Lecture-2-Python-Basic-Elements-Sep04-2018.pptx

15-110: Principles of
Computing
Basic Elements of Python Programs
Lecture 2, September 04, 2018
Mohammad Hammoud
Carnegie Mellon University in Qatar

Today…
• Last Session:
• Motivation
• Introduction to Hardware Basics & Programming Languages
• Writing Simple Python Commands
• Today’s Session:
• Basic Elements of Python Programs: Literals, Assignments, Datatype Conversion,
Identifiers, and Expressions
• Announcement:
• We will practice on the Python basic elements on Thursday, September 06, 2018
during the recitation

• In the following example, the parameter values passed to the print
function are all technically called literals
• More precisely, “Hello” and “Programming is fun!” are called textual literals,
while 3 and 2.3 are called numeric literals
>>> print("Hello")
Hello
>>> print("Programming is fun!")
Programming is fun!
>>> print(3)
3
>>> print(2.3)
2.3
Literals

• A literal is used to indicate a specific value, which can be assigned to
a variable
>>> x = 2
>>> print(x)
2
>>> x = 2.3
>>> print(x)
2.3
Simple Assignment Statements
 x is a variable and 2 is its value

• A literal is used to indicate a specific value, which can be assigned to
a variable
>>> x = 2
>>> print(x)
2
>>> x = 2.3
>>> print(x)
2.3
Simple Assignment Statements
 x is a variable and 2 is its value
 x can be assigned different values;
hence, it is called a variable

• A simple way to view the effect of an assignment is to assume that
when a variable changes, its old value is replaced
>>> x = 2
>>> print(x)
2
>>> x = 2.3
>>> print(x)
2.3
Simple Assignment Statements: Box View
2
Before
x = 2.3
2.3
After
x x

• Python assignment statements are actually slightly different from the
“variable as a box” model
• In Python, values may end up anywhere in memory, and variables are used to
refer to them
>>> x = 2
>>> print(x)
2
>>> x = 2.3
>>> print(x)
2.3
Simple Assignment Statements: Actual View
2
Before
x = 2.3
2
After
x x
2.3
What will
happen to
value 2?

• Interestingly, as a Python programmer you do not have to worry about
computer memory getting filled up with old values when new values
are assigned to variables
• Python will automatically clear old
values out of memory in a process
known as garbage collection
Garbage Collection
2
After
x
2.3
X
Memory location
will be automatically
reclaimed by the
garbage collector

• So far, we have been using values specified by programmers and printed
or assigned to variables
• How can we let users (not programmers) input values?
• In Python, input is accomplished via an assignment statement
combined with a built-in function called input
• When Python encounters a call to input, it prints <prompt> (which is a
string literal) then pauses and waits for the user to type some text and
press the <Enter> key
Assigning Input
<variable> = input(<prompt>)

• Here is a sample interaction with the Python interpreter:
• Notice that whatever the user types is then stored as a string
• What happens if the user inputs a number?
Assigning Input
>>> name = input("Enter your name: ")
Enter your name: Mohammad Hammoud
>>> name
'Mohammad Hammoud'
>>>

• How can we force an input number to be stored as a number and not as
a string?
• We can use the built-in eval function, which can be “wrapped around” the
input function
Assigning Input
>>> number = input("Enter a number: ")
Enter a number: 3
>>> number
'3'
>>>
Still a string!

Assigning Input
>>> number = eval(input("Enter a
number: "))
Enter a number: 3
>>> number
3
>>>
Now an int
(no single quotes)!

Assigning Input
>>> number = eval(input("Enter a
number: "))
Enter a number: 3.7
>>> number
3.7
>>>
And now a float
(no single quotes)!

• Here is another sample interaction with the Python interpreter:
Assigning Input
>>> number = eval(input("Enter an equation: "))
Enter an equation: 3 + 2
>>> number
5
>>>
The eval function will evaluate this formula and
return a value, which is then assigned to the variable “number”

• Besides, we can convert the string output of the input function into an
integer or a float using the built-in int and float functions
Datatype Conversion
>>> number = int(input("Enter a number: "))
Enter a number: 3
>>> number
3
>>>
An integer
(no single quotes)!

• Besides, we can convert the string output of the input function into an
integer or a float using the built-in int and float functions
Datatype Conversion
>>> number = float(input("Enter a number: "))
Enter a number: 3.7
>>> number
3.7
>>>
A float
(no single quotes)!

• As a matter of fact, we can do various kinds of conversions between
strings, integers and floats using the built-in int, float, and str functions
Datatype Conversion
>>> x = 10
>>> float(x)
10.0
>>> str(x)
'10'
>>>
>>> y = "20"
>>> float(y)
20.0
>>> int(y)
20
>>>
>>> z = 30.0
>>> int(z)
30
>>> str(z)
'30.0'
>>>
integer  float
integer  string
string  float
string  integer
float  integer
float  string

• Python allows us also to assign multiple values to multiple variables all
at the same time
• This form of assignment might seem strange at first, but it can prove
remarkably useful (e.g., for swapping values)
Simultaneous Assignment
>>> x, y = 2, 3
>>> x
2
>>> y
3
>>>

• Suppose you have two variables x and y, and you want to swap their
values (i.e., you want the value stored in x to be in y and vice versa)
>>> x = 2
>>> y = 3
>>> x = y
>>> y = x
>>> x
3
>>> y
3
X CANNOT be done with
two simple assignments

• Suppose you have two variables x and y, and you want to swap their
values (i.e., you want the value stored in x to be in y and vice versa)
>>> x = 2
>>> y = 3
>>> temp = x
>>> x = y
>>> y = temp
>>> x
3
>>> y
2
>>>

CAN be done with
three simple assignments,
but more efficiently with
simultaneous assignment
Thus far, we have been using
different names for
variables. These names
are technically called
identifiers

• Python has some rules about how identifiers can be formed
• Every identifier must begin with a letter or underscore, which may be
followed by any sequence of letters, digits, or underscores
>>> x1 = 10
>>> x2 = 20
>>> y_effect = 1.5
>>> celsius = 32
>>> 2celsius
File "<stdin>", line 1
2celsius
^
SyntaxError: invalid syntax
Identifiers

• Identifiers are case-sensitive
>>> x = 10
>>> X = 5.7
>>> print(x)
10
>>> print(X)
5.7
Identifiers

• Some identifiers are part of Python itself (they are called reserved words or
keywords) and cannot be used by programmers as ordinary identifiers
Identifiers
False class finally is return
None continue for lambda try
True def from nonlocal while
and del global not with
as elif if or yield
assert else import pass
break except in raise
Python Keywords

• Some identifiers are part of Python itself (they are called reserved words or
keywords) and cannot be used by programmers as ordinary identifiers
Identifiers
>>> for = 4
File "<stdin>", line 1
for = 4
^
SyntaxError: invalid syntax
An example…

• You can produce new data (numeric or text) values in your program
using expressions
Expressions
>>> x = 2 + 3
>>> print(x)
5
>>> print(5 * 7)
35
>>> print("5" + "7")
57
 This is an expression that uses the
addition operator

using expressions
Expressions
>>> x = 2 + 3
>>> print(x)
5
>>> print(5 * 7)
35
>>> print("5" + "7")
57
addition operator
 This is another expression that uses the
multiplication operator

using expressions
Expressions
>>> x = 2 + 3
>>> print(x)
5
>>> print(5 * 7)
35
>>> print("5" + "7")
57
addition operator
 This is another expression that uses the
multiplication operator
 This is yet another expression that uses the
addition operator but to concatenate (or glue)
strings together

using expressions
Expressions
>>> x = 6
>>> y = 2
>>> print(x - y)
4
>>> print(x/y)
3.0
>>> print(x//y)
3
>>> print(x*y)
12
>>> print(x**y)
36
>>> print(x%y)
0
>>> print(abs(-x))
6
Yet another
example…
Another
example…

Expressions: Summary of Operators
Operator Operation
+ Addition
- Subtraction
* Multiplication
/ Float Division
** Exponentiation
abs() Absolute Value
// Integer Division
% Remainder
Python Built-In Numeric Operations

• Data conversion can happen in two ways in Python
1. Explicit Data Conversion (we saw this earlier with the int, float, and str
built-in functions)
2. Implicit Data Conversion
• Takes place automatically during run time between ONLY numeric values
• E.g., Adding a float and an integer will automatically result in a float value
• E.g., Adding a string and an integer (or a float) will result in an error since
string is not numeric
• Applies type promotion to avoid loss of information
• Conversion goes from integer to float (e.g., upon adding a float and an
integer) and not vice versa so as the fractional part of the float is not lost
Explicit and Implicit Data Type Conversion

Implicit Data Type Conversion: Examples
>>> print(2 + 3.4)
5.4
>>> print( 2 + 3)
5
>>> print(9/5 * 27 + 32)
80.6
>>> print(9//5 * 27 + 32)
59
>>> print(5.9 + 4.2)
10.100000000000001
>>>
 The result of an expression that involves
a float number alongside (an) integer
number(s) is a float number

Implicit Data Type Conversion: Examples
>>> print(2 + 3.4)
5.4
>>> print( 2 + 3)
5
>>> print(9/5 * 27 + 32)
80.6
>>> print(9//5 * 27 + 32)
59
>>> print(5.9 + 4.2)
10.100000000000001
>>>
a float number alongside (an) integer
number(s) is a float number
values of the same data type will not result
in any conversion

• One problem with entering code interactively into a Python shell is
that the definitions are lost when we quit the shell
• If we want to use these definitions again, we have to type them all over again!
• To this end, programs are usually created by typing definitions into a
separate file called a module or script
• This file is saved on disk so that it can be used over and over again
• A Python module file is just a text file with a .py extension, which can
be created using any program for editing text (e.g., notepad or vim)
Modules

• A special type of software known as a programming environment
simplifies the process of creating modules/programs
• A programming environment helps programmers write programs and
includes features such as automatic indenting, color highlighting, and
interactive development
• The standard Python distribution includes a programming
environment called IDLE that you can use for working on the
programs of this course
Programming Environments and IDLE

• Programs are composed of statements that are built from identifiers and
expressions
• Identifiers are names
• They begin with an underscore or letter which can be followed by a combination
of letter, digit, and/or underscore characters
• They are case sensitive
• Expressions are the fragments of a program that produce data
• They can be composed of literals, variables, and operators
Summary

• A literal is a representation of a specific value (e.g., 3 is a literal
representing the number three)
• A variable is an identifier that stores a value, which can change (hence,
the name variable)
• Operators are used to form and combine expressions into more complex
expressions (e.g., the expression x + 3 * y combines two expressions
together using the + and * operators)
Summary

• In Python, assignment of a value to a variable is done using the equal
sign (i.e., =)
• Using assignments, programs can get inputs from users and manipulate
them internally
• Python allows simultaneous assignments, which are useful for swapping
values of variables
• Datatype conversion involves converting implicitly and explicitly between
various datatypes, including integer, float, and string
Summary

• Functions- Part I
Next Lecture…

Lecture-2-Python-Basic-Elements-Sep04-2018.pptx

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