Chapter 7Problem Solving with

Loops

CMPF144 Introduction to Problem Solving and Basic CMPF144 Introduction to Problem Solving and Basic ComputerComputer

ObjectivesObjectives1. Develop problems using loop logic structure

with decision and sequential logic structures2. Use problem-solving tools (IPO Chart,

Pseudocode, Flowchart) when developing solution using loop logic structure

3. Use counters and accumulators in problem solution

Objectives (Cont.)Objectives (Cont.)4. Distinguish different uses of two types of

loop logic structures5. Distinguish different uses of two loop

designs6. Use nested loop instructions to develop

problem solution7. Use recursion in simple problem

Loop Logic StructureLoop Logic Structure Also known as Repetition Control Structure Is used when we need to perform certain SAMESAME

actions REPEATEDLYREPEATEDLY Main DIFFICULTY: to identify which instructions should

be repeated Performing TWO (2)TWO (2) standard tasks:

Counting (incrementing @ decrementing) Accumulating ( calculating a sum @ total)

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Loop Logic Structure Loop Logic Structure (Cont.)(Cont.) Can be categorized into TWO (2)TWO (2) main types:

While/End_While Repeat/Until

Each type of loop structure has a specific use according to language / problem

Each type of structure DO NOT share the same Pseudocode and Flow chart

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Can be either INCREMENTINGINCREMENTING or DECREMENTINGDECREMENTING Incrementing is done by adding a constant, such as 1 or 2, to

the value of a variable Example: Decrementing is a process of subtracting a constant from a

current variable’s value Example: Variable names are the samesame on both sides of equal sign. Is a type of assignment instruction

i = i + 1

CountingCounting6

counter = counter - 2

AccumulatingAccumulating A process of summing up a group of numbers Similar to incrementing, except a variablevariable

instead of a constant is added to another variable

Example:

Is also a type of assignment instruction

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total = total + numbernumber

While/End_WhileWhile/End_While

Using the keyword of WHILE…WHILE…THEN….END_WHILETHEN….END_WHILE

Tells the computer that while the condition is TRUETRUE, it should repeat ALL instructions between the WHILE and END_WHILE (loop)

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While/End_While (Cont.)While/End_While (Cont.) Diamond is used for the whilewhile part, which represents

condition / decision Decision will be checked before executing any instructions

within the loop If resultant is FALSEFALSE at the start, the instructions within

the loop will NOT be executed If resultant is TRUETRUE, the complete set of instructions will be

executed, and computer will go back to the beginning of the loop and process the condition again, until it is FALSEFALSE

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While/End_While (Cont.)While/End_While (Cont.) Pseudocode Flow Chart

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Begin . . WHILE condition(s) THEN

Instruction Instruction

END_WHILE . .End

Exercise 1:Exercise 1:Develop an IPO chart, pseudocode, flowchart and complete Python program, to find the average age of all the students in a class

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IPO ChartIPO Chart

Input Process Outputage 1. Get age

2. Calculate average3. Display average

average

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BeginSet count = 0Set sum = 0.0Get ageWHILEWHILE age <> 0 THENTHENsum = sum + agecount = count + 1Get ageEND_WHILEEND_WHILEaverage = sum / countDisplay average

End

PseudocodePseudocode13

Tell me, Which one is:

•Primer Read??•Trip value @ flag??

Flow ChartFlow Chart14

TF

Set sum = 0.0

Begin

Set count = 0

count = count + 1

End

average = sum/count

Display average

Get age

age <> 0

sum = sum + age

Get age

sum = 0.0count = 0age = input("How old are you:")while age <> 0: sum = sum + age count = count + 1 age = input("How old are you:")average = sum/countprint "The average age is", average

Python CodePython Code15

Output:Output:

Repeat/UntilRepeat/Until

Using the keyword of REPEAT….UNTILREPEAT….UNTIL Tells the computer to repeat the set of

instructions between the Repeat and the Until, until a condition is TRUETRUE

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Repeat/Until (Cont.)Repeat/Until (Cont.) TWO (2) TWO (2) major differences between Repeat/Until loop

with While/End_While loop: In While/End_While loop, program continues to loop as long as

resultant of condition is TRUETRUE. Whereas Repeat/Until loop continues to loop while resultant of condition is FALSEFALSE

In While/End_While loop, condition is processed at the BEGINNINGBEGINNING. However, in the Repeat/Until loop, the condition is processed at the ENDEND, which cause the instructions in the loop to be processed at least ONCE, regardless of whether it is necessary (hence, While/End_While is often the preferredpreferred choice)

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Repeat/Until(Cont.)Repeat/Until(Cont.) Pseudocode Flow Chart

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Begin . . REPEAT

Instruction Instruction

UNTIL condition(s) . .End

Exercise 2:Exercise 2: Refer back to same scenario case in Exercise 1 IPO chart is still the same Pseudocode and Flow chart is different as

compared to solution in Exercise 1

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BeginSet count = 0Set sum = 0.0Get ageREPEATREPEATsum = sum + agecount = count + 1Get ageUNTIL UNTIL age == 0average = sum / countDisplay average

End

PseudocodePseudocode20

Flow ChartFlow Chart21

T

F

Set sum = 0.0

Begin

Set count = 0

count = count + 1

End

average = sum/count

Display average

Get age

age == 0

sum = sum + age

Get age

General Design of LoopGeneral Design of Loop

Two general designs: Automatic-counter loop Sentinel-controlled loop

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Automatic-Counter LoopAutomatic-Counter Loop

Also known as counter-controlled loop Is used when we need to execute a number of

instructions from the program for a finite, pre-determined number of time

It incrementsincrements or decrementsdecrements a variable (which acts as a counter) each time the loop is repeated

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The programmer uses a variable as a counter that starts counting at a specified number and increments the variable each time the loop is processed. Example:

The amount to be incremented is specified by the instruction. Example:

The set of instructions within the loop repeats until the counter is greater than greater than an ending number

count = 0

Automatic-Counter Loop Automatic-Counter Loop (Increments)(Increments)24

count = count + 2

Exercise 3:Exercise 3:Develop an IPO chart, pseudocode, flowchart and complete Python program, to find the average age of FIVE (5) users

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IPO ChartIPO Chart

Input Process Outputage 1. Get age

2. Calculate average3. Display average

average

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BeginSet count = 0Set sum = 0.0Get ageWHILEWHILE count < 4 THENTHENsum = sum + agecount = count + 1Get ageEND_WHILEEND_WHILEaverage = sum / countDisplay average

End

PseudocodePseudocode27

Flow ChartFlow Chart28

TF

Set sum = 0.0

Begin

Set count = 0

count = count + 1

End

average = sum/count

Display average

Get age

count< 4

sum = sum + age

Get age

sum = 0.0count = 0age = input(“Your age:")while count < 4: sum = sum + age count = count + 1 age = input(“Your age:")average = sum/countprint "The average age is", average

Python CodePython Code29

Output:Output:

The programmer uses a variable as a counter that starts counting at a specified number and increments the variable each time the loop is processed. Example:

The amount to be decremented is specified by the instruction. Example:

The set of instructions within the loop repeats until the counter is lesser than lesser than or not equal to not equal to an ending number

count = 10

Automatic-Counter Loop Automatic-Counter Loop (Decrements)(Decrements)30

count = count - 1

Exercise 4:Exercise 4:

Develop an IPO chart, pseudocode, flowchart and complete Python program, for a program which will prompt for the user to insert a positive integer. The program will then perform count down (decrement by ONE) towards the integer inserted. The message of “Happy New Year” will be displayed AFTER the integer is decreased to ZERO.

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IPO ChartIPO Chart

Input Process Outputnumber 1. Get number

2. Display number until 03. Display “Happy New Year”

• Number• Message of

“Happy New Year”

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BeginGet numberWHILEWHILE number >= 0 THENTHENDisplay numbernumber = number - 1END_WHILEEND_WHILEDisplay “Happy New Year”

End

PseudocodePseudocode33

Flow ChartFlow Chart34

T

F

Begin

number = number - 1

End

Display “Happy New Year”

Get number

Display number

number >= 0

number=input("Give me a number:")while number >= 0: print number number = number - 1print "Happy New Year"

Python CodePython Code35

Output:Output:

Sentinel-controlled LoopSentinel-controlled Loop

Is used when we need to execute a number of instructions from the program indefinitely until the user tells it to stop or a special condition is met

IndicatorIndicator (also known as flag, switches, trip values or sentinel values) needs to be set internally in the program to control when the processing of a loop should stop

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Exercise 5:Exercise 5:

Develop an IPO chart, pseudocode, flowchart and complete Python program, for a program which will read in a series of student’s score (as long as the score is not -99). Upon receiving the sentinel value -99, the program will stop reading in students’ score. It will then compute and display average score for all students and the total number of students whom have inserted scores into program.

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IPO ChartIPO Chart

Input Process Outputscore 1. Get score

2. Calculate average, count3. Display average, count

averagecount

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BeginSet sum = 0.0Set count = 0Get scoreWHILEWHILE score <> -99 THENTHENsum = sum + scorecount = count + 1Get scoreEND_WHILEEND_WHILEaverage = sum / countDisplay average, count

End

PseudocodePseudocode39

Flow ChartFlow Chart40

TF

Set sum = 0.0

Begin

Set count = 0

count = count + 1

End

average = sum/count

Display average, count

Get score

score <> -99

sum = sum + score

Get score

Python CodePython Code41

Output:Output:

Why don’t you try to

code??

Nested LoopsNested Loops

Loops can be nested like decision logic structure Each inner loop must be nested inside the loop

just outside it (outer loop) However, both loops do NOT have to come from

the same type of loop structure Example:

A While/End_While Loop can be nested inside a Repeat/Until Loop

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Exercise 6:Exercise 6:

Develop an IPO chart, pseudocode, flowchart and complete Python program, for a program which will prompt for two inputs from user:1.The number of multiplication timetables2.The number of rows required in each set of multiplication timetable The program will then generate a complete timetable set based on user’s requirement

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IPO ChartIPO Chart

Input Process Outputnumberrow

1. Get set2. Get row3. Calculate multiply4. Display multiply

multiply

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PseudocodePseudocode45

BeginGet numberGet rowset i = 1WHILEWHILE i <= number THENTHENSet j = 1WHILE WHILE j <= row THENTHENmultiply = i * jDisplay multiplyj = j + 1END_WHILEEND_WHILEi = i + 1Display “*******”

END_WHILEEND_WHILEEnd

Flow ChartFlow Chart46

It’s drawing time

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Sample Output:Sample Output:

Let’s try to code

RecursionRecursion Occurs when a module or a function calls back

itself The condition that ends the loop must be

within the module Will be covered in lab module 7

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More exercisesMore exercisespp.186, PROBLEMS

Question 1Question 3

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Loop ChallengesLoop Challenges50

CONSTRUCT appropriate Pseudocode and Flow chart for each scenario case @ sample output provided

Then CONVERT each Pseudocode @ Flow chart into complete Python program

Loop Challenge - 1Loop Challenge - 151

Loop Challenge - 2Loop Challenge - 252

Loop Challenge - 3Loop Challenge - 353

Loop Challenge - 4Loop Challenge - 4

Develop a program to calculate how many years does it take for a user to get RM10000 in his saving account, assuming that he deposited RM5000 into a bank, with yearly interest 2.00%

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55 Loop Challenge - Loop Challenge - 55

Loop Challenge - 6Loop Challenge - 656

Loop Challenge - 7Loop Challenge - 7 An applicant for a secretarial job will be given a

maximum of five typing tests. The applicant will be hired as soon as he or she scores over 50 words per minute on two tests. Write a program that allows a supervisor to enter each typing test score on completion of each test. The program should print Hire as soon as the applicant qualifies without asking for further tests. If after five test scores have been entered the typist still hasn’t qualified, the program should print reject.

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58 Loop Challenge – 7 (cont.)Loop Challenge – 7 (cont.)

Loop Challenge – 8Loop Challenge – 859