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Posted By: Lurleen Fischer     Category: C Programming     Views: 2738

This article explains about Accessing The Address of a variable, Declaring and Initializing Pointers, Accessing a Variable Through Pointer, Pointer Expressions, Pointer Comparison, Pointers and Arrays, Pointers and Char Strings, Pointers and Functions, Pointers and Structure.

-  To access variable outside function
-  More efficient way to handle data tables
-  Length and complexity can be reduced
-  Execution speed increased
-  Pointer array to char strings results in saving of data storage
-  Can though, cause problems

Understanding Pointers

  • Computer memory is a sequential collection of storage cells for data and instructions 
  • Each cell, has an address associated with it
  • The addresses are numbered contiguously 
  • The last address depends on memory size
  • During execution system always associates name of a variable with it’s address

Accessing The Address of a variable

  • The actual location of a variable in the memory is system dependent 
  • It may vary throughout the execution
  • Address operator is used to access it
  • &125, int x[10], &x, &(x+y) are illegal use of address operator
  • &x[0] or &[x+i] are valid operations

Declaring and Initializing Pointers

  •  int *p; // p contains address of an int 
  •  float *x // x contains address of a float
  •  p = &quantity // quantity is an int variable
  •  p = &f where f is float is an erroneous statement, because *p results in a wrong ans
  •  int x, *p = &x; // initializes p to address of x
  •  int *p=&x, x is invalid

Accessing a Variable Through Pointer

  • The indirection operator (*) is used to access the value of a variable by its ptr
  • * can be remembered as value at address 
  • int n = *p // int *p = &quantity is done 
  •  int n = *&quantity // is = quantity
  • *5445 where 5445 is a valid location does not yield the content at that address
  • *x=25 changes value of f indirectly

Pointer Expressions

  •  y = *p * *q // (*p) * (*q)
  •  sum += *p  , *p2 = *p2 + 10
  •  z = 5 * - *p2/ *p1 //(5 * (-(*p2)))/(*p1)
  •  in above /* is not acceptable
  •  p+1, p – 4,  p++, p- - are allowed
  •  p2 –p1 for the same array are no of elements in between them

Pointer Comparison

  •  p1 > p2 is true if p1 is having higher memory location than p2
  •  p1 == p2 and p1 !=p2 are also accepted as valid
  •  pointers to related variables like array or string elements makes sense
  •  Pointers can not be used in division, multiplication or addition

Pointers and Arrays

  • Array name is a pointer to itself
  • ANSI  standard makes &a = a
  •  int x[10], *p = x; makes p = &x[0]
  •  while ( p < &x[4]) { sum+=*p; p++ }
  • The compiler allocates contiguous space for all the elements row-wise in multidimensional arrays
  •  int x[10][20] and int *p = x is allowed
  • When we increment i by 1, p will be incremented by the size of the row and not the size of the element itself
  •  p is pointer to first row, p+i is pointer to ith row, *(p+i) is the first element of the ith row
  • *(p+i) + j is jth element of ith row
  • *(*(p+i)+j) value stored in cell (i,j)

Pointers and Char Strings

  •  char name[20], *cptr = name; is valid
  •  while (*cptr != ‘\0’) or (*cptr) is true until the end of the string is reached
  •  while (*cptr); length = cptr – name;
  •  char *name; name = “Delhi” is accepted
  •  Char arrays with variable row length are called ragged arrays, better handled by ptrs

Pointers and Functions

  • Passing a pointer to a function is known as call by reference and function works on actual variables in that case
  •  void copy(char *s1, char*s2)   { while ((*s1++ = *s2++) != ‘\0’ }
  • Pointer parameters are very common in string functions like above
  • *s++ and *++s are different!
  •  type (*ptr)() is a pointer to a function with return type as type
  •  It is different than type *ptr()
  •  double (*p1)(), mul();
  •  p1 = mul; 
  •  (*p1) (x,y) is same as mul(x,y)
  • It is used in system programming

Pointers and Structures

  •  struct inventory { char name[30];
  •   int number;
  •   float price; }
  • product[2],*p;
  •  p = product; // assign zeroth element
  •  p -> name is same as (*p).name
  •  ++ptr->count increments count
  •  (++ptr)->count increments ptr and then return count
  •  ptr++->count is legal and  increments ptr after accessing count 
  •  *ptr->p fetches whatever p points to
  •  *ptr->p++ increments p after accessing whatever it points to
  •  (*ptr->p)++ increments whatever p points to
  •  *ptr++->p increments ptr after accessing whatever it points to

Points on Pointers

  •  A pointer containing garbage can not be detected in certain cases
  •  Abundance of operators are to be used carefully with pointers
  •  Passing pointers to double dimension arrays need other than first size to be specified
  •  Pointer to auto array of a function can not be passed back


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Lurleen Fischer
Lurleen Fischer author of Pointers is from Frankfurt, Germany.
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