Arrays

Basics

Example:

char A[12];
char *B[8];
double C[6];
double *D[5];

Array	Element Size	Total Size	Start Address	Element i
A	1	12	$x_A$	$x_A+i$
B	4	32	$x_B$	$x_B+4i$
C	8	48	$x_C$	$x_C+8i$
D	4	20	$x_D$	$x_D+4i$

Pointer Arithmetic

If p is a pointer of type $T$, where L is the size of $T$, and the value of p is $x_p$, then p+i has the value $x_p + Li$.

Nested Arrays

int A[n][m];

is equivalent to the following declaration

typedef int rowm_t[m];
rowm_t A[n];

Array A contains n elements of array with m integers. Therefore, the total array size is $4nm$ bytes. It is analagous to a two-dimensional array where the reference starts from A[0][0] through A[n-1][m-1]. If the address of A[0][0] is $x_A$, then the address of A[i][j] is $x_A + L(mi+j)$.

Fixed-Size Arrays vs Variable-Size Arrays

Fixed-Size Arrays

The size of a fixed-size array is determined at compile time. This means that the array size is known and fixed when the program is compiled.

Fixed-Size Array Example

#include <stdio.h>

int main(){
    int N = 5;
    int fixedArray[N];
    return 0;
}

Variable-Size Arrays

The size of a variable-size array is determined at run time.

Variable-size arrays can be declared as local variables or as arguments to a function. The address computation for variable-size arrays is similar to that of fixed-size arrays, with two main differences:

1. The positions of the arguments on the stack are shifted due to the addition of the size parameter.
2. A multiply instruction is used to compute the address of an element rather than using the leal instruction.

Static variables, like fixed-size arrays, are any global variables and any other variable declared static. The compiler allocates the variables, giving them a constant address. To allocate storage of variable-size arrays dynamically need malloc or calloc.

For example:

int a;
int* b;
void foo () {
    b = (int*) malloc (10*sizeof(int));
    b[a] = a;
}

At compilation compiler allocates pointer b. At run time b calls malloc to get an address for the dynamic array.

Note in Java static int[] arr is not a static variable.