Python Programming - III. Controlling the Flow

PYTHON PROGRAMMING
III. Controlling the Flow Engr. Ranel O. Padon

PYTHON PROGRAMMING TOPICS
I
•  Introduction to Python Programming
II
•  Python Basics
III
•  Controlling the Program Flow
IV
•  Program Components: Functions, Classes, Modules, and Packages
V
•  Sequences (List and Tuples), and Dictionaries
VI
•  Object-Based Programming: Classes and Objects
VII
•  Customizing Classes and Operator Overloading
VIII
•  Object-Oriented Programming: Inheritance and Polymorphism
IX
•  Randomization Algorithms
X
•  Exception Handling and Assertions
XI
•  String Manipulation and Regular Expressions
XII
•  File Handling and Processing
XIII
•  GUI Programming Using Tkinter

CONTROLLING THE FLOW: STRUCTURES

DECISION-MAKING
our lives are filled with choices: 
 
1. Saan ako kakain?
2. Gi-gimmick ba ako sa Friday?
3. Gusto niya rin kaya ako?

DECISION-MAKING
many choices you make depend on other circumstances 

1. Saan ako kakain? 

May budget pa ba akong malupit? (Yes|No) 

2. Gi-gimmick ba ako sa Friday? 

Wala ba ako kelangang tapusin? (Yes|No) 

3. Gusto niya rin kaya ako?

Lagi ba syang nagpaparamdam sa akin? (Yes|No)

ALGORITHMS
Before writing a program to solve a particular problem,
it is essential to have a thorough understanding of the
problem and a carefully planned approach to solving the
problem.

The Elevator-Mirror Problem

ALGORITHMS
Any computing problem can be solved by executing a series of
actions in a specified order. 
 
An algorithm is a procedure for solving a problem in terms of 

1. actions to be executed

2. the order of execution

ALGORITHMS
“rise-and-shine” algorithm for getting out of bed and
going to work:  

(1) get out of bed

(2) take off pajamas

(3) take a shower,

(4) get dressed

(5) eat breakfast

(6) carpool to work.

ALGORITHMS
Suppose that the same steps are performed in a
slightly different order:

(1) get out of bed

(2) take off pajamas

(3) get dressed

(4) take a shower

(5) eat breakfast

(6) carpool to work

PSEUDOCODE
Pseudocode is an artificial and informal language that
helps programmers develop algorithms. 
 

Characteristics
q similar to everyday English
q convenient and user-friendly
q not an actual computer programming language

PSEUDOCODE
Pseudocode helps the programmer “plan” a program
before attempting to write it in a programming language 
 
A carefully prepared pseudocode program can be converted easily
to a corresponding Python program

PROGRAM CONTROL
Program Control 

specifying the order in which statements are to be executed
in a computer program

CONTROL STRUCTURES
By default, program execution is sequential. 
 
You could break this behavior using transfer of control. 
 
Transfer of Control makes use of Control Structures.

STRUCTURED PROGRAMMING
In the 1960’s, indiscriminate use of transfer of controls,
especially goto statements, resulted to spaghetti code. 
 
Structured programming is synonymous to goto elimination or
goto-less programming.

STRUCTURED PROGRAMMING
Structured programs are clearer, easier to debug and modify and
more likely to be bug-free in the first place.

CONTROL STRUCTURES
Structured programs could be written in terms of
three control structures: 
 

I. Sequence
 

II. Selection

III. Repetition

CONTROL STRUCTURES
I. Sequence Control Structure 
 
- the default flow of program

CONTROL STRUCTURES
II. Selection Control Structures 
 

a. if (single selection)

b. if/else (double selection)

c. if/elif/else (multiple selection)

CONTROL STRUCTURES
III. Repetition Control Structures 
 

a. while (single selection)

b. for (double selection)

CONTROL STRUCTURES
In Summary: 
 

I. Sequence
 

II. Selection (if, if/else, if/elif/else)

III. Repetition (while, for)

CONTROL STRUCTURES
Any Python program can be constructed from 6 different types of
control structures (sequence, if, if/else, if/elif/else, while and for)
combined in 2 ways (control-structure stacking and control-structure
nesting)

CONDITIONAL STATEMENT
A conditional statement is basically a simple yes or no question. 

‘Mahal mo ba ako?’:

‘Oo’:

“Pakasal na tayo.”

‘Hindi Eh’:

“Iyak na lang ako.”

CONDITIONAL STATEMENTS
Conditional statements are one of the most important programming
concepts: They let your programs react to different situations and
behave intelligently.

CONDITIONAL STATEMENTS
Conditional statements are also called “if/then” statements,
because they perform a task only if the answer to a question is
true:

‘If may sapat akong pera, then bibili ako ng Windows 8!’

if SELECTION STRUCTURE
if (kondisyon):

# mga gagawin kung totoo ung kondisyon 

 
if (maitim == True):
print “maitim ako!”

Note: Parentheses are optional.

Pseudocode:

Code:

Flowchart:

if/elif/else SELECTION STRUCTURE

A nested if/else structure is faster than a series of single-selection if
structures because the testing of conditions terminates after one of
the conditions is satisfied.

In a nested if/else structure, place the conditions that are more likely
to be true at the beginning of the nested if/else structure.

This enables the nested if/else structure to run faster and exit earlier

EMPTY STATEMENT
if gender == “sirena”:
pass

COUNTER-CONTROLED REPETITION
Because floating-point values may be approximate, controlling the
counting of loops with floating-point variables may result in imprecise
counter values and inaccurate tests for termination.

Programs should control counting loops with integer values.

SENTINEL-CONTROLED REPETITION
Sentinel Value:

also called as dummy value, signal value or flag value

SENTINEL-CONTROLED REPETITION
In a sentinel-controlled loop, the prompts requesting data entry
should explicitly remind the user of the sentinel value.

NESTED CONTROL STRUCTURES
Write a program to summarize the exam results of 10 students. Next
to each name is written a I if the student passed the exam and a 2 if
the student failed. 
 
1. Input each test result (i.e., a 1 or a 2). Display the message

“Enter result” on the screen each time the program
requests

another test result.
2. Count the number of test results of each type.
3. Display a summary of the test results: the number of students

who passed and the number of students who failed.
4. If more than 8 students passed the exam, print the message

“Raise tuition.”

Sample Run 1:

Sample Run 2:

The most difficult part of solving a problem on a computer is
developing an algorithm for the solution.

Once a correct algorithm has been specified, the process of
producing a working Python program from the algorithm normally is
straightforward.

for REPETITION STRUCTURE
the built-in range(start, end, step) function returns a list containing
integers in the range of start to end-1. 

 
What is range(10, 0, -1)?

PRACTICE EXERCISE: 
 
using for loop print the following number series:

a.) 7, 14, 21, …, 70, 77

b.) 20, 18, 16, …, 4, 2

c.) 99, 88, 77, …, 11, 0

Compound Interest

STRUCTURED PROGRAMMING SUMMARY

break AND continue STATEMENTS
programmer could also alter the flow of a loop
using the break and continue statements

break STATEMENT
breaks or causes immediate exit from while or for structure

continue STATEMENT
skips the remaining statements in the body of a while or for structure
and proceeds with the next iteration of the loop

continue STATEMENT
causes immediate exit from while or for structure

break AND continue STATEMENTS
break and continue statements are also used to improve performance
by minimizing the amount of processing by causing early exit or
avoiding unneeded computations or cases.

LOGICAL OPERATORS
Relational Operators:

<, >, <=, >=, ==, !=

Logical Operators:

and, or, not

AUGMENTED ASSIGNMENT
Assignment Expressions could be abbreviated.

AUGMENTED ASSIGNMENT
The += symbol adds the value of the expression on the right of the
+= sign to the value of the variable on the left of the sign and stores
the result in the variable on the left of the sign.

PYTHON 2.2 KEYWORDS
special/reserved words used for control structures &
other Python features

they could not be used as variable names

PRACTICE EXERCISE 1
Write a program that reads a positive integer and
determines if it is a prime number or not.

PRACTICE EXERCISE 2
A palindrome is a number or a text phrase that reads the same
backwards or forwards.

For example, each of the following five-digit integers is a palindrome:
12321, 55555, 45554 and 11611.

Write a program that reads in a five-digit integer and determines
whether it is a palindrome. (Hint: Use the division and modulus
operators to separate the number into its individual digits.)

PRACTICE EXERCISE 3
Write a program that reads a nonnegative integer and computes and
prints its factorial. 
 
The factorial of a nonnegative integer n is written n!

n! = n· (n - 1) · (n - 2) · … · 1

(for values of n >= 1) and

n! = 1

(for n = 0).

For example, 5! = 5 · 4 · 3 · 2 · 1, which is 120.

PRACTICE EXERCISE 4
What is the output of this? Explain why. 

for row in range(0,5):
print “*”

PRACTICE EXERCISE 5

for column in range(0,3):
print “*”
print

PRACTICE EXERCISE 6

print “*”,
print

PRACTICE EXERCISE 7

for group in range(0,3):
print "*",
print
print

To be able to control the flow is powerful!

REFERENCES
q  Deitel, Deitel, Liperi, and Wiedermann - Python: How to Program (2001).
q  Disclaimer: Most of the images/information used here have no proper source
citation, and I do not claim ownership of these either. I don’t want to reinvent the
wheel, and I just want to reuse and reintegrate materials that I think are useful or
cool, then present them in another light, form, or perspective. Moreover, the
images/information here are mainly used for illustration/educational purposes
only, in the spirit of openness of data, spreading light, and empowering people
with knowledge. J

Python Programming - III. Controlling the Flow

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