Lecture 6.1 flow control selection

1 of 66Module 6 : Flow control
Introduction to
Computational Thinking
Module 6 : Flow control #1
Asst Prof Chi‐Wing FU, Philip
Office: N4‐02c‐104
email: cwfu[at]ntu.edu.sg

Topics
• Basic Concepts: Why Selection and Repetition
• Selection (Branching)
• Basic concepts
• Case study: Paper Scissor Rock
• Syntax
• Examples in Python
• Repetition (Looping)
• Basic concepts: for and while
• Syntax: while
• The range function
• Syntax: for
• Nested loops
• break, continue, and pass
• Case Study: Visual Example - Path of a projectile

Basic Concepts
• The Power of Computer Programs
• Program Execution
• Why Selection?
• Why Repetition?
• What is Flow Control / Control Flow?

The Power of Computer Programs
• Help automation
• Can repeat computation (which may be routine
job) with high reliability and reusability
• Reliability – low error rate (compared to human)
• Reusability – same piece of code run again and again
• Can be embedded to enhance many daily life
devices: phone, TV, fridge, cooker, etc.
• How many computers you now have?

Program Execution
• Given a computer program, CPU runs its
instructions one by one
This program can automate the computation of circle area
and circumference; no matter how many times we run it,
it can reliably do the same computation consistently

Program Execution
• But… This program only shows a basic kind of
control flow, called “sequence”
• If we only have “sequence” in programming…
Is it enough? What cannot be done?
Step 1 Step 2 Step 3 ……

Sequence alone?
If I ask you to write a program for …
1. Computing the average height of
students in a class?
2. A simple computer game such as
paper scissor rock?

Sequence alone?
1) Computing the average height of
students in a class?
• Basic idea for the algorithm:
Sum all height values and Divide it by total
• If we only have sequence…
We need to repeat certain instruction, e.g.,
accumulate height of students, exactly N times,
where N is the total number of students

Sequence alone?
Implementation:
sum = 0.0
height_student = ask user input
sum += height_student
height_student = ask user input
sum += height_student
...
average = sum / N
But…
• Different classes have different number of students!!!
• So, the program is not reusable and has to be
changed for different classes and years. Tedious!
Need to
repeat this
N times

Sequence alone?
2) A simple computer game such as
paper scissor rock?
• Basic idea for the algorithm here:
1. First, the computer program has to
randomize a choice out of the three
2. Then get user input (user’s choice)
3. Finally, the computer program
compares the two choices and
determines who wins

Sequence alone?
But… in the final step…
• How can a computer program compare and
tell who wins?
• With “sequence” …Every step is pre-planned
and fixed; there is ONLY one possible
consequence (control flow) in the program
• But… sometimes
human wins; sometimes
computer wins...

So… we need
• Selection – A computer program can
dynamically choose which instruction(s) to be
executed next based on certain condition(s)
during the program runtime
• Different instructions can be selected to run at
different time
• BUT the program is the same (we do not need to
change and compile it again for different situations;
we already define possible program responses for
different cases when writing the program)

So… we need
• Looping – A computer program can
dynamically choose how many times it
repeats certain instruction(s) during the
program runtime
• Program instructions can be repeated dynamically;
sometimes 3 times, sometimes 1000 times, or
sometimes even 0 times
• Again, the program is the same (we do not need to
change it for different number of repetitions)

Computational thinking - Looping
1) To compute the average height of the
students in a class
• We may repeat the height accumulation
based on the value of N
sum = 0.0
REPEAT N times
ASK user for next student’s height
sum += height
END of REPEAT
average = sum / N
Same program can work
no matter how many
students; more reusable

Computational thinking - Selection
2) Game: paper scissor rock
• A program can compare choices from human and
computer, and then determine who wins
IF user_choice == paper
and computer_choice == scissor
print("computer wins")
IF user_choice == scissor
and computer_choice == paper
print("human wins")
... # for other cases

What is Flow Control / Control Flow?
• It is to control which instruction to be
executed next
• By default, it is defined by the “sequence”
concept, i.e., one after the other
• But selection and repetition can alter the flow…
When you write/read a program,
make sure you understand the flow!!!
i.e., what is to be executed next for every step…
The flow control in a program -> Its Logic!!!

Topics
• Basic concepts
• Syntax
• Syntax: while
• Syntax: for
• Nested loops

Basic Concept: IF
• Form #1: IF statement
IF condition is True
THEN
DO THIS
END IF
……
Hence… Program can make decisions!!!
Whether to
execute it or not depends on the
condition during program runtime
After that, continue the sequence
and execute the next instructionan IF statement

Basic Concept: IF
• Form #1: IF statement
THEN
DO THIS
END IF
……
Adopted in many programming languages with
proper indentation! Python forces you!!!
We usually indent these
statement(s) to improve the
code readability that this part
becomes to the true condition

Examples
• Example #1
IF body temperature > 37.5
THEN
print("Fever!")
print("Time to see doctor!")
END IF
……
The power of IF statement is that the
program can selectively run the block
based on the runtime condition!!!
Sometimes run it! Sometimes skip it!
In Python, this group
of indented statement(s)
is called a suite/block.
(compound statement:
a set of statements
being used as a group)

Basic Concept: IF-ELSE
• Form #2: IF-ELSE
THEN
SUITE1
ELSE
SUITE2
END IF
……
Execute true part
if condition is true
After that, continue the sequence
and execute the next instruction
Execute false part
if condition is false (also indented)
A (single) IF-ELSE statement in the top level

Examples
• Example #2.1
IF body temperature > 37.5 THEN
print("Fever! See doctor")
END IF
IF body temperature <= 37.5 THEN
print("Normal!")
END IF
…… Here we use two consecutive
IF statements for true and false sides
Any issue? Efficiency? Redundancy?

Examples
• Example #2.2
IF body temperature > 37.5
THEN
print("Fever! See doctor")
ELSE
print("Normal!")
END IF
……
The power of IF-ELSE
statement is that the program
becomes more efficient!
JUST ONE checking
(temperature > 37.5) can let us
know which way to go!!
Avoid redundant check!

Basic Concept: IF-ELIF-ELSE
• Form #3: IF-ELIF-ELSE
IF condition1 is True THEN
DO SUITE A
ELIF condition2 is True THEN
DO SUITE B
ELSE
DO SUITE C
END IF
……
Execute A if condition 1 is true
then skip B and C
No matter after which case, we
continue here (sequence concept)
Execute B if condition 1 is false
and condition 2 is true, then skip C
Execute C if both conditions
1 and 2 are false
Else if
The whole statement

• Form #3: IF-ELIF-ELSE
DO SUITE A
DO SUITE B
ELSE
DO SUITE C
END IF
……

• Form #3: IF-ELIF-…-ELSE statement
DO SUITE A
DO SUITE B
DO SUITE C
ELSE
DO SUITE D # conditions 1,2,3 are all false
END IF
……
You may have
many … many
ELIF blocks
The whole statement

Examples
• Example #3
Let’s write a simple number guessing game.
The computer program randomly pick a
number in-between 0 and 50 (inclusively).
You can make a guess and the computer
can tell you whether your guess is the same,
too large, or too small.

Examples
• Example #3.1
……
IF my_guess > computer_num THEN
print("Your guess is too large")
END IF
IF my_guess < computer_num THEN
print("Your guess is too small")
END IF
IF my_guess == computer_num THEN
print("Bingo!!! Correct")
END IF
…… 1) Is this program logically correct?
2) Efficient? Any redundant check?
Three consecutive
IF statements

Examples
• Example #3.2
……
ELIF my_guess < computer_num THEN
ELIF my_guess == computer_num THEN
END IF
……
1. Is it better?
If condition 1 is true,
no need to check
others (they must be F)
2. But can you make
it even better?

Examples
• Example #3.3
ELIF my_guess < computer_num THEN
ELSE
END IF
……
When comparing two
numbers, there are
only three possibilities!!!
If it is not the first two
cases, must be the
third case!!!

Basic Concept: Nested IF
• Form #4: Nested IF statement
• Recall that for each IF statement, there is an
associated block for its TRUE part
DO THIS SUITE
END IF
……

• Inside this True block (even the False block),
you may have another IF statement(s)
DO THIS SUITE
ENDIF
END IF
……
This is called
nested IF
* Note the proper
indentation

• And… inside the TRUE block of the TRUE
block, you may still use IF statement(s)
DO THIS
ENDIF
ENDIF
END IF
We DO THIS only if
condition 1 is true,
condition 2 is true,
and condition 3 is true
SIMILAR to AND’ed

• Or even with ELSE inside…
DO SUITE A
ELSE
DO SUITE B
END IF
END IF

• Or use it on the False side…
DO SUITE A
ELSE
DO SUITE B
ELSE
DO SUITE C
END IF
END IF
Which one to use?
It’s all about the logic
for problem solving

Topics
• Basic concepts
• Syntax
• Syntax: while
• Syntax: for
• Nested loops

Case Study: paper scissor rock
• Recall our paper scissor rock game
• Assume:
Variable comp_choice stores computer’s
choice on paper, scissor, or rock, while
Variable your_choice stores your choice

Case Study: paper scissor rock
• Let’s analyze the problem…
• Without using IF-ELSE and nested-IF, if we
only use IF statements, how many possible
combinations of choices?
There are
3 x 3 possible
combinations

1st attempt
• So… Pseudo code #4.1
IF comp_choice == paper and your_choice == paper THEN
print("draw")
END IF
IF comp_choice == paper and your_choice == rock THEN
END IF
IF comp_choice == paper and your_choice == scissor THEN
print("you win")
END IF
…… # just continue with the
# remaining six cases ….
Is it logically correct?
Good program?
Efficient?
We exhaust all possible
combinations of input

1st attempt
• So… Pseudo code #4.1
IF comp_choice == paper and your_choice == paper THEN
print("draw")
END IF
IF comp_choice == paper and your_choice == rock THEN
END IF
IF comp_choice == paper and your_choice == scissor THEN
print("you win")
END IF
……
Notice anything in common?
… the first three IF statements?
Do we check them redundantly

2nd attempt
• Pseudo code #4.2
IF comp_choice == paper THEN
IF your_choice == paper THEN
print("draw")
END IF
IF your_choice == rock THEN
END IF
IF your_choice == scissor THEN
print("you win")
END IF
END IF
……
We can combine them by
using nested IF… then, can
avoid redundant comparison!!!
Exercise: Draw the flowchart

3rd attempt
print("draw")
ELIF your_choice == rock THEN
ELIF your_choice == scissor THEN
print("you win")
END IF
END IF
……
Further use ELIF (else if) to
avoid redundant checks!
What check(s) may be avoided?

4th attempt
print("draw")
ELIF your_choice == rock THEN
ELSE
print("you win")
END IF
END IF
……
Further use ELSE to avoid
one more redundant check!
(it slightly differs from previous two)

Another approach - 5th attempt
IF ( comp_choice == paper and your_choice == paper )
or ( comp_choice == rock and your_choice == rock )
or ( comp_choice == scissor and your_choice == scissor ) THEN
print("draw")
ELIF ( comp_choice == paper and your_choice == rock )
or ( comp_choice == rock and your_choice == scissor )
or ( comp_choice == scissor and your_choice == paper ) THEN
ELSE
print("you win")
END IF We may have many other solutions!
Different ways of computational thinking!
Think: based on
possible outcomes

Another approach - 6th attempt
• Pseudo code #4.6… Even better
IF comp_choice == your_choice THEN
print("draw")
ELIF ( comp_choice == paper and your_choice == rock )
or ( comp_choice == rock and your_choice == scissor )
or ( comp_choice == scissor and your_choice == paper ) THEN
ELSE
print("you win")
END IF Hope that you enjoy computational
thinking and programming…
(note: we are not using Python yet)
Further simplified

Topics
• Basic concepts
• Syntax
• Syntax: while
• Syntax: for
• Nested loops

Python Syntax
Now… let’s look at how Python implements
• IF statement
• IF-ELSE statement
• IF-ELIF-ELSE statement
• Nested IF

Syntax #1
• IF statement in Python
Syntax:
if <a boolean expression> :
one or more indented statements
Example:
if a > b:
print( "a > b" )
MUST use
colon here
MUST use
indentation for the
entire true block

Syntax #1 cont.
Note:
1) Colon : marks the
beginning of a block!
2) In Python, indentation -- not just for readability
but also for defining the scope of a block!
3) Number of whitespaces for indentation is
flexible but should be consistent for the same
level in the same program
More keywords in Python
• else (see syntax #2)
• elif (see syntax #3)

Syntax #2
• IF-ELSE statement
if a > b:
print("a > b")
else:
print("a <= b")
In Python, always
Remember that colon
marks the beginning
of a block/suite
*** Don’t forget it!!!
else syntax error

Syntax #3
• IF-ELIF-ELSE statement
if a > b:
print("a > b")
print("case 1 here")
elif a < b:
print("a < b")
else:
print("a == b")
In Python, indentation
defines a block!!!
So if you continue to
use the same amount
of indentation, it is
still in the same block

Nested IF – No new syntax
• Comparing a, b, and c:
if a >= b:
if a >= c:
print("maximum value is ",a)
else
print("maximum value is ",c)
else
if b >= c:
print("maximum value is ",b)
else
end
Any bug(s)
here?
(no logic error)

Bugs fixed
• Nested IF
if a >= b:
if a >= c:
print("maximum value is ",a)
else:
else:
if b >= c:
print("maximum value is ",b)
else:
six syntax
errors fixed!
need
proper
indentation

Syntax #4
• In Python
• 0, ‘’, [ ] or other “empty” objects are equivalent
to False; anything else is equivalent to True
Run it
More info: http://docs.python.org/library/stdtypes.html#truth-value-testing

Topics
• Basic concepts
• Syntax
• Syntax: while
• Syntax: for
• Nested loops

Python Implementation
• Now… Let’s revisit the examples we went
through in real Python code
• See the beauty and simplicity of Python

Examples Revisit in Python
• Example #1
- Compare with the pseudo code
we saw previously
- Python code is simple and clear

• Example #2.2
- Remember to put colon also for else!
(it marks the beginning of a block)

• Example #3.3
Use this print()
temporarily for
code testing
when there is a
random scenario;
When done,
just comment it
Note:

• Example #4.1
Very tedious

• Example #4.4
Code Simplified

• Example #4.6
By analyzing the pattern, we can
further simplify the code
statement
continuity

Exercises!
Trace these code carefully and understand its control flow.
Can you draw the flow charts for these code?
What is/are the conditions for reaching each print()?
Note: condition_A, condition_B, etc., are boolean variables

More Exercises!

Take Home Messages
• Computational thinking or algorithm design requires the
careful and thoughtful applications of sequence, selection
(branching) and repetition (looping) to control the flow
• #1 Understand: be able to trace code and understand the
control flow inside a given program
• #2 Analysis: Given a problem, think and analyze carefully
different possibilities/combinations and the logic
• #3 Apply: Furthermore, transform the logic appropriately
into IF-ELSE, IF-ELIF-ELSE and nested IF
• #4 Test: Lastly, use sufficient test data to evaluate your
program with different consequences
Last word… practice practice practice

Reading Assignment
• Textbook
Chapter 2: Control
2.1 to 2.4
Note: Though some material (2.3 and 2.4) in
textbook is not directly related to the lecture
material, you can learn more from them.

Lecture 6.1 flow control selection

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