- C has
static
arrays
- In contrast: Java has dynamic arrays
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- Arrays in programming languages:
- There are 2 kinds of
arrays that programming languages can provide:
- Static arrays
- The size (number of elements)
associated with the
name of the array variable
cannot be changed
in
static arrays
(After an array is created, the size of the array cannot be changed)
- The size (number of elements)
associated with the
name of the array variable
cannot be changed
in
static arrays
- Dynamic arrays
- The size (number of elements)
associated with the
name of the array variable
can be changed
in
dynamic arrays
(After an array is created, the size of the array can be changed - the array size can grow or shrink)
- The size (number of elements)
associated with the
name of the array variable
can be changed
in
dynamic arrays
- Static arrays
- There are 2 kinds of
arrays that programming languages can provide:
- Static array:
- Static arrays are implemented
using
an (array)
identifier
- The name of the array marks the staring location of the array in memory.
- The array identifier (= name) is
directly associated with the
starting location of the array
as depicted in the following figure:
Explanation:
- When you define an array with
double a[10] ;the compiler will:
- first allocate
memory cells for the array element (say starting at
memory address 5000)
and
- make the symbolic name a equal to the value 5000
- first allocate
memory cells for the array element (say starting at
memory address 5000)
and
- When you define an array with
- When more variables are
created, the static array
will be boxed in
by reserved memory cells
and have no way to expand:
- Static arrays are implemented
using
an (array)
identifier
- Dynamic array: (Java !)
- Dynamic arrays are implemented
using
an array object
reference variable
- You first define the
array object reference variable:
- The identifier is
associated with the
array object reference variable !!
- Then in the second step,
you create (allocate space) the
actual array and
store its location
in the
array object reference variable:
- You can expand a
dynamic array by
allocating space else where
in the memory and
make the object reference variable
point to
the new array:
(Remember, you may not be able to expand the old array because it could be boxed in by reserved memory cells.
But you can always try to find a bigger chuck of unreserve memory elsewhere !)
- Dynamic arrays are implemented
using
an array object
reference variable
- Dynamic arrays: (Java !)
- Advantage:
flexibility
- Disadvantage:
slow
- To access an array element
a[i]
(assuming you already know
the index i),
you need
at least 2 memory accesses:
- One memory access to read the
array object reference variable
- A second memory access to access the array element itself
- One memory access to read the
array object reference variable
- To access an array element
a[i]
(assuming you already know
the index i),
you need
at least 2 memory accesses:
- Advantage:
flexibility
- Static arrays: (Most programming languages)
- Advantage:
speed
- To access an array element a[i] (assuming you already know the index i), you only need 1 memory access
- Disadvantage:
inflexible
- You cannot change the size of the (static) array after you have created the array.
- Advantage:
speed