Python Lecture 2

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BS GIS Instructor: Inzamam Baig
Lecture 2
Fundamentals of Programming

Values and Types
A value is one of the basic things a program works with, like a
letter or a number
e.g 1, 2, and “Hello, World!”
2 is an integer, and “Hello, World!” is a string, so called because it
contains a “string” of letters

Type
If you are not sure what type a value has, the interpreter can tell
you
>>> type('Hello, World!')
>>> type(17)
>>> type(3.2)

Values and Types
What about values like ’17’ and ‘3.2’?
>>> type('17')
>>> type('3.2')

>>> print(1,000,000)
Python interprets 1,000,000 as a commaseparated sequence of
integers, which it prints with spaces between

Variables
A variable is a name that refers to a value
An assignment statement creates new variables and gives them
values:
>>> message = ‘Department of Computer Science'
>>> n = 17
>>> pi = 3.1415926535897931

Print()
To display the value of a variable, you can use a print statement:
n = 17
pi = 3.141592653589793
>>> print(n)
17
>>> print(pi)
3.141592653589793
The type of a variable is
the type of the value it
refers to.
>>>type(message)
>>>type(n)
>>> type(pi)

Variable Names and Keywords
Programmers generally choose names for their variables that are
meaningful and document what the variable is used for
Variables Names can contain both letters and numbers, but they cannot
start with a number
It is legal to use uppercase letters, but it is a good idea to begin variable
names with a lowercase letter
The underscore character ( _ ) can appear in a name
Variable names can start with an underscore character1

Variable Naming
>>>76trombones = 'big parade'
SyntaxError: invalid syntax
>>>more@ = 1000000
>>>class = 'Advanced Theoretical Zymurgy'
SyntaxError: invalid syntax 1

Keywords
and del from None True as
elif
global nonlocal try assert else if
not
while break except import or with
class
False in pass yield continue finally
is
raise async def for lambda return
await

Statements
A statement is a unit of code that the Python interpreter can
execute
A script usually contains a sequence of statements

Operators and Operands
Operators are special symbols that represent computations like addition and multiplication
The values the operator is applied to are called operands.
The operators +, -, *, /, and ** perform addition, subtraction, multiplication, division, and
exponentiation
20+32
hour-1
hour*60+minute
minute/60
5**2
(5+9)*(15-7)

/ Operator in Python 3 and Python 2
The division operator in Python 2.0 would divide two integers and
truncate the result to an integer
>>>minute = 5
>>>minute/2
2

/ Operator in Python 3 and Python 2
In Python 3.x, the result of this division is a floating point result:
>>>minute = 5
>>>minute/2
2.5
To get the floored division in python 3.x use //
>>>minute = 5
>>>minute/2
2

Expressions
An expression is a combination of values, variables, and
operators
A value all by itself is considered an expression, and so is a
variable, so the following are all legal expressions
17
x
x + 17

Expressions
In Interactive Mode:
the interpreter evaluates it and displays the result:
>>> 1 + 1
2
In Script:
But in a script, an expression all by itself doesn’t do
anything

Order of operations
the order of evaluation depends on the rules of precedence
For mathematical operators, Python follows mathematical convention
The acronym PEMDAS is a useful way to remember the rules
Parentheses (1+1)**(5-2)
Exponentiation 3*1**3
Multiplication and Division 6+4/2
Addition and Subtraction 5-3-1

Modulus Operator
The modulus operator works on integers and yields the
remainder
>>>quotient = 7 // 3
>>>print(quotient)
2
>>>remainder = 7 % 3
>>>print(remainder)
1

String Operations
+ operator performs concatenation, which means joining the strings by
linking them end to end
>>>first = 10
>>>second = 15
>>>print(first+second)
25
>>>first = '100‘
>>>second = '150'
>>>print(first + second)
100150

String Operations
The * operator also works with strings by multiplying the content
of a string by an integer
>>>first = 'Test '
>>>second = 3
>>>print(first * second)
Test Test Test

Asking the User for Input
Sometimes we would like to take the value for a variable from the user
via their keyboard
Python provides a built-in function called input that gets input from the
keyboard
When this function is called, the program stops and waits for the user
to type something
When the user presses Return or Enter, the program resumes and
input returns what the user typed as a string

Asking the User for Input
>>>inp = input(‘Enter Your Name: ’)
Enter Your Name: Adnan
>>>print(inp)
Adnan

Numbers Input
speed = input('Enter the speed in Miles: ')
miles_to_km = speed / 0.62137
print(miles_to_km)
Enter the speed in Miles: 1
Traceback (most recent call last):
File "C:/Users/Inzamam Baig/Desktop/speed.py", line 2, in
<module>
miles_to_km = speed / 0.62137
TypeError: unsupported operand type(s) for /: 'str' and
'float'

Comments
As programs get bigger and more complicated, they get more difficult to
read
it is a good idea to add notes to your programs to explain what your code in
doing
In Python they start with the # symbol
# compute the percentage of the hour that has elapsed
percentage = (minute * 100) / 60
Everything from the # to the end of the line is ignored; it has no effect on the
program

Comments
v = 5 # assign 5 to v
v = 5 # velocity in meters/second

Choosing mnemonic variable names
follow the simple rules of variable naming, and avoid reserved
words
a = 35.0
b = 12.50
c = a * b
print(c)
hours = 35.0
rate = 12.50
pay = hours * rate
print(pay)
x1q3z9ahd = 35.0
x1q3z9afd = 12.50
x1q3p9afd = x1q3z9ahd * x1q3z9afd
print(x1q3p9afd)

Choosing mnemonic variable names
We call these wisely chosen variable names “mnemonic variable
names”.
The word mnemonic2 means “memory aid”.
We choose mnemonic variable names to help us remember why
we created the variable in the first place

Debugging
>>>bad name = 5
>>> month = 09
File "", line 1
month = 09 ^
SyntaxError: invalid token

Debugging
>>>principal = 327.68
>>>rate = 0.5
>>>interest = principle * rate
NameError: name 'principle' is not defined
>>>1.0 / 2.0 * pi
Variables names are case sensitive
Apple is not same as apple or APPLE

Python Lecture 2

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