Structured Programming

1. Defn: This is an approach to program development that produces programs of high quality that are easy to test, debug, modify and maintain. This technique emphasizes the modular approach to program development. Structured Programming Modular Programming: Using this approach a large complex problem is broken up into sub-problems, and if necessary these are further divided until the tasks to be solved are simpler or easier for programming code to be developed. – Top Down Design ADD A MEMBER SEARCH FOR A MEMBER DELETE A MEMBER

2. StructuredProgramming • Defn: A structure chart is ideally a map of your program which shows how variables pass data between modules in a program. It helps the programmer to produce a top-down design of the program • Steps: • Place the main module as the root of an upside-down tree • Determine the set of tasks necessary for solving the problem. • Place these below the root and connect the root to each task • Break down each task into even smaller tasks if necessary. • Eventually the program is broken down to a point where the “leaves” of the structure chart “tree” represent simple modules that can be coded with just a few statements.

3. Structure Charts Automatic Teller Activate Customer Accounts Handle transactions Validate Customer Pull customer record Check Transactions Savings Transactions Exit Transactions Get Valid Card # Get Valid Code # Handle Queries Handle Deposits Handle withdrawals Fig 2. A structure chart,. Each box (node) in the chart represents a block of code called a module. The root node (box at the top) represents the main section of the program. Each connecting line represents a call to a module. The connecting lines could be variables (data) which are passed to and from modules.

4. Structured Programming • Most of the programs written at the ordinary and advanced level courses are likely to be short; however, in the IT arena most problems will require thousands, if not millions of lines of code. (Windows 2000 – over 35 millions lines of code). The importance of splitting a problem into a series of self-contained modules then becomes obvious. A module should not exceed 100 lines, and preferably short enough to fit on a single page or screen. • ADVANTAGES OF MODULAR PROGRAMMING • Allows a problem to be split in stages and for a team of programmers to work on the problem, also enabling a faster solution. • Senior programmers can be given the more complex modules to write, and the junior programmers the simpler ones. • Program modification is easier since changes can be isolated to specific modules. • Modules can be tested and debugged independently. • Since a module performs a specific well defined task, it is possible to develop and place commonly used modules in a user library so that they can be accessed by different programs. (e.g. Validation, Uppercase, Text color…Pascal) • If a programmer cannot continue through the entire project, it is easier for another programmer to continue working on self contained modules.

5. Structured Programming • In the Pascal programming language the concept of structured programming is expressed using ProceduresandFunctions. • Procedures are portions of code that perform a specific task such as draw a square on screen, prompt a user to enter values, display results on screen etc. They can exist as independent statements e.g. textcolor(red), inc(x), clrscr; • Functions are also portions of code that perform a specific task. However, they return a value or result, such as an integer, a real number, a string , etc. Consequently, they can only be used as components of statements e.g. ch:=upcase (ch); y:= abs(z); Consider the following simple example. Program Example1: Procedure open_screen; Begin writeln(‘ Welcome to Cyber Core….’); end; Begin open_screen; End. Program header Procedure header End of Procedure Start of main program Calling the procedure open_screen End of program

6. Structured Programming Examine the Pascal program below which prompts a user for two names and outputs the one that is alphabetically smaller. The program uses a procedure which creates a border (of 20 asterisks) before and after the output. Note also that the procedure is called twice from the main program and that procedures and functions are written before the main program. Program smaller_name; Var name1,name2:string; Procedure Border; Var i:integer; Begin for i:= 1 to 20 do write(‘*’); writeln; End; Begin writeln(‘ Enter two names ‘):readln(name1,name2); Border; If (name1<name2) then writeln(name1) else writeln(name2); Border; End. Program header Global Variables Local Variable End of Procedure Start of main program Call Procedure Call Procedure again

7. Structured Programming • Notice that the procedure will only print 20 asterisks, no more and no less. If 10 or even 40 asterisks are required then the same procedure can be used, though some modification will be required. This can be achieved with the use of a parameter list with one variable, say Num_asterisks . The procedure now looks as follows: • Procedure Border(Num_asterisks:integer); • Var i:integer; • Begin • for i:= 1 to Num_asterisks do • write(‘*’); • writeln; • End; • On examining the new format the number of asterisks is passed to the procedure through the parameter list in the procedure header. Consequently, when the procedure needs to be called, the value of the parameter is placed in brackets. So calls in the main program would look like Border(10); or Border(40) ; for printing 10 or 40 asterisks respectively. • Global variables exist throughout the scope of a program. They are declared at the start of the program and values may be accessed at any time and position during the execution of the program. • Local Variables are declared within a procedure or function. They are created when the subroutine is started and only exist when that portion of code is executed. When the procedure or function is exited the memory space which was reserved for the local variables is released for other use. N.B to minimize the constraints for the running of a program more local declarations should be made where possible. Parameter List

8. Structured Programming In the Pascal programming language Functions are very similar to Procedures except that they return a value. The golden rule is that if a value is not to be returned then a procedure should always be used. When programming the reserved word Function is used instead of the word Procedure and the data type of the returning value must be stated. Format of a Pascal Function Function Name _of _function (parameter_listing) :datatype Var any_local_variables; Begin statement1; statement2; Name_of_Function:= value; End; Function is_even (num : integer):boolean; Begin if (num mod 2 = 0) then is_even:=true else is_even:= false; end Integer, string, char, boolean Must be given a value consistent with the data type stated

9. Structured Programming • Consider the Pascal program below and answer the questions that follow. • Program test1; • Var ans, num:integer; • Procedure cubed(num: integer; var ans:integer); • Begin • ans:= num * num * num; • End; • Begin • Writeln(‘Enter number : ‘); readln (num); • cubed(num , ans); • Writeln(‘Number Cubed : ‘,ans); • readln; • End. • What is the name of the program? • Write the names of the global and local variables. • State the parameter list in the procedure. • For each parameter , identify whether it is a variable or value parameter and why. • What will be the output if the following numbers are entered (a) 4 (b) 3.2 ? • Do you think cubed should be written as a Function? If so why? • Assume you decide to write cube as a Function, rewrite the above portion of code. • Program test1; • Var num:integer; • Function Cubed(num: integer) :integer; • Begin • cubed:=num * num * num; • End; • Begin writeln (‘Enter number : ‘); readln (num); • writeln(‘Number Cubed : ‘, cubed(num)); • readln; • End.

10. Structured Programming • Consider writing a program where a user is required to enter integer values M and N and to calculate and output M raised to the power N. In analyzing the problem, three cases need to be considered: • the case of N being Zero • the case of N being positive and • finally the case of N being negative • The modular breakdown of the problem now looks as follows: Power Raising M + n M -n M0 It is now possible to focus on one module at a time and to develop the internal workings (stepwise refinement). For the Zero case the body of the module would need only contain the statement setting the answer to 1 (Any number raised to the power of Zero equals 1!) For the Positive Case the following portion of code could be used ans:= 1; for i:= 1 to N do ans:= ans * M For the Negative Case it is observed that M – nis equal to that of 1/ M + n. Thus, use could be made of previously written code for the positive case. (all that is necessary is for the procedure to be appropriately called)