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Introduction
What is C Identifiers Characters in C Datatypes & Variables Structure of C Program How to install turbo C in windows XP? How to install turbo C in windows 7/8?
Introduction [cont'd]
How to Compile and Run? Header files C Tokens Escape sequence Keywords Storage class specifier Constants
Introduction [cont'd]
Volatile
Operators
Operators Arithmetic Operators Relational operators Logical operators Increment & Decrement Operators Assignment Operators Bitwise operators
Operators [cont'd]
Conditional Operator Sizeof Operator
Input Output Functions
printf Scanf Getch putch getche Putchar getchar
Input Output Functions [cont'd]
gets puts
Decision Statements
If If-Else Else-If Nested If-Else Switch Goto
Looping Statements
For While Do-While Break Continue
Arrays
What is Array? One Dimensional Two Dimensional Multi Dimensional
Strings
What is String? Strlen Strcpy Strcmp Strlwr Strupr Strchr
Strings [cont'd]
Strstr Strcat Strrev
User-defined Functions
What is Functions? Function prototype Defining a Function Simple Arguments Return Value Arguments with Return value
User-defined Functions [cont'd]
Call by value Call by reference Recursion Nesting Functions Arrays as Arguments
Structures and Unions
Define Structure Copying Structures Comparing two Structures Arrays of Structures Nested Structures Structure with Functions Define Union
Structures and Unions [cont'd]
Different between Struct and Union
Pointers
Define Pointers & Operator Operators in pointers Pointers Array Pointers as Function Arguments Functions Returning Pointers Pointers and Structures
Memory
Dynamic Memory Allocation Malloc Calloc Realloc Free
Files
What is Files? Write Files Read Files Append Files Error handling
# Preprocessor
What is preprocessor in C? #include #define #if and #else #elif #ifdef #ifndef
# Preprocessor [cont'd]
#undef
#ifndef in # Preprocessor of Programming in C
2311 Views
#ifndef 
#ifndef is used to find constant is defined or not. If given constant defined already then returns 0 otherwise 1. It is indirect to #ifdef.
Syntax 
#ifndef (condition)
statement;
#endif
Program
	#include
	#ifndef PI
	#define PI 3.14
	#endif
	void main(){
	   printf("%f",PI);
	}
Output 
3.140000
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