1. Intro to Programming using Python Lecture #1
Yosef Mendelsohn
With many thanks to Dolores Kalayta for the original version of these notes.

2. Python development environment

3. Getting started with Python
Install Python on your computer
The latest release of Python version 3
Download the Python software from:
Select the appropriate installer for your operating system (Windows, Linus/Unix, Mac OS X)
Find and click on the IDLE shortcut

4. The Python shell
Whether in Command Window or IDLE, Python is waiting for us to “say” something
>>> (this is the Python prompt)
In this mode, Python will instantaneously INTERPRET our statements and return an “answer”
This is known as "interactive mode".
As opposed to 'script mode'
Interactive mode means that each statement is executed once you hit 'enter'
Open IDLE in Windows
This is the Python interactive Shell (IDE integrated development environment); the interpreter that translates and runs Python.
>>> is the Python prompt. The interpreter uses the prompt to indicate that it is ready for instructions.
Type
The interpreter evaluated the expression and replied 17.3 and on the next line gave a new prompt, by indicating it is ready for more input.
Script Mode: Alternatively, we can write a program in a file and use the interpreter to execute the contents of a file. Such a file is called a script. Scripts have the advantage that as they are files, they can be saved to disk, sent to others, collaborated on, etc.
For example: we can create a file named ‘test.py’
By convention, files that contain Python programs have names that end in .py
Click File  Open file;
Type
print(‘this program adds two numbers’)
print ( )
We could then go to a command prompt (more on this later) and type:
python test.py
and the entire script would be executed

5. Open IDLE
What are the expected results for: >>> >>> 5 – 1.3 >>> 5 * 1.3 >>> 5 / 1.3 >>> > Note the unusual result… >>> 2 ** 3
> this is known as a "floating point error".
We will discuss this later in the course.
Most of these operators are familiar.
Note that '**' gives you an exponent calculation

6. Arithmetic expressions, data types and functions
Some Python language basics
Using Python to evaluate algebraic expressions
Examine the core data types: integer and float numbers
Operations on numbers and notation used
Built- Functions

7. Data Types / Python Numeric values Integer Floating point
Values without a decimal point
Python data type int
Floating point
Values with a decimal point
Python data type float
Every values has a specific “data type”.
For examples, numbers are classified as integer or floating-point
Numeric values with a decimal point are known as floats
Numeric values without a decimal point are classified as integer
Groups of text are known as strings. We make text a string by putting it in quotes (single or double quotes)
All of the following are strings: “hello how are you?” “John Smith” “I am 35 years old” “q” “ “

8. Numerical Operations / Symbols
Addition +
Subtraction -
Multiplication *
Division (returns float) /
Division Quotient (returns int) //
Division Remainder (returns int) %
Exponentiation **
Review above: symbols used to perform operation. Use * (asterisk) the ‘x’ to indicate multiplication
Open IDLE and do
3 + 4 returns 7; add 2 integers returns an integer
3 – 4 returns -1; subtract 2 integers returns an integer
6 * 6 returns 36; multipy 2 integers return an integer
5 / 3 returns float data type; the result of division, even between two ints is always a float
4 / 2 returns 2.0
5 // 3 returns 1 int quotient
5 % 3 returns 2 int remainder
4 % 2 returns 0
4 % 3 returns 1
6 * 6 returns 36
6.0 * 6.0 returns 36.0
6 * 6.0 return 36.0
6 * 6 * 6 * 6 * 6 = returns 7776 that is really 65
6 ** 5 returns 7776
6 *** 5 returns a syntax error
Remainder division is also known as modulus operator
Do not forget about the // and % operators --> they turn out to be surprisingly useful at times!

9. Operator precedence () ** * / // % + -
() ** * / // % + -
Operators with the same precedence are evaluated from left-to-right.
Use IDLE for exploring and experimenting with expressions like this.
What does (4-1) + 2 * 3 ** 2 evaluate to?
Operations are: parentheses, addition, multiplication, exponent.
Order is: Parentheses, THEN exponent THEN multiplication THEN addition
What does (4-1) + 2 * 3 ** 2 evaluate to?

10. Result Data Type DT Op Result DT int + - * / float // % **
Float arithmetic
Float operator float returns float e.g. 3.0/2.0 = 1.5
Float operator int returns float e.g. 3.0/2 = 1.5
Try in IDLE
2/3 = so int/int = float
2//3 = 0 integer division, returns the quotient int//int = int
2%3 = 2 the remainder int, int%int = int
2.0%3.0 = 2.0 float
3.5 // 2.4 = 1.0
3.5 % 2.4 = 11 this is what it should be; python gives you 1.1 floating point arithmetic is an issue in all languages – do not go there
All operations between floats always result in a float
If an expression contains ints and there is at least one float, the result is always a float
This information may seem pedantic, but it's not!
Be sure that you are clear on the concept.

11. Practice
For each of the following expressions predict the result and it’s data type; then execute the expression in IDLE to determine if you prediction is correct.idle
4 + 2
6 * 7 – 9/3
6 * 7 – 3
2.5 / (1 + 4)
* 3 – 10/5
( ) * (3 – 10/5)
Answers:
4 + 2 = 6
= 6.0
6 * 7 – 9/3 = 39.0
6 * 7 – 3 = 39
2.5 / (1 + 4) = 0.5
* 3 – 10/5 = 28.2
( ) * (3 – 10/5) = 14.2

12. Built-in functions abs(x) Returns the absolute value of a number
float(x)
Returns a floating point number constructed from a number
format(value [, format_spec])
Convert a value to a “formatted” representation, as controlled by format_spec
int(x)
Returns an integer value constructed from a number
min (arg1, arg2,…)
Returns the smallest of two or more arguments
max(arg1, arg2,…)
Returns the largest of two or more arguments
round(number[, ndigits])
Returns the floating point value number rounded to ndigits after the decimal point
Many more at –
The Python language has a number of functions and types built into it that are always available. (i.e. we do not need to "import")
More in 'import' later
Python is "case sensitive". That is, round(3.4) will work. Round(3.4) will not.
abs(-10.0) = 10.0
abs (-10) = 10
min ( 2, 4, -4, 0) = -4
max (2, 4, -4, 0) = 4
max ( 2, 4.0, 4, -4, 0) = 4.0
float(10) = 10.0
Int(10.6) --> Error
Examples

13. Python Standard Library
This is a standard library of functions that are present with every installation of Python. As your programming needs become more advanced, you may need to install additional libraries separately.
Python has many, many built-in functions as part of the standard library. Because there are so many functions, we organize them into groups calls 'modules'. In order to use functions that are in a certain module, we must first import that module.
For example, if we wanted to use some more advanced mathematical functions that live in a module called 'math', we would first type:
import math
Numeric and Mathematical Modules
math
fractions
random
>>> import math
>>> math.pi
>>>
The Python Standard Library consists of functions and classes organized into components called modules.
Each module contains a set of functions and/or classes related to a particular application domain.
In idle
>>> import math
In addition to functions such as log (logarithm), factorial, trigonometric functions, etc, some modules also include constants.
For example,
>>> math.pi
Want to calculate the area of a circle Area = 𝜋𝑟^2
Type into IDLE
pi * 5 ** 2
NameError - name ‘pi’ is not defined
Use math library
import math
math.pi * (5 ** 2)
Thought question: Do we need the parentheses above?
Answer: No….
why not?
Try it:
The area of a circle is pi * radius2
Suppose you have r=5, get Python to output the area.
Solution is in the PPT notes.

14. Variables

15. Variables
Suppose we performed a complex calculation in Python and we wanted to SAVE the result for future use
In order to do that we need to have a “container” in memory where we can store our piece of data
Such “containers” are called variables

16. Variables
In the code below, we have two variables, one that holds a value in pounds, and a second that holds a value in kilograms. pounds = 163 kilograms = pounds / 2.2 print(kilograms)
dollars = 50
euros = dollars*0.84
print(euros)
Try it:
Create a variable called 'dollars' and assign it a value of 50.
Then create a variable called 'euros' and assign it a value of dollars*0.84, and output the result.
Solution is in the notes below.

17. Variables: assignment statements
Formula for calculating compound interest
A is the value after t periods
P is the principal amount (initial investment)
r is the annual nominal interest rate ( not reflecting the compounding)
n is the number of times the interest is compounded per year
t is the number of years the money is invested
If an amount of $5,000 is deposited into a savings account at an annual interest rate of 1%, compounded monthly, the value of the investment after 10 years can be calculated as follows
Type into IDLE first with using only the A variable
Then type in using intermediate variables.
Do in Idle first without and then with variables.
>>> A = 5000 * ( ) ** (12*10)
A =
Let's round it:
round(A, 2)
Now try it with variables:
P = 5000
r = 1/100 = .01
n = 12
t = 10
A = P * (1 + r/n) ** (n*t)
Must type in variables before the formula
print (A) --> or $
balance = round(A,2) --> round to 2 decimal places
print(‘The investment balance after 10 years is ‘, balance)
We will learn more about the print() function as we progress.
Given: n = 12 months, t = 10 years, P = 5000, r = 1/100
Write a line of code to determine the value and assign it to the variable ‘A’

18. Try it in IDLE without variables

19. Try it in IDLE using variables

20. Variables: assignment statements
We have done the following in memory for each variable:
An object (container) was created for the variables
For the first variable, we put a label “n” on the object
The type label of the object 'n' is int
The number 12 was stored as this object n 12 t 10 r
.01
Recall: everything must be in main memory before it can be executed by the cpu.
These values will stay in memory until we close(quit) Idle
Main Memory P
5000 A
What is the type for label A?

21. Legal Python variable names
A variable name a.k.a. identifier, can be made up of
Letters
Digits
The underscore character _
Can not begin with a digit
Can be as long as you want
Variable names are case sensitive
EVERYTHING in Python is case sensitive!
MyStuff and mystuff are entirely different variables
Awareness of case sensitivity in Python is very important!
Most commonly used programming languages are case sensitive

22. Reserved words
Besides variable names, that we make up, there are other words that are used in Python
These special identifiers are called “reserved words”. They are words that already have a predefined meaning / usage in Python
A reserved word cannot be used as an identifier (e.g. as a variable name)

23. Python reserved words and as assert break class continue def del elif
else
except
false
finally
for
from
global if
import in is
int
lambda
none
nonlocal
not or
pass
raise
return
true
try
while
with
yield

24. Multi-word variable names
There are two conventions for names with multiple words:
Using underscore as delimiter
shoe_size
Using “camelCase” capitalizing the initial of the second (third,…) word
shoeSize
You can choose one style and stick with it
Be consistent within each program

25. Python program
As you saw, we can enter multiple statements in IDLE, each statement is executed when you hit (enter/return). Now let’s turn this into a program.
We write a program in a file and use the interpreter to execute the contents of the file. Such a file is called a script. Working directly in the interpreter is convenient for testing short bits of code because you get immediate feedback. Think of it as scratch paper used to help you work out problems. Anything longer than a few lines should be put into a script.
Let’s write a program that adds 2 numbers

26. Go to IDLE Click File  New File
If you have not already saved the program, click OK and give this file a name. When you click save the program will execute
Create a folder named Python and place it in a location that is easy for you to get to. By convention files that contain Python programs have names that end with .py.
I will use c:\myPythonPrograms\ FirstProgram as my file name. Click save and the program wi

27. IDLE Shell The output is returned to the IDLE Shell 3 + (-12)
Returns -9

28. Variables: assignment statements
Formula for calculating compound interest
A is the value after t periods
P is the principal amount (initial investment)
r is the annual nominal interest rate ( not reflecting the compounding)
n is the number of times the interest is compounded per year
t is the number of years the money is invested
Now, let’s write a program that does this calculation.
Open a new file, save as compound_interest.py
Given: n = 12 months, t = 10 years, p = 5000, r = 1/100

29. compound_interest_v1.py RESULTS:
Save as compound_interest_v1.py
RESULTS:
======= RESTART: C:\Users\ymendels\Dropbox\401\compound_interest_v1.py =======
The investment balance is
The interest amount is
>>>

30. Practice Problem You scored x/100, y/100, z/100 on homework
What is your total homework percentage?
See notes:
( x + y + z)/300
(x + y + z) - Min (x, y z) /200 leave this for a homework exercise:
The instructor drops the lowest homework score; what is your homework percentage?

31. Create algorithm
We want the program to use 3 variables and calculate the average percentage
Step 1: Assign a value to score_1
Step 2: Assign a value to score_2
Step 3: Assign a value to score_3
Step 4: Calculate Average by summing the scores and dividing by total possible points
Step 5: Multiply Average by 100 to get percentage
Step 6: Print percentage
Save your program
Prepare your test criteria: i.e. the 3 scores you want to use
Calculate the results manually.
Run your program; Does your program results match your predicted results.
## Calculate average percentage of 3 homework scores
score_1 = 100
score_2 = 85
score_3 = 87
average = (score_1 + score_2 + score_3)/300
percentage = average * 100
print('The average percent is', percentage)

32. Strings
In addition to number and Boolean values, Python supports string values ( i.e. non-numeric values)

33. Strings
A string value is represented as a sequence of characters enclosed within quotes
A string value can be assigned to a variable.
String values can be manipulated using string operators and functions
+ is the string concatenation operator
String and generally enclosed in single quotes, but double quotes are acceptable.
Double quotes are necessary if you have a single quote within the string
For example print (“That’s all folks”)

34. Explicit conversion of int to string
Adding two or more strings together to make a longer string is known as concatenation
This is used a lot in programming
Only string data can be concatenated
To change an int data type to a string use str()
Example:
>>> age = 16 >>> print('Adam is ' + age + ' years of age') TypeError: must be str, not int >>> print ('Adam is ' + str(age) + ' years of age') Adam is 16 years of age
>>> age = 16
>>> print ('Adam is ' + age + ' years of age')
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
print ('Adam is ' + age + ' years of age')
TypeError: must be str, not int
>>> print ('Adam is ' + str(age) + ' years of age') Ad
Adam is 16 years of age
There are many, many more operations and functions for strings,