We are now quite happy to use the basic mathematical expressions, however in engineering we use scientific functions i.e Sin , Cos  , ln etc . Within C we have the following functions

    double atan(double);
    double cos(double);
    double exp(double);
    double fabs(double);
    double log(double);
    double sin(double);
    double sqrt(double);
    double tan(double);
    double acos(double);
    double asin(double);
    double atan2(double, double);
    double ceil(double);
    double cosh(double);
    double floor(double);
    double fmod(double, double);
    double frexp(double, int *);
    double ldexp(double, int);
    double log10(double);
    double modf(double, double *);
    double pow(double, double);
    double sinh(double);
    double tanh(double);
 
  These are similar to the scientific functions on a calculator. We can print out the square root of 4 by assigning the function to a variable i.e.
 
    A =4;
    answer = sqrt(A);   or

    answer = sqrt(4);
 
The overall format of any function is a function name and the value (parameters) passed to it and returned. Within  C  we have only have a single type of function and one value can only be returned ( Although there are tricks to get around this

The syntax of a function is
 
type Function name(type variable, type variable etc )
  {
    Any local variables used by the function
    Function program
 
    return ( variable or value ); 
  }
 
Some important points regarding functions are that the Variables declared are local which means that the main program doesn't see them. The return commands variable is the only parameter returned. Within C all functions should have a prototype description. This is a single line statement which informs the compiler of the type of variables used in the function. In fact the description of the maths functions are in fact prototype headers. This is time to introduce a new type i.e. 'void', by default C takes no type as int, therefore void means no variable.