Lecture 2 – Elements of C Language



Control Structures

Control Structures are the structures which control the flow of the program in any programming language. There are three basic control structures in any programming language.

  1. Sequence or Statements 
  2. Selection or Decisions 
  3. Iteration or Loops

Sequence or Statements

These structures are responsible for the linear flow of the program. Following are the types of Sequence Structures
  •  Input – these statements are responsible for taking input from the user. For example scanf, gets, getch, getche etc. 
  • Output – these statements are responsible for giving output to the user. For example printf, puts etc. 
  • Expressions – these statements constitutes of program logic. For example y = mx +c; or c= a+ b; etc.

Selection or Decisions

There are some situations where we have to take decision based upon some condition. These structures are used to transfer the control of your program based upon a condition. For example if you want to go to your home and you stuck in a traffic jam, you opt for an alternate route. Similarly we make decisions based upon a certain condition. Examples of decisions are if, if-else, if-else-if and switch. They will be discussed later.

Iterations or Loops

There are situations where we need to repeat some statements. These structures help us to achieve that. There are two types of loops
1.       Counter controlled loops – We have only one loop in this category i.e. the ‘for’ loop. This loop repeats a statement or a block of statements for the specified number of times.
2.       Sentinel controlled loops – These loops repeat a statement of a block of statements based upon a condition. We have two loops in this category while, and do-while loop.

Now! I shall discuss the first control structure, i.e. statements
 
Figure 1 - Basic Structure of a C Program

Elements of a C Program

Preprocessor Directive

It is the instruction for the compiler. It tells the compiler to do it first. Preprocessor directive starts with a # symbol followed by a keyword like include, define and if etc. The keyword which is followed by # symbol is NOT necessarily a keyword but it is a part of the preprocessor directive.

Header File

It is the file which contains the definition of the most functions. These header files are pre-programmed and are shipped with the compiler itself. Dot H is the extension for the header file. The header file stdio.h contains the definitions of the functions like printf and scanf. (Functions will be discussed later)

Include Folder

It is specified by the angle brackets <>.
If the header file name is specified between these angle brackets (e.g. #include <stdio.h> )then the linker will search the file from the include folder which is specified in the IDE settings. The default location of Include folder is C:\TC\INCLUDE it may be changed if required from the IDE options.
If the header file is included using inverted commas (e.g. #include “stdio.h” ) then the linker would search the file in the CURRENT WORKING DIRECTORY, normally C:\TC\BIN 

Return Type

It is the type of a variable which is returned when the function’s body ends. For example, if I say “Dear Student! Bring me a glass of water”. Now let us breakup this statement so that we would be in a better position to understand what is a function, return type and parameter. (Return types will be discussed with functions)
·         Bring me – it is analogous to a function call
·         A glass of water – it is analogous to a parameter passed

Main function

It is the entry point to a C program. In most languages there’s a main function to start with, like C, C++, Java, and Visual C++. If there’s no main function there’s no C program. The compiler tries to locate the main function. If there is more than one function in a C program then this main function stands out from the rest, telling the compiler to start from here (i.e. main).
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