More on Arrays: MultiDimensional Arrays, Arrays are Parameters

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More on ArraysMulti-Dimensional Arrays,Arrays
are Parameters

• Soon Tee Teoh
• CS 49C

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Remember 2D Arrays?
int m20050 // 2D array float
far102032 // 3D array void func()
int i, j for (i0ilt200i) for
(j0jlt50j) mij 10 i
2j
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2D Arrays in Memory

• This is how a 2D array is kept in memory.
• Each element is kept in consecutive positions in
  memory.

int m32
Memory
m00
m01
m10
m11
m20
m21
Size of an int
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Pseudo 2D Arrays

• You can also define a one-dimensional array in
  the following way.
• You use it as though it is a two-dimensional
  array.
• You index the array by calculating your own
  offset index to the array.

define WIDTH 5 define HEIGHT 3 int iarray
HEIGHT WIDTH int n,m int main ()
for (n0nltHEIGHTn) for
(m0mltWIDTHm)
iarraynWIDTHm(n1)(m1)
return 0
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Passing Array as Argument

• We have done this before.
• Remember, the name of the array also refers to
  the address of the first element of the array.
• Therefore, we can pass the name of the array as
  though it is a pointer.

void printarray (int arr, int length)
for (int n0 nltlength n)
printf(d , arri)
printf(\n)
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Clearer Way

• The preceding way, though it works, is kind of
  weird.
• It requires you to think of a pointer as an
  array, and vice versa.
• When writing a function that accepts an array as
  an argument, it makes more sense for the
  programmer to write it in such a way that make it
  clear that it is expecting an array.

void printarray (int arr, int length)
for (int n0 nltlength n)
printf(d , arri)
printf(\n)
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Multi-Dimensional Arrays as Arguments

• A procedure in C can also accept a
  multi-dimensional array as an argument.
• The number of elements in the higher dimensions
  have to be specified in the function signature.

void procedure (int myarray34)
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malloc and free
void malloc ( size_t size )
void free ( void ptr )

• malloc requests some memory from the Operation
  System and returns the address of the memory
  allocated.
• free frees up the space previously received by
  malloc.

void func(int n) int i int iptr
iptr (int )malloc(nsizeof(int)) for
(i0iltni) iptri 15 i 2
free(iptr)
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The NULL Pointer

• malloc returns the address of the memory that has
  been allocated.
• However, sometimes it fails.
• For example, when the heap is full, or when
  malloc requests too much memory, the OS cannot
  give malloc the memory.
• In this case, the return value of malloc is the
  NULL pointer.

void func(int n) int i int iptr
iptr (int )malloc(nsizeof(int)) if
(iptr ! 0) // check that malloc did not return
the NULL pointer for (i0iltni)
iptri 15 i 2
free(iptr)