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, PhilipOffice: 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 = 0.0height_student = ask user inputsum += height_studentheight_student = ask user inputsum += 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 thisN 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-plannedand 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 - Looping1) To compute the average height of the

students in a class• We may repeat the height accumulation

based on the value of N

sum = 0.0REPEAT N times

ASK user for next student’s heightsum += height

END of REPEATaverage = sum / N

Same program can workno matter how many students; more reusable


Computational thinking - Selection2) Game: paper scissor rock

• A program can compare choices from human and computer, and then determine who wins

IF user_choice == paperand computer_choice == scissor

print("computer wins")IF user_choice == scissor

and computer_choice == paperprint("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!!!







Basic Concept: IF• Form #1: IF statement

IF condition is TrueTHEN

DO THISEND IF……

Hence… Program can make decisions!!!

Whether toexecute it or not depends on thecondition during program runtime

After that, continue the sequenceand execute the next instructionan IF statement


Basic Concept: IF• Form #1: IF statement


DO THISEND IF……

Adopted in many programming languages with proper indentation! Python forces you!!!

We usually indent thesestatement(s) to improve the code readability that this partbecomes to the true condition


Examples• Example #1

IF body temperature > 37.5THEN

print("Fever!")print("Time to see doctor!")

END IF…… The power of IF statement is that the

program can selectively run the blockbased 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


SUITE1ELSE

SUITE2END IF……

Execute true partif condition is true

After that, continue the sequenceand execute the next instruction

Execute false partif condition is false (also indented)

A (single) IF-ELSE statement in the top level


Examples• Example #2.1

IF body temperature > 37.5 THENprint("Fever! See doctor")

END IFIF body temperature <= 37.5 THEN

print("Normal!")END IF…… Here we use two consecutive

IF statements for true and false sidesAny issue? Efficiency? Redundancy?



IF body temperature > 37.5THEN

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 THENDO SUITE A

ELIF condition2 is True THENDO SUITE B

ELSEDO SUITE C

END IF……

Execute A if condition 1 is truethen skip B and C

No matter after which case, wecontinue here (sequence concept)

Execute B if condition 1 is falseand condition 2 is true, then skip C

Execute C if both conditions 1 and 2 are false

Else if

The whole statement


Basic Concept: IF-ELIF-ELSE• Form #3: IF-ELIF-ELSE



ELSEDO SUITE C

END IF……


Basic Concept: IF-ELIF-ELSE• Form #3: IF-ELIF-…-ELSE statement



ELIF condition3 is True THENDO SUITE C

ELSEDO SUITE D # conditions 1,2,3 are all false

END IF……

You may have many … manyELIF blocks

The whole statement


Examples• Example #3

Let’s write a simple number guessing game.The computer program randomly pick anumber 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.



……

IF my_guess > computer_num THEN print("Your guess is too large")

END IFIF my_guess < computer_num THEN

print("Your guess is too small")END IFIF my_guess == computer_num THEN

print("Bingo!!! Correct")END IF…… 1) Is this program logically correct?

2) Efficient? Any redundant check?

Three consecutiveIF statements



……


ELIF my_guess < computer_num THEN print("Your guess is too small")

ELIF my_guess == computer_num THENprint("Bingo!!! Correct")

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?




ELIF my_guess < computer_num THEN print("Your guess is too small")

ELSEprint("Bingo!!! Correct")

END IF……

When comparing two numbers, there areonly three possibilities!!!If it is not the first twocases, 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

IF condition1 is True THENDO THIS SUITE

END IF……



• Inside this True block (even the False block), you may have another IF statement(s)

IF condition1 is True THENIF condition2 is True THEN

DO THIS SUITEENDIF

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)


IF condition3 is True THENDO THIS

ENDIFENDIF

END IF

We DO THIS only ifcondition 1 is true, condition 2 is true, and condition 3 is trueSIMILAR to AND’ed



•Or even with ELSE inside…


DO SUITE AELSE

DO SUITE BEND IF

END IF



•Or use it on the False side…


ELSEIF condition2 is True THEN

DO SUITE BELSE

DO SUITE CEND IF

END IF

Which one to use?It’s all about the logicfor problem solving







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 possiblecombinations


1st attempt• So… Pseudo code #4.1

IF comp_choice == paper and your_choice == paper THENprint("draw")

END IF

IF comp_choice == paper and your_choice == rock THENprint("computer wins")

END IF

IF comp_choice == paper and your_choice == scissor THENprint("you win")

END IF…… # just continue with the

# remaining six cases ….

Is it logically correct?Good program?Efficient?

We exhaust all possiblecombinations of input


1st attempt• So… Pseudo code #4.1

IF comp_choice == paper and your_choice == paper THENprint("draw")

END IF

IF comp_choice == paper and your_choice == rock THENprint("computer wins")

END IF

IF comp_choice == paper and your_choice == scissor THENprint("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 THENIF your_choice == paper THEN

print("draw")END IFIF your_choice == rock THEN

print("computer wins")END IFIF 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• Pseudo code #4.3


print("draw")ELIF your_choice == rock THEN

print("computer wins")ELIF your_choice == scissor THEN


END IF……

Further use ELIF (else if) to avoid redundant checks!What check(s) may be avoided?

Exercise: Draw the flowchart


4th attempt• Pseudo code #4.4


print("draw")ELIF your_choice == rock THEN

print("computer wins")ELSE


END IF……

Further use ELSE to avoidone more redundant check!

Exercise: Draw the flowchart(it slightly differs from previous two)


Another approach - 5th attempt• Pseudo code #4.5

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


print("you win")END IF We may have many other solutions!

Different ways of computational thinking!

Think: based onpossible outcomes


Another approach - 6th attempt• Pseudo code #4.6… Even better

IF comp_choice == your_choice THENprint("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


print("you win")END IF Hope that you enjoy computational

thinking and programming…(note: we are not using Python yet)

Further simplified







Python SyntaxNow… 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 usecolon here

MUST useindentation for theentire 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 beginningof 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")print("case 2 here")

else:print("a == b")print("case 3 here")

In Python, indentationdefines a block!!!So if you continue to use the same amountof 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)elseprint("maximum value is ",c)

end

Any bug(s)here?

(no logic error)


Bugs fixed• Nested IF

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:print("maximum value is ",c)

six syntaxerrors 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







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 codewe saw previously

- Python code is simple and clear


Examples Revisit in Python• Example #2.2

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



Use this print()temporarily forcode testing when there is arandom scenario;When done,just comment it

Note:



Very tedious



Code Simplified



By analyzing the pattern, we canfurther simplify the code

statementcontinuity


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 appropriatelyinto 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: Control2.1 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

Technology

end ifend

true thenif

average height

flowcontroltopics

range function

pass case

nested loops

aspaper scissor