Problem Solving and Design

Problem Solving and Design

1. What is a computer system?

   • Made up of:
     • Hardware
     • Software
     • Data
     • Communications
     • People
   • Can be divided into subsystems
     • Continue to be divided into subsystems until each subsystem performs a single action
       • can also be shown using FLOWCHARTS
         • shows diagrammatically the steps required for a subsystem in the order to be performed
         • also called an ALGORITHM
           • sets out steps to complete a given task
         • Can be shown using PSEUDOCODE
       • can be shown using TOP DOWN DESIGN
         • Demonstrates modular construction of the system
           • can be shown using flowcharts or pseudocode
         • The breaking down of a computer system into a set of subsystems which are broken down until each subsystem performs a single action
           • AKA each problem broken down into smaller, more managable problems
           • Called STEPWISE REFINEMENT
         • works for any size computer systems
           • several programmers can work independantly at the same time
             • reduces development and testing time
         • Showing top down design in a structured form is called STRUCTURE DIAGRAMS
           • shows design of a computer system in a hierarchical way
           • each level giving more detail breakdown of the system
       • can be described using SUB-ROUTINE
         • a set of programming instructions for a given task that forms a subsystem, not the whole system

2. Test Data

   • used in order to determine whether a solution is working
     • usually, each subsystem tested separately before the whole system is tested
   • a SET OF TEST DATA
     • is all the items of data required to work through a solution
     • used together with the results expected
       • this is called NORMAL DATA
         • should be used to work through solution to find actual results and see whether they are same as expected results
   • also done to prove solution does not give incorrect results
     • called ERRONEOUS or ABNORMAL TEST DATA
     • should be rejected by solution

3. Validation and Verification

   • VALIDATION
     • Performed automatically by computer system to ensure only reasonable data is excepted
     • different types of check for same piece of data
       • Format check
         • data entered conforms to a predefined pattern
       • Range check
         • Checks that only numbers within a certain range are accepted
       • Type check
         • data entered of a specific caracter type
       • Presence check
         • ensure that some data has been entered and value not blank
       • Character check
         • checks that data does not have any invalid characters or symbols
       • Length check
         • data contains exact number of characters - eg: password
         • data entered has reasonable amount of characters
       • Check digits
         • calculated from all the other digits in the code
         • used in
           • barcodes
           • product codes
           • ISBN
           • Vehicle Identification Numbers
         • used to identify errors in data entry caused by mistyping or misscanning a barcode.
           • Incorrect digit
           • Transposition errors
           • omitted or extra digits
           • phonetic errors
   • VERIFICATION
     • checking that data has been accurately copied
     • Methods used:
       • Double entry
       • screen / visual check
         • manual check by user who is entering data
       • parity check
       • checksum

4. Trace tables

   • can be used to record results from each step in an algorithm
   • can be used to record the value of an item each time it changes
     • also called DRY RUN
   • set up with a column for each variable and a column for any output