Arrays 1

1
Arrays (1)

• You create an array in LISP by using the function
  (make-array ltarray_dimensiongt).
• All elements are initially set to nil.
• To create a 1-dimensional array of size 3
• (make-array 3)
• To create a 2-dimensional array of size 3 x 3
• (make-array (3 3))
• To create a 3-dimensional array of size 3 x 3 x 3
• (make-array (3 3 3))
• Array index always starts from 0.

2
Arrays (2)

• In the previous examples, after you have created
  the arrays, there was no way by which you could
  refer to it.
• Here is how you can assign an array to a
  variable
• (setq an_array (make-array 3))
• How would you then set and access the values of
  each cell within an array?

3
Aref

• To access a particular element in an array, use a
  function called aref.
• For example, if you have a 1 dimensional array
  named an_array 10 elements, then to access the
  nth element, you could do this
• (aref an_array n)
• For example (aref an_array 9)
• (aref an_array 7)

4
Setf

• Setf is the only way to set the elements of an
  array.
• To set the nth element of an_array to a
  particular value, do as follows
• (setf (aref an_array n) value)
• For example (setf (aref an_array 5) Hello)

5
Incf (an interesting function?)

• Incf reads a NUMBER from an array at a specified
  position, increments it, and then writes it back.
  
• Assume an_array (hello NIL 3)
• Then (incf (aref an_array 2)) will give you
  (hello NIL 4)
• What happens if you do this
• (incf (aref an_array 0))
• (incf (aref an_array 1))