CSI 101 Elements of ComputingSpring 2009

Lecture #5

Designing with Pseudocode

Wednesday, February 4th, 2009

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Design Deficiencies

• Many design methods missing key constructs:– No inherent looping or recursion ability

• Looping is the process of repeating the same instructions multiple times in a row

– Limited ability to define where data comes from

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Design Needs

• Inexperienced programmers needed more guidance

• Insufficient to review– Lack of detail made it too difficult to gauge

correctness and efficiency

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New Design

• Called “pseudocode”– Literally means “Fake code”

• Uses verbal descriptions and code-like structures– Code used is based on Basic, which evolved

into Visual Basic

• Pseudocode often used in addition to other design methods– Pseudocode is an example of Low-level design

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Pseudocode constructs

• Execution block

• Input and Output

• Operations

• Decision

• Looping

• Branching

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Execution Block

• BEGIN to start

• END to conclude

• Used to start a pseudocode application, as well as separate repeated or recursion blocks

• Each BEGIN must have a corresponding END

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Execution Block Example

BEGIN MyProgram: (These are ellipses. They denote that

: statements exist, but I am not listing them)

BEGIN RepeatBlock:

END RepeatBlock:

END MyProgram

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Input and Output

• READ … [FROM source]– Can read directly into variables– If you need to specify source, use the optional

FROM keyword

• WRITE … [TO location]

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I/O Examples

• READ Num1, Num2 FROM keyboard

• WRITE Total TO File “Out.txt”

• WRITE Pay as Format Currency– This last version starts blurring the line

between design and code, as you will see later in the course

– Best to keep pseudocode DESCRIPTIVE, but not just like code

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Operations

• SET – Used to put a value into a variable– Not used so much anymore– Mostly use equation sign as equation

• CALL – Invoke a stored routine– A stored routine is one that has been

previously created and stored in the system

• Examples:– SET Ave = Sum/Count– CALL 3Sort(High, Med, Low)

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Decision

• IF statement

• Condition must be stated to have only TRUE or FALSE as the answer

• Must have statement or statements to run if condition is TRUE

• Must close with ENDIF

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IF statement forms

• Minimum: – IF condition THEN action

• Provide alternative– IF condition THEN action ELSE alternative

• Multiple checksIF condition THEN actionELSEIF new-condition THEN new-actionELSE alternativeENDIF

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Decision Examples

• Check for higher value:IF A >= B THEN High = AELSE High = BENDIF

• Equal special case:IF A > B THEN PRINT “A Higher”ELSEIF A < B THEN PRINT “B Higher”ELSE PRINT “Equal”ENDIF

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Looping

• DO Statement

• Two types:– DO WHILE(condition) … ENDWHILE– DO … UNTIL(condition) ENDDO

• WHILE tests condition before performing action(s) between DO and ENDWHILE

• UNTIL performs actions then tests– Guarantees that statements run at least once

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DO WHILE Example

• Add first ten values:SET Count = 0SET Sum = 0DO WHILE(Count< 10)

READ Value FROM fileSum = Sum + ValueCount = Count + 1

ENDWHILE

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DO UNTIL Example

• Read items from file:DO

READ Element FROM file

CALL ProcessElement(Element)

UNTIL(EOF)

ENDDO

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Branching

• ON condition GOTO label

• Most often used for Error Handling:ON Error GOTO Cleanup

:

Cleanup: READ Error type from Error structure• Note Cleanup: is a label. Labels are immediately

followed by a colon

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Pseudocode Example

• Let’s revisit the user login process

• This time, since we have a loop structure, we can give the user 3 chances

• We’ll use functions to check for valid userid and password

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Algorithm

• Initialize count and validity flag

• Loop 3 times or until valid:– Get userid and password– Check for userid existence– Check for valid password– Both OK, set flag– Otherwise, ask user to try again

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Solution

BEGIN login

SET Count = 0, valid = False

DO

READ userid, password FROM user

exists = UserExists(userid)

IF exists THEN valid = CheckPwd(userid, password)

ENDIF

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Solution, cont

Count = Count + 1

IF valid = False THEN PRINT “Invalid login information. Try again”

ENDIF

UNTIL(valid OR Count = 3)

ENDDO

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Solution, concluded

IF valid = False THEN

BEGIN Terminate

PRINT “Invalid login attempt”

Terminate user

END Terminate

ENDIF

END login

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Another Example

• Let’s design the Bubble Sort algorithm discussed in Lecture #3

• Takes a list of numbers, compares two at a time, and switches their positions if necessary

• Loops through list again if a switch was made

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Bubble Sort design

BEGIN BubbleSort

SET switched to False

Loop: Item = 1

DO

If Item > Item(+1) Then switch Items

SET switched to True

End If

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Bubble Sort Design, cont

Increase Item by 1

UNTIL all items have been compared to its neighbor

ENDDO

ON (switched set to True) GOTO Loop

END BubbleSort