1. And now for something completely different . . .
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2. Part 4 More on Data Processing and Introducing Selection
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3. Python Data Processing and Selection
4.1 Python Program Structure
4.2 Memory Concepts
4.3 Arithmetic Expression Operators
4.4 Relational and Logical Operators
4.5 Selection
4.6 De Morgan’s Laws
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4. Python Data Processing and Selection
4.1 Python Program Structure
4.2 Memory Concepts
4.3 Arithmetic Expression Operators
4.4 Relational and Logical Operators
4.5 Selection
4.6 De Morgan’s Laws
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5. 1. Programs are composed of modules
Python Program Structure
1. Programs are composed of modules
2. Modules contain statements
3. Statements contain expressions
4. Expressions create and process objects
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6. Python Data Processing and Selection
4.1 Python Program Structure
4.2 Memory Concepts
4.3 Arithmetic Expression Operators
4.4 Relational and Logical Operators
4.5 Selection
4.6 De Morgan’s Laws
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7. Memory Concepts in Python
A variable is a name that refers to a value. The assignment statement creates new variables and gives them values: number1 = input("Enter first number:\n")
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8. Variables refer to Objects
Variable names such as number1 actually refer to Python objects.
Every object has a data type (e.g. int), a size (e.g. 4 bytes), a value (e.g. 10) and a location in the computer's memory. . .
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9. Creation of an object
number1 = input("Enter first number:\n")
Python first creates an object to hold the user-entered string and places the object into a memory location
"10"
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10. Creation of an object number1 "10"
number1 = input("Enter first number:\n")
The assignment symbol (=) then associates the name number1 with the newly created object.
number1
"10"
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11. A program cannot change an object's type or location
In Python, a program cannot change an object's type or location.
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12. A program cannot change an object's type or location
In Python, a program cannot change an object's type or location.
Only with a small selection of object types can the value of the object be changed. . .
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13. These objects are said to be immutable.
Python program statements cannot change the value of a number or string object.
These objects are said to be immutable.
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14. Creating a string object
10 is entered
number1 = input("Enter first number:\n")
print (number1, type(number1))
The output is:
10 <class 'str'>
number1
"10"
04-01.py
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15. Converting from one data type to another
number1 = input("Enter first number:\n")
print (number1, type(number1))
The int function creates a new object to store the integer value of the string entered in the first line.
number1 = int(number1)
number1
"10" 10
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16. Converting from one data type to another
number1 = input("Enter first number:\n")
print (number1, type(number1))
number1 = int(number1)
The new object has a different address which is then associated with the variable name number1 .
number1
"10" 10
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17. Converting from one data type to another
number1 = input("Enter first number:\n")
print (number1, type(number1))
Once an object no longer has a reference to it, its memory is released.
number1 = int(number1)
print (number1, type(number1))
number1
"10" 10
04-02.py
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18. Converting from one data type to another
number1 = input("Enter first number:\n")
print (number1, type(number1))
number1 = int(number1)
print (number1, type(number1))
Once an object no longer has a reference to it, its memory is released.
number1 10
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19. Displaying an objects memory location
number1 = input("Enter first number:\n")
print (number1, type(number1), id(number1))
number1 = int(number1)
print (number1, type(number1) ), id(number1))
A representation of the memory location of an object can be obtained by using the id function.
04-03.py
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20. Python Data Processing and Selection
4.1 Python Program Structure
4.2 Memory Concepts
4.3 Arithmetic Expression Operators
4.4 Relational and Logical Operators
4.5 Selection
4.6 De Morgan’s Laws
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21. Numeric Expressions (int)
2 + 4
6 - 4
6 * 3
6 / 3
6 % 3
6 // 3 -5
3**2
04-04.py
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22. Numeric Expressions (float)
6.0 * 3.0
6.0 / 3.0
6.0 % 3.0
6.0 // 3.0
-5.0
3.0**2.0
04-05.py
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23. Mixed Numeric Expressions
6 * 3.0
6 / 3.0
6 % 3.0
6 // 3.0
-5.0
3**2.0
04-06.py
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24. Python Data Processing and Selection
4.1 Python Program Structure
4.2 Memory Concepts
4.3 Arithmetic Expression Operators
4.4 Relational and Logical Operators
4.5 Selection
4.6 De Morgan’s Laws
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25. Relational operators relate two operands
7 > 10
4 < 16
4 == 4
4 <= 4
4 >= 4
4 != 4
These are Boolean expressions. The result of these expressions is either true (1) or false (0).
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26. Boolean expressions result in values true or false
7 > 10
4 < 16
4 == 4
4 <= 4
4 >= 4
4 != 4
04-07.py
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27. A Boolean Example number = 10 isPositive = (number > 0)
a boolean expression
04-08.py
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28. A Boolean Example number = 10 isPositive = (number > 0)
# the value true (1) is
# assigned to isPositive
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29. Combining Boolean expressions
You can combine Boolean expressions. For example, if you need to know if a person's age is greater than 21, AND they have a salary greater than 50 thousand dollars…..
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30. Combining Boolean Expressions with a Logical Operator (and)
age = 25
salary = 55000
print ((age > 21) and (salary > 50000))
04-09.py
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31. Logical operator: and (age > 21) and (salary > 50000)
The and is known as a logical operator.
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32. Logical (Boolean) Operators
and or
not
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33. Truth Tables of Boolean Operators
value of A value of B resulting value of
A and B
false false false
false true false
true false false
true true true
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34. Truth Tables of Boolean Operators
value of A value of B resulting value of
A or B
false false false
false true true
true false true
true true true
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35. Truth Tables of Boolean Operators
value of A resulting value of
not A
false true
true false
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36. When writing boolean expressions or arithmetic expressions, it is usually best to indicate the order of operations by using parentheses (brackets).
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37. If parentheses are not used in an expression, the computer will perform the operations in an order determined by the precedence rules. . .
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38. Precedence Rules lowest highest logical or logical and logical not
relational operators: <, >, <=, etc
+, -
*, /, %, //
-x, + x
( )
highest
Note: expressions in parentheses (brackets) are evaluated first.
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39. Python Data Processing and Selection
4.1 Python Program Structure
4.2 Memory Concepts
4.3 Arithmetic Expression Operators
4.4 Relational and Logical Operators
4.5 Selection
4.6 De Morgan’s Laws
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40. The if statement
The if statement makes use of a Boolean expression to decide which statement(s) to execute.
x = 'spam'
if x == 'spam':
print ('Hi spam')
else:
print ('not spam')
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41. The if statement The Boolean expression in this example is:
x == 'spam'
The expression has a value of true or false (1 or 0).
x = 'spam'
if x == 'spam':
print ('Hi spam')
else:
print ('not spam')
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42. The if statement
x = 'spam'
if x == 'spam':
print ('Hi spam')
else:
print ('not spam')
The Boolean expression is also known as the condition of the if statement.
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43. The if statement
x = 'spam'
if x == 'spam':
print ('Hi spam')
else:
print ('not spam')
If the condition is true, the first print statement is executed and the second one is skipped.
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44. The if statement
x = 'spam'
if x == 'spam':
print ('Hi spam')
else:
print ('not spam')
If the condition is false, the first print statement is skipped and the second one is executed.
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45. The if statement
x = 'spam'
if x == 'spam':
print ('Hi spam')
else:
print ('not spam')
It's possible to have multiple statements in the true or false sections of an if statement...
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46. The if statement
x = 'spam'
if x == 'spam':
print ('Hi spam')
print ('Hi Anne')
else:
print ('not spam')
print ('bye Anne')
If the condition is true, the first two print statements are executed and the second set are skipped.
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47. The if statement - syntax
The if statement starts with the keyword if followed by a Boolean expression, followed by a colon (:).
x = 'spam'
if x == 'spam':
print ('Hi spam')
print ('Hi Anne')
else:
print ('not spam')
print ('bye Anne')
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48. The if statement - syntax
Beneath the if line, the statements to be run if the condition is true are entered after pressing the Tab key or typing a few space characters.
x = 'spam'
if x == 'spam':
print ('Hi spam')
print ('Hi Anne')
else:
print ('not spam')
print ('bye Anne')
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49. The if statement - syntax
The statements to be run must be indented to the same level. It's recommended to press the Tab key before typing the statements.
x = 'spam'
if x == 'spam':
print ('Hi spam')
print ('Hi Anne')
else:
print ('not spam')
print ('bye Anne')
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50. The if statement - syntax
The else part of an if statement is optional, but if included, must be followed by a colon (:), and then the indented statement(s) to be executed if the condition is false.
x = 'spam'
if x == 'spam':
print ('Hi spam')
print ('Hi Anne')
else:
print ('not spam')
print ('bye Anne')
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51. The if statement 04-10.py 04-11.py 04-12.py x = 'spam' if x == 'spam':
print ('Hi spam')
print ('Hi Anne')
else:
print ('not spam')
print ('bye Anne')
04-10.py
04-11.py
04-12.py
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52. The nested if / elif / else statement
Nested if/else statements test for multiple cases by placing if/else selection structures inside other if/else selection structures…
score = input("Enter score: ")
score = int(score)
if score >= 80:
grade = 'A'
else:
if score >= 70
grade = 'B'
grade = 'C'
print ("\n\nGrade is: " + grade)
04-13.py
04-14.py
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53. The nested if statement
Nested if/else statements can be written using an alternate if/elif/else construct.
Program
04-14.py is exactly equivalent to py.
score = input("Enter score: ")
score = int(score)
if score >= 80:
grade = 'A'
elif score >= 70:
grade = 'B'
elif score >= 55:
grade = 'C'
elif score >= 50:
grade = 'Pass'
else:
grade = 'Fail'
print ("\n\nGrade is: " + grade)
04-15.py
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54. Python Data Processing and Selection
4.1 Python Program Structure
4.2 Memory Concepts
4.3 Arithmetic Expression Operators
4.4 Relational and Logical Operators
4.5 Selection
4.6 De Morgan’s Laws
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55. De Morgan’s Laws
1. A not and is equivalent to an or with two negated inputs.
2. A not or is equivalent to an and with two negated inputs.
04-16.py
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56. This presentation uses the following program files:
04-01.py
04-02.py
04-03.py
04-04.py
04-05.py
04-06.py
04-07.py
04-08.py
04-09.py
04-10.py
04-11.py
04-12.py
04-13.py
04-14.py
04-15.py
04-16.py
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57. End of Python3_Processing_Selection.ppt
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58. Last updated: Friday 15th May 2009, 09:13 PT, AHD
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