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\noindent {\bf FUNDAMENTAL MATHEMATICAL CONSTANTS:}

	A number of mathemtatical constants are stored in IDL's
``internal variables'', which are characterized by the first
character being ``!''. 
	
Try the following:

	{\bf print, !pi} (yep! this is just $\pi$)

	{\bf help, !dtor} (degrees times !dtor gives radians)

	{\bf print, !radeg} (radians times !radeg gives degrees)

\noindent There's no variable for $e$, the base of natural logarithms;
to get its numerical value, you have to take $e^1$:

	{\bf print, exp(1)} (base of natural logarithms)

\noindent {\bf THE HELP FACILITY}

	Hypertext on-line documentation is provided by IDL's {\bf HELP}
facility.  To access {\bf HELP}, type

	{\bf ?}

\noindent and after some huffing and puffing a hypertext window will
come up.  Click on ``Navigate'', then on ``Index Search'', which puts
the alphabetical list of help index words on the screen. 

	The first batch are those beginning with ``!'', i.e.  the system
variables.  You can click on one to get the information.  To access
others, either use the scrollbar or type in the word. 

	For example, in the following pages we will use the random
number generator {\bf RANDOMU}.  Type this into the Index Search and
there you are.  The first portion gives information about the function
or procedure.  Then it defines the required input parameters.  {\bf
RANDOMU} needs two input parameters, the ``seed'' and the number of
random numbers to generate.  (All random number generators use an input
number, called a ``seed'', to begin the process of generating random
numbers; they have to start somewhere!).  In IDL, if you don't specify
the numerical value of the seed, it sets {\bf seed} equal to the time
from the system clock, which means the numbers differ each time you call
{\bf RANDOMU}.  For example, with RANDOMU if you want 230 numbers
distributed randomly between 0 and 1, type

	{\bf output = randomu(seed, 230)}

\noindent This generates a 230-element array called {\bf output}.

	Next, {\bf HELP} lists the keywords, which are optional.  With
{\bf RANDOMU} you can not only generate numbers that are distributed
uniformly, but also with other distributions.  For example, you can get
a normal (also called ``Gaussian'') distribution by setting the {\bf
normal} keyword equal to unity:

	{\bf output = randomu(seed, 230, normal=1)}

\noindent or, equivalently when a keyword is equal to unity,

	{\bf output = randomu(seed, 230, /normal)}

\noindent After all this, the help facility provides an example, and
then it gives related items that you can click on if you wish. 

\end


