Program Units and Data

1. Program Units and Data • Program Unit Structure • Declarative Part: Data Structures • Procedural Part: Statements • Procedures • Calling the procedures or functions • Passing parameters • Parameter handling in procedure

2. Program Unit Structure • A program unit has a well defined structure • Consider the C++ programs that you have developed • #include <iostream.h> • void main (void) • { • } • Let us look at a short program

3. Cookie Monster • #include <iostream.h> • void main(void) • { • int cookies; • cookies=12; • while (cookies>=1) { • cout<<“Cookies are for me!!”<<endl; cookies--;} • }

4. Data Declaration Part • The first part of the program unit is called the declarative part • Here all the data items to be used in the program are declared • For example int cookies; • Data items to be used in the program are conceptually shaped according to the needs

5. Data Shapes • Data can be kept in the form of one dimensional arrays, two dimensional tables, trees, linked lists and records • There are appropriate data structures provided in programming languages

6. AN ARRAY OF CHARACTERS IS A ONE DIMENSIONAL ROW A B C D E A B C D E A CIRCULARLY LINKED LIST

7. Example of Tables

8. Heterogeneous Data With a Single Name

9. Procedural Part • The procedural part of program unit contains two types of statements • Assignment statements • For example, cookies=12; • Control statements • For example, while (condition) { }

10. Procedures (Functions in C++) • We focus on the program unit to understand the way the modular software is developed • Main program unit can call procedures to carry out some work. In C++, these are called functions • In C++, the main program unit is our • void main(void) • It is where the execution begins

11. Procedures • A procedure can be called from the main program unit(In C++, any function can call any other function) • Each procedure or function has the same structure as we have defined for program unit. Each procedure or function contains declarative part and procedural part • Let us see the way the main program unit calls a procedure

12. A Program Unit Calling a Procedure

13. Procedure Call Steps • The calling program unit reaches a certain stage in execution where it needs to call a procedure • When the procedure call is executed, control is transferred to the procedure • The procedure carries out the work and then transfers control back to the calling program unit

14. Parameters • When the program unit calls the procedure, it can pass some data to be used by the procedure • For example, a procedure is to be called to draw a circle on the screen • The procedure must be told where to draw the circle and how big the circle should be • call draw_circle(center,radius)

15. Parameter Handling • The data sent to the procedure can be handled by the procedure in two ways • The procedure can get a copy of the data. In this way, the procedure can only read the data and cannot modify it • The procedure can get a reference to the original data. In this case, the procedure can modify the data