plot modulus function

[message]

Readers,

An introductory maths book describes the concept of a modulus function 
as a simplest description of a prescription (piecewise) function, 
providing the following example:

f(x)=|x| defined as
f(x)={x,x>=0
        -x,x<0

The octave manual is now being read (chapter 1 so far :) ). What 
sections would describe how to generate values of x, so that a graph can 
be plotted (the plan would be to use gnuplot).

Thanks.