Different kinds of array variables used in programming language

  • Programming languages can provide 2 kinds of array variables:

      1. Static arrays

      2. Dynamic arrays

  • Static arrays (e.g.: in C programs)

      • The location of static arrays cannot be changed after the array has been created/allocated

      • The size (= number of elements) of the array variable cannot be changed

  • Dynamic arrays (e.g.: in Java programs)

      • The location of dynamic arrays can be changed after the array has been created/allocated

      • The size (= number of elements) associated with the array variable can be changed

Review: Java's syntax to define an array

  • Java defines an int array A of 10 elements as follows: 

         int[] A;
   A = new int[10];

     

     

     

     

     

     

     

Review: Java's syntax to define an array

  • Java defines an int array A of 10 elements as follows: 

         int[] A;
   A = new int[10];

    int[] A will reserve memory for the reference variable A:

               

Review: Java's syntax to define an array

  • Java defines an int array A of 10 elements as follows: 

         int[] A;
   A = new int[10];

    new int[10] reserves memory for 10 int and returns the base address:

               

Review: Java's syntax to define an array

  • Java defines an int array A of 10 elements as follows: 

         int[] A;
   A = new int[10];

    "A = " assigns the return value (= addr of array) of new to the ref var A:

               

Review: Java's syntax to define an array

  • Java defines an int array A of 10 elements as follows: 

         int[] A;
   A = new int[10];

    Result: the location of the array is stored in a program variable

               

Why is Java's array dynamic ?
 

  • Dynamic arrays use a reference variable to point to the actual array:
     

Dynamic arrays are not the norm in high level programming languages

Why is Java's array dynamic ?
 

  • The location and size of the array can be changed by updating the reference variable:

Dynamic arrays are not the norm in high level programming languages

Why is C's array static ?
 

  • A static array uses a identifier (= symbolic constant) to define location of an array:
     

  • A symbolic constant cannot be change after the array has been defined

Why is C's array static ?

  • Example: you cannot update an array "variable" in C (but is allowed in Java):

    C's array identifier is a constant Java's array identifier is a variable 
     int main() 
 {   
     double a[10];
     double b[10];
    
     a = b;  // Illegal to update
             // a CONSTANT
             // (analogy: 9 = 4;)
 }   

     public static void main(String[] args)
 {
     double[] a = new double[10];
     double[] b = new double[10];

     a = b;  // You can update
             // an variable
             // Analogy: a = 4;
 }

DEMO: /home/cs255001/demo/C/set1/staticArray.c + dynArray.java

Summary: how are static and dynamic arrays implemented

Static array   (C) Dynamic array   (Java)
The array variable (A) is a (symbolic) constant for the location (= address) of the array and cannot change (therefore, the array is static) The array variable (A) is a reference variable that contains the base address of the array and can be updated with a new location

Pros and cons of static vs dynamic arrays

  • Dynamic arrays: (Java !)

      • Advantage: flexible, you can increase the size if needed

      • Disadvantage: slower (need one extra memory access per array operation)

  • Static arrays: (C)

      • Advantage: faster

      • Disadvantage: not flexible (cannot change array size)