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CSI 101 Elements of Computing

Fall 2009

Lecture #4Using Flowcharts

Monday February 2nd, 2009

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Let's Talk about Designing

● Why Design?● Proper Preparation Prevents Poor Performance● Determine flow and function

● What happens when● Think through before coding

● Prevents trial and error● Prevents massive corrections

● Determine most efficient way● Fewer places to insert errors● Faster processing

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Advantages to Designing

● Ensure completeness● By working through design, can see if you have

covered everything requested

● Can see where requirements are vague or lacking

● Provide focus

● Important for large projects with multiple people

● Identify key flow points and transitions

● Can plan adequate and complete test plans

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Design for Testing?

● To completely test, must be able to completely know flow

● Large projects have multiple tests● And therefore multiple testers● Rarely is developer the same person who tests

● Checkpoints● Key spots in program where status of data is

known

● Variation● Condition or branch that causes different results

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Examples of Variations

● Conditions● Something greater or less than● Need to test how many states?

● 3 – Greater, lesser, or equal● Flows

● Looping until a particular event or successful test● How many variations?

● 3 – starting, continuing, and stopping● Transitions

● Calling a function● How many variations?

● 2 normal (before and after) and error restore

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Designing made Easy

● Designs don't have to be large or complex● Some companies write out big documents

● Detail is important to the person writing● Most people only need highlights● The simpler a design is, easier to read and

understand● That's the philosophy behind Flowcharts

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Flowcharts

● Pictorial● If you know the symbols, you can read

● Machine independent● Can be enlarged for emphasis● Uses simple recognizable shapes● Few constructs to remember

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Symbols

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Sample Flowchart (without text)

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Terminator

● Starts and ends a process● Only one flow, either departing or entering

START

STOP

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Process

● Contains calculation or process● Can contain a sequence of consecutive

operations● Has a single flow entering and a single flow

exiting

Calculate average Sum = A+B+C

Ave = Sum/3

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Decision

● Condition whose result causes one of multiple calculations or processes

● Stated as question with only YES or NO as answer

● One flow entering and two flows exiting

Is A > B?

Yes

No

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Input/Output

● Read input or write output● Only one operation, but can have multiple

inputs or outputs● One flow entering and one flow exiting

Read A,B,C

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Connector

● Brings together separate flows that are coming together to perform the same operation

● Multiple flows entering, one flow exiting● Note there is no “disconnector”

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B

Link

● Continues flow on another page● Note this is used ONLY to allow a flow to move

onto another page. There is never a need to have two links with the same letter or number on the same page!

● Link is labeled with a letter or number, and has a matching link on another page

● Only one flow, either entering or exiting● Enters at existing page, exits on other page

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External Reference

● Not Input or Output, which use local devices● Memory transfer from database or other

external device● Denotes Load into register● One flow entering, one flow exiting

Get next Employee record

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Predefined Function

● Denotes CALL● Perform a function that has previously been

written and stored in a library● If it's a function that produces a result, the

result can be stored into a variable● One flow entering, one flow exiting● In box below, MIN is the predefined function

● Low is the variable containing the result

Low = MIN(A,B,C)

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Let’s try one

● Let’s write a flowchart to detail the process of a user logging into a computer account

● What’s the first step?● Determine input and output● Input: userid and password● Output: one of three situations:

● Accepted● Incorrect userid● Incorrect password

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Other Considerations

● Password is encrypted. Must use system’s routine to check it

● The file containing user names cannot be “open” text● Would mean any process could get list of names● There must be a system routine to check for

existence

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So what are our steps?

● INPUT userid and password● CALL check of userid● DECISION: is userid valid?

● No, send message to user and exit● Yes, continue

● CALL check of password, passing userid● DECISION: OK?

● No, send message to user and exit● Yes, send message of success and exit

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AnswerSTART

Input userid, pwd

Exists = UserCheck (userid)

Userid exists?

No

Yes

A

B

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Answer, part 2

A

OK = PwdCheck( userid, pwd)

Pwd ok?

“User does not exist”

No

“Begin work”

Yes

B

“Invalid password”

END

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Extra consideration

● Some systems allow users three chances, in case they misspelled something

● How would that change this flowchart?● Have a loop

● Must use a counter, as there is no loop structure in flowcharts

● Set counter, flow to decision● Decision tests if counter <= 3● Yes, goes through process again● No, connect to connector before END terminator

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Bonus Assignment

● Implement the “3 tries” extra consideration mentioned on the previous slide● In other words, recreate the flowchart on Slides

21 and 22 but with the “loop” that allows the user three tries

● This is not required; do it if you would like● Due the same time as Homework

Assignment #1● Counts as an extra 50 points