1 CSI 101 Elements of Computing Fall 2009 Lecture #4 Using Flowcharts Monday February 2nd, 2009

2 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

3 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

4 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

5 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

6 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

7 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

8 Symbols

9 Sample Flowchart (without text)

10 Terminator ● Starts and ends a process ● Only one flow, either departing or entering START STOP

11 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

12 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

13 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

14 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”

15 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 2

16 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

17 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)

18 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

19 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

20 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

21 Answer START Input userid, pwd Exists = UserCheck (userid) Userid exists? No Yes A B

22 Answer, part 2 A OK = PwdCheck( userid, pwd) Pwd ok? “User does not exist” No “Begin work” Yes B “Invalid password” END

23 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

24 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