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L7 decision tree & table
This document discusses various techniques for structuring decisions: structured English, flowcharts, decision tables, and decision trees. It provides examples and explanations of each technique. Structured English uses an IF/THEN statement format. Flowcharts show the sequence of steps using standard symbols. Decision tables illustrate decision rules in a table format. Decision trees use a branching diagram to represent multiple conditional decisions. The document emphasizes that these techniques help ensure decisions are complete, avoid errors, and reduce necessary condition testing.
L7 decision tree & table
- 1. Major Topics Structured English Flowcharts Decision Tables Decision Trees Decision Criteria
- 2. Structured English Structured English is a technique used to describe algorithmic procedures and is sometimes considered an alternative to flowcharts.
- 3. Example IF Hours isgreater than 40 Y N THEN Hours > 40 Compute Overtime Pay ELSE Compute Compute Compute Regular Pay Overtime Regular ENDIF
- 4. What are Flowcharts? They describe a procedure or program by specifying the sequence diagrammatically in which individual steps must be executed. An ideal means for documenting complex algorithms so that users can refer to them as procedural guides.
- 5. Flowchart Symbols Terminal Input/ (Start/Stop) Output Process Decision or Test Flow Line Connector
- 6. Simple Example Start A Print Headings Print Number, EOF? Square N Read a Number Stop Procedure reads numbers from a file and produces a Table of the numbers and their squares. A
- 7. Decision Tables A Decision table is a table of rows and columns, separated into four quadrants and is designed to illustrate complex decision rules Condition Stub – upper left quadrant Rules Stub – upper right quadrant Action Stub – bottom left quadrant Entries Stub - bottom right quadrant
- 8. Decision Table Layout Standard format used for presenting decision tables. Decision Rules Stub Stub Action Entries Stub Stub
- 9. Decision Table Example 1 2 3 4 Like Boss? Y Y N N 20% Pay Raise? Y N Y N Stay Another Year X X Quit after 2 Months X Quit next Week X
- 10. Developing Decision Tables Process requires the determination of the number of conditions (inputs) that affect the decision. The set of possible actions (outputs) must likewise be determined The number of rules is computed Each rule must specify one or more actions
- 11. Number of Rules Each condition generally has two possible alternatives (outcomes): Yes or No In more advanced tables, multiple outcomes for each condition are permitted The total number of rules is equal to 2 no. of conditions Thus, if there are four conditions, there will be sixteen possible rules
- 12. Building the Table For each rule, select the appropriate action and indicate with an ‘X’ Identify rules that produce the same actions and attempt to combine those rules; for example: Condition 1 Y Y Condition 1 Y Condition 2 Y N Condition 2 - Action 1 X X Action 1 X
- 13. Cleaning Things Up Check the table for any impossible situations, contradictions, and redundancies and eliminate such rules Rewrite the decision table with the most reduced set of rules; rearranging the rule order is permissible if it improves user understanding
- 14. Decision Table example: combine and reduce Conditions and Actions 1 2 3 4 5 6 7 8 Order from Fall Catalog Y Y Y Y N N N N Order from Christmas Catalog Y Y N N Y Y N N Order from Special Catalog Y N Y N Y N Y N Mail Christmas Catalog X X X X Mail Special Catalog X X Mail Both Catalogs X X The four gray columns can In addition, Rules 1&5 and be combined into a single Rules 3&7 can be combined. rule. Note that four each, Each pair produces the same there was NO order placed action and each pair shares two from the Special Catalog. common conditions.
- 15. Decision Table example~ Final Version Conditions and Actions 1 2 3 Order from Fall Catalog -- -- -- Order from Christmas Catalog Y -- N Order from Special Catalog Y N Y Mail Christmas Catalog X Mail Special Catalog X Mail Both Catalogs X Eliminates the need to check for every possible case.
- 16. Decision Table example: checking for completeness and accuracy Conditions and Actions 1 2 3 4 Salary > $50,000 per year Y Y N N Salary < $2,000 per month Y N Y N Award Double Bonus X Award Regular Bonus X Award no Bonus X Although the Y-N Combinations suggest a rule, in this case, it is impossible for conditions 1 & 2 to exist simultaneously. They are in complete contradiction with each other. In the final version of the table, Rule 1 will disappear and Rules -4 will become Rules 1-3.
- 17. Importance of Decision Tables Aids in the analysis of structured decisions Ensures completeness Checks for possible errors (impossible situations, contradictions, and redundancies, etc.) Reduces the amount of condition testing that must be done
- 18. Decision Trees Decision trees are useful when multiple branching occurs in a structured decision process, although they can be quite effective when only two decision paths are called for. They are helpful when necessary to maintain a certain order for a series of decisions.
- 19. Drawing Decision Trees Drawn horizontally Circle is analogous Root of tree is to the to the condition part left side of an IF statement Square Nodes Square is analogous indicate actions to the consequent of Circle Nodes an IF statement (the represent possible ‘THEN’ part) conditions “IF Circle THEN Square”
- 20. Decision Trees Assume thefollowing discount policy: “ Bookstores get a trade discount of 25%; for orders from libraries and individuals, 5% allowed on orders of 6-19 copies per book title; 10% on orders for 20-49 copies per book title;15% on orders for 50 copies or more per book title.” Using the Decision Table describe the above mentioned discount policy.
- 21. Decision Tree Example 50 25% Discount O size >= 4 2 Osi ze< re 50 Nil Discount s to 5 ok bo 1 lib ra ri 5% Discount 6 es 9 & 1 6- in : ze di si vi O du 10% Discount al Osize:2 0-49 7 s 3 Osi ze> 50 15 % Discount 8