Pointers and Arrays

Pointers

• A pointer is a variable that holds an address.
• A pointer can store addresses from the stack, heap, or read-only memory.
• Pointers hold the address of only one memory location. However, often the memory location is one element in an array. Therefore, pointers can be used to access arrays.
  
  
• Declarations:
  int *iptr; Creates a pointer to an int named iptr
  char *cptr; Creates a pointer to a char named cptr
  float *fptr; Creates a pointer to a float named fptr
  int **pptr; Creates a pointer to a pointer to an int named pptr
  
  These declarations create pointer variables on the stack that hold arbitrary values. They are not currently holding usable addresses.
  
  
• Assignments:
  iptr=(int *)malloc (5*sizeof(int)); Allocates 20 bytes on the heap and stores the address in iptr
  cptr="hello"; Creates a string in read-only memory and stores the address in cptr
  
  float pi=3.14;
fptr = π Stores the address of pi in fptr
  pptr = &iptr; Stores the address of iptr in pptr
  
  
• Usage
  
  1. * has two pointer-related functions:
     1. * is used in the creation of pointer variables (above).
     2. * is used to get the value stored at the pointer's address. Getting the value that the pointer points to is called dereferencing the pointer.
     
     
  2. *iptr = 5; Sets the value of the location pointed to by iptr to 5.
     int i = *iptr; Sets i to the value pointed to by iptr.
     *(iptr+1) = 10; Sets the value of the second element in the iptr array to iptr to 10.
     iptr[2] = 15; Sets the value of the third element in the iptr array to iptr to 15.
     
  
  
• Notes
  
  • Array and pointer notation are interchangeable: *(array+2) means array[2].
  • Unlike arrays, pointers can be assigned to point to new locations. Arrays cannot be assigned to new addresses.
  • & is the opposite of *. & gets the address of a variable. * gets the value pointed to by the address.