By Sidhant Garg

 C was developed between by Dennis Ritchie at Bell Laboratories for use with the Unix Operating System.  Unlike previously developed languages C provides facilities for structured programming and allows lexical variable scope and recursion  C is one of the most popular programming languages and there are a very few computer architecture for which the C compiler does not exist.

C programs basically contains the following: Preprocessor Commands Variables Constants Input/output Functions

The #include pre-processor directive is used to link required library files to the main program file (.cpp) Some of the common header files and their functions are as follows: Stdio.h : Standard input /output functions Math.h: Basic math functions such as tan, log, pow(x,y) etc. String.h: String operations such as strcpy, strcat etc. Stlib.h: Standard functions such as malloc, rand, srand etc. Ctype.h: Character classification functions such as isalnum, isalpha, isdigit etc. Iso646.h: To use macros for logical operators E.g. #include void main() { int x; scanf(“%d”,x); //needs the stdio.h header file }

 #include  /* print Fahrenheit-Celsius table  for fahr = 0, 20,..., 300 */  main()  {  int fahr, celsius;  int lower, upper, step;  lower = 0; /* lower limit of temperature scale */  upper = 300; /* upper limit */  step = 20; /* step size */  fahr = lower;  while (fahr <= upper) {  celsius = 5 * (fahr-32) / 9;  printf("%d\t%d\n", fahr, celsius);  fahr = fahr + step;  }

In C, variables can be broadly of the following data types: char – character type char stringname [x] (x=1,2..n)- string of ‘x’ characters int array [x] – array containing ‘x’ elements int- integer type float- real numbers including decimals etc. double- extended form of float (larger range) The above can also be used in conjunction with the following ‘datatype’ modifiers: long short signed unsigned cont…

In addition to the previously described standard datatypes, C allows the following user defined datatypes: typedef- Allows users to change the name of a standard data type. For e.g. typedef float money will allow the user to use money as a data type enum- Enumerated data types similar to C++ e.g. enum color{red,blue,white} struct- Structures are heterogeneous user-defined datatypes They are often used to create nodes and lists They are like the data members of classes but do not have the concept of associated functions `e.g. struct features{ int height; double weight; char[5] color; }

 In C, constants can be defined in the following two ways-  Using #define e.g. #define pi 3.14  Using const e.g. const double x=3.0  Constants are often declared globally instead of being used directly in the main program.

User input from the keyboard is done using ‘scanf ‘ Output to the screen is done using ‘printf’ Here are the format specifiers that are required %d - int %c - char %f - float %lf -double %s -string %x- hexadecimal For e.g. #include void main() { int x; scanf(“Enter integer: %d”,x); printf(“The integer you entered is: %d,x,”. Goodbye!”); }

 Functions are modules or subprograms that accept certain parameters and return values to the calling function / main program  They have the following format: Return_type function_name(parameters) { local variables; C statements; return return_value; }

 In C all the function arguments are passed by value. This means that the functions are given the values of it arguments in temporary variable rather than the originals.  It usually leads to more compact programs with fewer extraneous variables, because parameters can be treated as conveniently initialized local variables in the called routine

For example, here is a version of power that makes use of this property. /* power: raise base to n-th power; n >= 0; version 2 */ int power(int base, int n) { int p; for (p = 1; n > 0; --n) p = p * base; return p; } The parameter n is used as a temporary variable, and is counted down (a for loop that runs backwards) until it becomes zero; there is no longer a need for the variable i. Whatever is done to n inside power has no effect on the argument that power was originally called with.

 Operators and Expressions  Arrays  Linked Lists  Pointers  Stacks and Queues  Hash Tables