CSE 2031 Lecture 6

Arrays and pointers

• An array name by itself is
  
  • An address
    
  • A pointer value
    
• A pointer is
  
  • A variable taking addresses as values
    
• An array name is
  
  • A particular fixed address
    
  • Like a constant pointer
    
• When an array is declared, the compiler allocates
  
  • A base address
    
    • The address of element 0
      
• And sufficient memory allocation for the rest of the elements
  

Array indexing and pointer arithmetic

#define MAXSIZE 1024

int A[MAXSIZE], *p;

/*

*space allocated for A

* but p – has not been given a value , even if it did here wouldn't necessarily be any space allocated *where it pointed

*/

Equivalent Statements

• P = A;
  
• P = A+I;
  
• P = &A[0];
  
• P=&A[i];
  
• P[0]
  
  • is the same as saying *p
    
• p[3]
  
  • is the same as saying *(p+3)
    

Summing the Array

• 4 forms
  
• 1
  
  • 
    

  sum =0;
  

  int i;
  

  for (i=0; i<maxSize; i++)
  

  sum += A[i];
  

  
   

• 2
  

sum = 0;

for(p=A; p<&A[MAXSIZE];p++)

sum+= *p;

• 3
  

sum = 0;

For(i=0;i<MAXSIZE;i++

sum += *(A+i);

• 4
  

Sum = 0;

P=A;

For(i=0 i<MAXSIZE;i++)

Sum += p[i]; /*no dereferencing*/

• Practice adding pointers and casting them
  
• When you minus the two you get how many blocks of allocated memory exist between them
  
• If you cast them and minus them you get the mem address casted to ints then you get the difference between that
  

Arrays as function parameters

• In a function header
  
  • An array isn't declared w/ a fixed size in a function header
    
  • Int A[] is equivalent to int *A
    
    • This ONLY applies to a function parameter
      
  • Otherwise Int A[] is is NOT equivalent to int *A
    
    • Int *A; creates a pointer variable
      
    • Int A[] creates a constant pointer and no storage
      

Pointers and strings

• Strings are arrays of char and are ended with \0
  
  • \0 is a null character
    
• "ABC"
  
  • Type is char*
    
  • Value is address of the 'A'
    
• Char word[] = "xyz"
  
  • Is the same as char word[] = {'x','y','z','\0'};
    
• Char *p = "xyz";
  
  • Word is an array of length 4.
    
  • P is a pointer and points (for now) to an array with 4 elements
    
  • P can change reference
    
  • *p doesn't allocate new space
    

Dynamic Memory Allocation

• Memory allocated while the program is running
  
• Stack
  
  • When there is a function call there are parameters and local variables
    
  • Space set aside to hold the values for returns when functions are called
    
    • Stacks grow during recursion
      
    • Goes from the top down
      
    • LIFO
      
  • A stack grows and shrinks automatically
    
• Heap
  
  • Declares space and the space remains allocated
    

Malloc

• Void *malloc ( size_T n) /* memory allocation*/
  
  • Returns pointer to n bytes
    
  • Parameter is an integer type
    
    • Signed or unsigned
      
  • Sets aside an amount of space
    
  • Set aside contiguously , like an array
    
  • What is returned is a pointer to the space allocated
    
  • If space cannot be allocated a null pointer is returned
    
  • The storage is uninitialized
    
    • Not necessarily emptied space
      
  • Returns NULL if it can't be done
    
• Void *calloc (size_t n, size_t element_size)
  
  • Allocate enough space for this many elements that take 'so much space'