The SYNTAX errors must be corrected before the program will compile.  Examples of compiled languages are C# and C++ programming languages. 

With languages such as PYTHON, which are ”Interpreted”, the computer takes one line of code and translates it.  If it finds an error, it sends you a message for that one line of code only.  The SYNTAX error must be corrected for that one line of code, then the interpreter can begin again checking each line of code until everything works.

Once the program has been interpreted correctly, then the computer can "run"  your computer program to perform the tasks that each statement tells it to do.  This now may include using data from the keyboard, or from an external storage source (such as a disc) to make the code perform the required tasks.  As the computer runs,  it uses data throughout the program to complete its' tasks.  At the end of "running" the program - it will produce an output.  This may be printed on the screen,  printed to the printer, or stored on disc, CD, DVD, etc. 

If you do not get the correct output that you expected from your program, then you have a LOGIC ERROR.  A logic error is one where you did NOT follow the correct logic in writing your computer program originally.  For instance, you ask to read data that doesn't exist, or you have a mathematical statement that doesn't work correctly.  You then need to "debug" your program and go back to your original program, figure out where you made your "logic" error, rewrite the program, send the program back through the interpreter to be sure your SYNTAX is correct, and then rerun the program - Hopefully producing the correct output that you expect!!  Many programmers make the mistake of getting output but not actually checking its validity.

PYTHON

Python is an interpreted language.  You will install the PYTHON language on your computer (or use the ones in our lab).  It contains the interpreter for the language.  You will write your PYTHON program one line at a time.   >>> is the prompt you get and you will then type in your one line of PYTHON code.  This is the "interactive" mode to enter programs.

Another mode for your programs is "script" mode.  You use a simple editor such as Notepad and write all of your PYTHON lines of code and save them in a file with the .py extension at the end of the file name you created.    To "run" (interpret and run) your saved file you use the command:  python  filename.py

Another way to write, save, interpret and run your programs, is to use an integrated development environment such as IDLE.  This environment gives you all of the tools you need.    You type Python statements within the environment (at the prompt >>>), and you are using the text editor and it displays such things as keywords in different colors!  You then can save your completed programs and run them. 

How do you get python?

https://software.rc.fas.harvard.edu/training/scraping/install/

In this module we will be covering the concepts of how to design a program - life cycle.  We will be looking at Input-process-output logic.  We will learn some of the basic PYTHON commands, and will looking at "reading" input, producing output and performing calculations.

Program Development Life Cycle

One of the MOST important things a programmer needs to learn is the Program Development Life Cycle.  Just like we should follow steps when building a house - we need to follow steps in building a computer program.  The Program Development Life Cycle has 5 major steps:

1.  Design the program - this step is done BEFORE starting to write your program.  You need to a) understand the task that the program needs to perform, and 2) determine the steps (LOGIC) in their correct order that must be taken to perform the task.  You need to break down the task into a series of steps - IN THE CORRECT ORDER - this is called an ALGORITHM!!!  You then may create a "visual" to help you - sometimes you can use a computer flowchart (see chapter and link 1), or you can do pseudocode (see chapter and link 2).  Either a flowchart or pseudocode  can map your statements into what the computer needs to do.  Many programmers try to skip this step - DON'T fall into that trap!!!!

     a)  write out the steps in their correct sequence order

     b)  create either a flowchart or pseudocode

2.  Write the code - you will be using PYTHON as your programming language to write your computer programs

3.  Interpret or compile your program - we will be using the interpreter; Correct any SYNTAX errors and put back through the interpreter until all SYNTAX errors are corrected.

4.  Execute/Run the program - the text calls this "testing" - you are seeing if the correct Output is being produced.  If the correct output is okay you are done, if not - go to step 5.

5.  Correct logic errors - go back to your original program, make corrections in PYTHON language, put back through the interpreter and re-run (execute).

The Overall "logic" of all programs

All computer programs follow the same basic 3-step process:  input-process-output.  The computer program asks for input data to use in the program, the program processes the data (does to the data what you want it to do), then the program produces the outputs that you want (screen, printer, disc, etc.).

Functions

In PYTHON (and many other languages) your keywords are actually prewritten code that is stored as part of the PYTHON language.  You have a one word "function" that will tell the computer to perform some type of action.  The pre-written code is "called" by the computer program.  In this case - to "call" a function is simply to tell the computer to follow that functions pre-written code.

Our first PYTHON function

The first PYTHON function will be "print".  The print function will display something on the computer screen for you to see.  The syntax for the print function statement is:

>>> print ('what you want to print')

example:   >>> print ('My name is Sally Smith')   *********Notice that since you want actual letters printed you MUST put these letters in single quotation marks (this is part of the SYNTAX rules of PYTHON).

Strings and Literals

Sequences of letters, etc. are called "strings".  A "string literal" is what we have in the single quotation marks.  You can also use the double quotation marks also in PYTHON.  When you need to use a quotation mark as part of your string literal - you use double quotation marks around the whole string and embed the single quotation mark where it needs to be in the string.

An example:  >>> print ("Don't forget to meet me at 1:00 today")   Don't has a single quote, so you must use the " " to enclose the string.

NOTE - you can use numbers inside any string literal and they are treated as alphabetic types of numbers - not ones to do mathematical functions with.

Comments

You MUST use comments throughout your PYTHON program to explain to others who might look at your computer program what you intended to do in this part of the program.  The PYTHON interpreter ignores comments (doesn't "execute" them).

A comment starts with the # character.

example:   #This program prints out the date

                print ('January 30, 2017')

Variables

A variable is a name that you create that will represent a value that will be stored in the computer's memory at the time the program is executing.  Your program will use variables to use data that has previously been stored that you want to use for your program, or new data that you want to use for your program.  REMEMBER - you come up with your own variable names for your program.  Choose variable names that make sense.  For example, if you have a number that represents someone's age - you would probably use "age" as your variable name.

We need to get actual data into the variable.  Remember you just have the NAME of the variable - you need to put some data into it.  This is called an Assignment statement - to put a value into a variable name.

variable = expression

example:  age = 25       you CAN'T change this order -    25 = age   is a SYNTAX error

A program example:

>>> # This is my first program

>>> width = 10   press the enter key on your keyboard

>>> length = 5   press the enter key on your keyboard

>>> print ('width')     press the enter key on your keyboard

width    (will be displayed on your screen)

>>> print (width)    press the enter key on your keyboard - NOTICE  NO quotation marks - you are asking to print the value inside the variable WIDTH

10

>>> 

Variable Naming Rules

You must follow the rules when you create the names of your variables:

1.  You can't use one of PYTHON's key words  ( see the links and slides )

2.  A variable name cannot contain spaces

3.  The first letter MUST be one of the letters A-Z, a-z or the underscore (_) character

4.  After the first letter you may use the letters A-Z, a-z, digits 0-9 or the underscore

5.  Uppercase and lowercase characters are DIFFERENT.  This means the variable name Age  and age are NOT the same!!!

Displaying Multiple Items with print Function

Within your parenthesis for your print statement you can combine the literal part of the statement - things within quotation marks, and the actual name of the variable (so you get the value contained in the variable printed):

example:   #This program prints out my room number

                 room = 1750

                 print ('I am staying in room number', room)

I am staying in room 1750

Reassigning values to the same variable in your program

Within a single program - you can reassign a different value to your variable after assigning the original value to the variable.

Numeric Data Types and Literals

In PYTHON we have several different "data types".  These types of data include - int  is used for integers, and float  is used for real numbers (with decimals) for numbers.

room = 1750

dollars = 5.76

We also have the str   for our string variables.

first_name  (notice you can use the underscore character in your variable name) = 'Barry'

Reading Input from the Keyboard

We can get data "into" our programs by either typing is from the keyboard (direct entry), or from stored data.

The SYNTAX for reading data that is entered from the keyboard is:  variable = input (prompt)

example:  age = input('What is your age?   ')

example:  >>> age = input('What is your age?    ')   - notice I left some blank spaces before the last quotation mark  -   press Enter

What is your age?   43         displays on the screen, type in 43   and press Enter

               >>> print (age)     press Enter

43      displayed on the screen

Reading numbers with Input Function

When data is typed from the screen as input it ALWAYS comes in as a string.  Thus is we input the number 72 is actually comes in as the string '72'.  If we wanted to do any type of math with this value, we need to first convert it to the integer or real number equivalent.  There are two built in functions to accomplish this task.

int(item)    will send the string value  - this is called Passing an Argument to the function - and return the equivalent integer value

float (item)     will send the string value - passing the argument to the function - and return the equivalent real number value

Calculations

Python has many mathematical operators that can be used:

+      addition

-       subtraction

*      multiplication

/      division           gives result as a floating point value

//     integer division      gives result as an integer value

%    remainder

**    exponent

Mathematical Operator Precedence

You need to remember the precedence order for your math operators; these are the same that you learned in math classes.

1.  exponentiation   **  is done first

2.  multiplication, division, and remainders     *, /, //, %   are done second

3.  addition and subtraction       done third

REMEMBER - you use parenthesis to change this order!!!!

Math formulas and programming statement

Remember that you have to follow the SYNTAX rules for everything.  The normal way we write math formulas needs to be written so that the computer understands the order properly.

Escape Characters

An escape character is a special character that is preceded with a backslash (\) within a string literal.  Please see page 67 to see the various escape types of characters.  These allow you to better control your output from your print function.