Introduction to Python 3, Variables
and Expressions
Difficulties : Two Things at Once
1. First you must solve the problem. When
solving a problem, I’d suggest being as far
away from a computer as possible. Think
about how you would solve the problem
yourself and write down those steps.
2. Once you have those basic steps down, you
can translate those into something that that
the computer can understand.
What is a good Program?
● Solves the problem.
● It should be easily readable.
● It should contain comments for anything non
obvious.
The Three Rules
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Rule 1: Think before you program
Rule 2: A program is a human-readable essay
on problem solving that also happens to
execute on a computer
Rule 3: The best way to improve your
programming and problem solving skills is to
practice.
Types of Computer Languages
● Machine Language - Punch cards, switches.
● Machine language is exactly what the computer
understands. It’s all 1’s and zero’s.
● Higher-Level Languages C++, C, Java, Python,
C#, Javascript
Python!
Who uses Python?
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Google
Yahoo
Dropbox
Instagram
Facebook
Netflix
Disney
and a bunch more
Why Python
● Features
○ Low cognitive load for students. Very high level English-like
language.
○ Easy to apply to problems. You don’t have to write dozens
of lines of code to accomplish simple tasks.
○ Broad support. Can be used by different professionals in
different settings on different OS’s.
● Open Source
○ Community supported
○ Multi-platform
○ Free
Programming Using Python
• Python interpreter: is a computer program that
executes code written in the Python programming
language.
• An interactive interpreter: is a program that allows
the user to execute one line of code at a time.
• The interactive interpreter displays a prompt (">>>")
that indicates the interpreter is ready to accept code.
Executing a Python Program
• The Python interactive interpreter is useful for simple operations
or programs consisting of only a few lines.
• Programmer can write Python code in a file, and then provide that
file to the interpreter. The interpreter begins by executing the first
line of code at the top of the file, and continues until the end is
reached.
• Statement: is a program instruction. A program mostly consists of a
series of statements.
• Expressions: are code that return a value when evaluated.
• A new variable is created by performing an assignment using the =
symbol.
• print() :displays variables or expression values.
• '#' characters denote comments.
Computer Program Basics
• Program consists of instructions executing one at a time.
• Basic instruction types:
– Input: program gets data from file, keyboard, network.
– Process: program performs computations on that data ( add,
subtract,…etc).
– Output: program puts that data somewhere ( file, screen,
network,.. etc).
• Note:
• Programs use variables to refer to data
–
–
–
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First character must be _ or A-Z, a-z.
Any remaining characters can be 0-9, _, A-Z or a-z
Can not be a keyword
It is case sensitive.
• A sequence of instructions that solves a problem is called an
algorithm
Basic Input and Output
• input() function:
– It causes the interpreter to wait until the user enters some
text(numbers, letters, special characters) and has pushed
the return key.
– It will read text entered by the user, and assign the entered
text to a variable.
Ex: name=input()
- Reading from input always results in a string type.
However sometimes we need to input integers or decimal
numbers (int(),float())
Ex: my_int = int('123')
miles=float(input('Enter distance in miles'))
Ex:salary = int(input()) You can combine input() and
int() to read in a string from the user and then convert that
string to an integer.
Basic Input and Output
•
Print() functions:
• It is used to print output to screen.
• Each print() will output on a new line.
Ex: print('hello world')  A string literal can be surrounded by matching single or double quotes.
Note:
 print('Hello', end=' ')
 print('CS101', end=' ')
Specifying end=' ', keeps the next print's output on the same line separated by a single space.
Output:
Hello CS101
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•
Printing multiple items using a single print statement:
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salary=1000
Print('Salary is: ',salary) # Comma separates multiple items and each item in the
output will be separated by a space
 Output:
Salary is: 1000
Newline characters:
– print('Hello\nCS101') #(\n moves output to new
line)
output:
Hello
CS101
- Using print() by itself without any text also prints a single newline.
Errors
• Syntax Error: The program contains invalid code that can not
be understood.
• Indentation Error: The lines of the program are not properly
indented.
• ValueError: An invalid value is used. Ex: giving letters to an
int()
• NameError: The program tries to use a variable that does not
exist.
• TypeError: An operation uses incorrect types. (adding an int
to a string)
• Logic Errors: Known as bug.
Example
wage = 20
hours = 40
weeks = 50
salary = wage * hours * weeks
print('Salary is:', salary)
hours = 35
salary = wage * hours * weeks
print('New salary is:', salary)
Output:
Salary is: 40000
New salary is: 35000