Module Cohesion and Coupling

1. Module Cohesion and Coupling Lecture 10

2. How to determine a good module? Cohesion: a measure of the internal strenght of a module – how closely the elements and statements of a module are associated with each other

3. Module Cohesion

4. Coincidental cohesion File_processing Open employee updates file Read employee record Print_page_headings Open employee master file Set page_count to one Set error_flag to false END No meaningful relationship each other

5. Logical Cohesion Read_all_files(file_code) CASE of file_code 1 : Read customer transaction record IF not EOF increment customer_transaction_count ENDIF 2 : Read customer master record IF not EOF increment customer_master_count ENDIF 3 : Read product master record IF not EOF increment product_master_count ENDIF ENDCASE END Three Read instructions perform three separate functions

6. Temporal Cohesion Initialisation Open transaction file Issue prompt `Enter today‘s date – DDMMYY` Read todays_date Set transaction_count to zero Read transaction record IF not EOF increment transaction_count ENDIF Open report file Print_page_headings Set report_total to zero END Element of the module are grouped because they Are related by time.

7. Procedural Cohesion Read_student_records_and_total_student_ages Set number_of_records to zero Set total_age to zero Read student record DOWHILE more records exist Add age to total _age Add 1 to number_of_records Read student record ENDDO Print number_of_records,total_age END Indicate the module performs more than one function.

8. Communicational Cohesion Validate_product_record IF transaction_type NOT = ´0´ THEN error_flag = true error_message = ´invalid transaction type` Print_error_report ENDIF IF customer_number is NOT numeric THEN error_flag = true error_message = ´invalid customer number` Print_error_report ENDIF IF product_no = blanks OR product_no has leading blanks THEN error_flag = true error_message = ´invalid product no` Print_error_report ENDIF END Operate on the same piece of data

9. Sequential Cohesion Process_purchases Set total_purchases to zero Get number_of_purchases Do loop_index = 1 to number_of_purchases get purchase add purchase to total_purchases ENDDO sales_tax = total_purchases * sales_tax_percent amount_due = total_purchases + sales_tax END

10. Functional Cohesion Calculate_sales_tax IF product is sales exempt THEN sales_tax = 0 ELSE IF product_price < $50.00 THEN sales_tax = product_price * 0.25 ELSE IF product_price < $100.00 THEN sales_tax = product_price * 0.35 ELSE sales_tax = product_price * 0.5 ENDIF ENDIF ENDIF END All of the element module contribute to the performance of a single specific task.

11. Conclusion • When designing the structure of an algorithm: • Try to form modules that have a single problem-related function. • In reality, it is not easy to construct a program where every module has functional cohesion No worry, your prime consideration is to produce modules and programs that are easy to understand and modify the higher the cohesion, the more likely it is that you achieved the aim.

12. Module Coupling • Coupling : a measure of the extent of information interchange between modules. • Tight coupling implies large dependence on the structure of one module by another because higher number of connections, more paths along which errors can extend into other parts of the program.

13. Module Coupling

14. Common Coupling Global Data Structure Read_customer_record Read customer record IF EOF THEN set EOF_flag to true ENDIF END Module A Module B Modules refer the same global data structures. Validate_customer_record IF customer_numbers is NOT numeric THEN error_message= ´invalid customer number` Print_error_report ENDIF END

15. External Coupling Global Data Variable Calculate_Sales_tax IF product is sales exempt THEN sales_tax = 0 ELSE IF product_price < $50.00 THEN sales_tax = product_price * 0.25 ENDIF ENDIF END Calculate_amount_due amount_due = total_amount + sales_tax END Module B Module A Modules access the same Global data variable (sales_tax)

16. Control Coupling Module A Process_input_code Read input_code Choose_appropriate_action (input_code) . . . END flag Module B A Modules are cohesive because of the passing of the control parameter (input_code) Choose_appropriate_Action(input_code) CASE OF input_code 1 : Read employee record 2 : Print_page_headings 3 : Open employee master file 4 : Set page_count to zero 5 : error_message = ´Employee number not numeric ` ENDCASE END B

17. Stamp Coupling Module A Process_transaction_record . . IF transaction record is for a male THEN Process_male_student(current record) ELSE Process_female_student(Current_record) ENDIF END Process_male_student(current_record) increment male_student_count IF student_age > 21 THEN increment mature_male_count ENDIF END Data structure Module B B Module passes a non-global data structure to another module (current_record) A

18. Data Coupling Module A A Process_customer_record Calculate_sales_tax(total_price, sales_tax) END Elementary Data Item Module B Calculate_sales_tax(total_price, sales_tax) IF total_price < $10.00 THEN sales_tax = total_price * 0.25 ELSE IF total_price < $100.00 THEN sales_tax = total_price * 0.3 ELSE sales_tax = total_price * 0.4 ENDIF ENDIF END Module passes a non-global Elementary data variable to another module (total_price and sales_tax) B

19. Coupling Conclusion • When designing solution algorithms, aim towards module independence and a minimum of information interchange between modules. • If possible, uncouple module from its surroundings by: • Passing data to a subordinate module in the form of parameters, instead of global data • Writing subordinate module as a self-contained unit that can: • Accept data passed to it • Operate on it without reference to other parts of the program • Pass information back to the calling module, if required.

20. EXERCISE • Design an algorithm that will produce a reorder list of products from a product inventory file. Each input product record contains the item number, the quantity on hand for the item, the quantity on order, the minimum inventory level for that item, and an obselete code (´X´if the item is obselete, blank if it is not)