Presentation © Copyright 2002, Bryan Meyers Top-Down, Structured Program Design Chapter 5

Programming in RPG IV Third Edition 2 Objectives Determine how to design and write structured RPG programs Use structured RPG operations that perform sequence, selection, and iteration Use subroutines Design a control break program

Programming in RPG IV Third Edition 3 Structured Design A methodology that limits its control to 3 basic structures –Sequence –Selection (decision) –Iteration (repetition, looping) Each of these control structures has a single entry point and a single exit point

Programming in RPG IV Third Edition 4 Sequence Control Structure Statement

Programming in RPG IV Third Edition 5 Selection Control Structure Condition? Statement(s) TrueFalse

Programming in RPG IV Third Edition 6 Iteration Control Structure Condition? Statement(s) True False

Programming in RPG IV Third Edition 7 Relational Codes SymbolCodeMeaning >GTGreater than <LTLess than =EQEqual to <>NENot equal to <=LELess than or equal to >=GEGreater than or equal to

Programming in RPG IV Third Edition 8 Collating sequence Character by character comparison EBCDIC Digits are compared based on algebraic value Letters are smaller than digits Lowercase letters are smaller then uppercase letters A blank is smaller than any other displayable character

Programming in RPG IV Third Edition 9 Selection Operations IF Conditional expression –ELSE –ENDIF AND/OR Nested IFs –ELSEIF SELECT –WHEN –ENDSL

Programming in RPG IV Third Edition 10 IF / ENDIF If the comparison is true all statements between the IF and ENDIF are performed If the comparison is false the statements are bypassed * /FREE IF Age >= 65; SeniorCnt = SeniorCnt + 1; ENDIF; /END-FREE

Programming in RPG IV Third Edition 11 IF / ELSE / ENDIF If the comparison is true all statements between the IF and ELSE are performed If the comparison is false all the statements between the ELSE and the ENDIF are performed * /FREE IF Hours <= 40; EVAL(H) TotalPay = Hours * PayRate; ELSE; EVAL(H) TotalPay = 40 * PayRate + (Hours - 40) * PayRate * 1.5; ENDIF; /END-FREE

Programming in RPG IV Third Edition 12 AND / OR AND: Both relationships must be true OR: One or the other or both relationships must be true * /FREE IF Age >= 65 AND Status = 'R'; // This block of code is executed only if Age >=65 and Status =R. ELSE; // If one or both conditions are not met,this code is executed. ENDIF; IF Age >= 65 OR Status = 'R'; // This block is executed if one or both conditions are true. ELSE; // This block is executed only if both conditions are false. ENDIF; /END-FREE

Programming in RPG IV Third Edition 13 Nested IFs Build IFs within IFs –Each IF requires an ENDIF * /FREE IF Sales <=5000; Rate =.005; ELSE; IF Sales <=10000; Rate =.0075; ELSE; IF Sales <=20000; Rate =.01; ELSE; Rate =.015; ENDIF; /END-FREE

Programming in RPG IV Third Edition 14 Nested IFs ELSEIF combines function of ELSE and IF statement –Single ENDIF ends IF/ELSEIF blocks * /FREE IF Sales <=5000; Rate =.005; ELSEIF Sales <=10000; Rate =.0075; ELSEIF Sales <=20000; Rate =.01; ELSE; Rate =.015; ENDIF; /END-FREE

Programming in RPG IV Third Edition 15 SELECT Appears on a line alone to identify the start of a case construct SELECT is followed by one or more WHENs –Each WHEN specifies a condition to be tested –Each WHEN is followed by one or more statements to be performed –As soon as it encounters a true condition, the computer executes the operation(s) following a WHEN Then control falls to ENDSL WHEN conditions can be coupled with AND/OR operations OTHER means “in all other cases” –OTHER should be the final catch all * /FREE SELECT; WHEN Sales <=5000; Rate =.005; WHEN Sales <=10000; Rate =.0075; WHEN Sales <=20000; Rate =.01; OTHER; Rate =.015; ENDSL; /END-FREE

Programming in RPG IV Third Edition 16 IF and Page Overflow Built in overflow indicators –OA thru OG and OV Use OFLIND File Specification keyword to associate indicator with report file Indicator will automatically come *ON when printer reaches overflow line at bottom of page

Programming in RPG IV Third Edition 17 Iteration Operations DOW Conditional expression - ENDDO DOU Conditional expression - ENDDO FOR - ENDFOR

Programming in RPG IV Third Edition 18 Do While Loop Leading decision loop Condition? Statement(s) Stop Start False True

Programming in RPG IV Third Edition 19 DOW Establishes a leading decision loop, based on a comparison –Allows AND and OR to form compound conditions Operations between DOW and ENDDO repeat as long as condition remains true * /FREE Number = 0; // Initialize Number to zero. Sum = 0; // Initialize Sum to zero. DOW Number < 100; // Loop while Number is less than 100. Number = Number + 1; // Increment Number by 1. Sum = Sum + Number; // Add Number to accumulator Sum. ENDDO; /END-FREE

Programming in RPG IV Third Edition 20 Do Until Loop Trailing decision loop Condition? Statement(s) Stop Start False True

Programming in RPG IV Third Edition 21 DOU Establishes a trailing decision loop, based on a comparison DOU repeats until the condition becomes true –Statements always execute at least once * /FREE Number = 0; // Initialize Number to zero. Sum = 0; // Initialize Sum to zero. DOU Number = 100; // Loop until Number equals 100. Number = Number + 1; // Increment Number by 1. Sum = Sum + Number; // Add Number to accumulator Sum. ENDDO; /END-FREE

Programming in RPG IV Third Edition 22 FOR Initiates a count-controlled loop Specify four things –Counter –Counter starting value Optional, defaults to 1 –Counter limit value (TO or DOWNTO) Optional, defaults to 1 –Counter increment value (BY) Optional, defaults to 1 * /FREE Sum = 0; FOR Number = 1 TO 100; Sum = Sum + Number; ENDFOR; /END-FREE

Programming in RPG IV Third Edition 23 Early Exits from Loops ITER skips the remaining instructions in the loop and causes the next repetition to begin LEAVE terminates the loop –Sends control to statements following ENDDO * /FREE DOW NOT %EOF; READ CustFile; IF %EOF; LEAVE; ELSE; EXCEPT Detail; ITER; ENDIF; ENDDO; /END-FREE

Programming in RPG IV Third Edition 24 Subroutines Subroutine: a block of code with an identifiable beginning and end Subroutines must have a unique name formed with the same rules that apply to fields Subroutines may execute other subroutines, but a subroutine should never execute itself Subroutines cannot contain other subroutines Subroutines appear after other calculation specifications –Organize subroutines alphabetically

Programming in RPG IV Third Edition 25 BEGSR/ENDSR BEGSR begins a subroutine –Include subroutine name ENDSR ends a subroutine * /FREE BEGSR CalcTax; EVAL(H) FICA = Gross *.0751; EVAL(H) StateTax = Gross *.045; IF Gross > 5000; EVAL(H) FedTax = Gross *.31; ELSE; EVAL(H) FedTax = Gross *.25; ENDIF; ENDSR; /END-FREE

Programming in RPG IV Third Edition 26 EXSR Executes subroutine named in Factor 2 * /FREE EXSR CalcTax; // EXSR causes control to drop to subroutine CalcTax. // Control returns here when the subroutine finishes.... BEGSR CalcTax; EVAL(H) FICA = Gross *.0751; EVAL(H) StateTax = Gross *.045; IF Gross > 5000; EVAL(H) FedTax = Gross *.31; ELSE; EVAL(H) FedTax = Gross *.25; ENDIF; ENDSR; /END-FREE

Programming in RPG IV Third Edition 27 Points to Remember Structured program design means developing program logic with flow of control tightly managed –Generally by using only structured operations Top-down methodology requires hierarchical program design –Work out broad logic first –Later, attend to detailed processing

Programming in RPG IV Third Edition 28 Points to Remember Decision logic operations –IF / ELSE / ELSEIF –SELECT / WHEN / OTHER Iteration (looping) operations –DOW (leading decision) –DOU (trailing decision) –FOR (counter controlled) Use ITER and LEAVE to provide an early exit from a loop Most of the structured operation of RPG require the use relational operators (>, =, <>)

Programming in RPG IV Third Edition 29 Points to Remember Subroutines allow an identifiable block of code to be executed BEGSR and ENDSR begin and end a subroutine –Coded after other calculations EXSR executes a subroutine