Chapter 7' C Pointers

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Chapter 7. C Pointers
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7.2 Pointer variable declarations and
initialization

• A pointer is a variable that contains a memory
  address as its value
• A variable directly references a value while a
  pointer indirectly references a value

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7.2 Pointer variable declarations and
initialization

• The following declaration declares a pointer and
  an integer
• int countPtr, count
• The variable countPtr is declared as type int
  (a pointer pointing to an integer value)
• Pointers can be declared to point to any type of
  data

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countPtr
count
7
count directly references the value 7 countPtr
indirectly references the value 7
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7.2 Pointer variable declarations and
initialization

• A pointer can be initialized as an address, a
  NULL, or a 0. Using 0 is equivalent to using
  NULL. However, NULL is preferred.
• int ptr1NULL
• int ptr20
• int ptr3y / will be explained later /

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7.3 Pointer operators

• The operator (address operator) returns the
  address of a variable. For example, the following
  statement assigns the address of y to yPtr
• yPtry

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7.3 Pointer operators

• The following statements results in the following
  figure
• int y5
• int yPtry

yPtr
y
5
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7.3 Pointer operators

• The operator (dereferencing operator) returns
  the value pointed by a pointer
• For example (see the figure and statements in the
  previous slide), the following statement will
  output the value 5.
• printf(d,yPtr)

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7.3 Pointer operators

• The operators and are complements of one
  another (see the program in the next slide)
• Figure 7.5 shows the precedence of the operators
  and

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Simple exercise

• Use pointers to switch the values of two variable

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7.4 Calling functions by reference

• In C, functions are generally called by value.
  Nevertheless, pointers can be used to simulate
  calling by reference
• See the next slide for an example

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include ltstdio.hgt void aa(int , int) main()
int a10,b10 printf(Before calling aa, ad,
bd\n,a,b) aa(a,b) printf(After calling
aa, ad, bd\n,a,b) void aa(int x, int
y) x 2 y 2 return
what will be outputted by this program?
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aa
main
x
a
10
b
y
10
10
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7.4 Calling functions by reference

• See Figures 7.6 through 7.9 for a more detailed
  explanation

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7.4 Calling functions by reference

• As mentioned in the previous chapter, passing an
  array to a function results in calling by
  reference. Actually, passing an array corresponds
  to passing the address of the first array element
• When passing an array, the notation and can
  be used interchangeably (see the next slide)

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void aa(int , int) main() int a10 . .
. aa(a,10) . . . void aa(int x, int y)
. . .
void aa(int , int) main() int a10 . .
. aa(a,10) . . . void aa(int x, int y)
. . .
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Simple exercise

• Use pointers to switch the contents of two arrays
  in a function other than main

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7.6 Bubble sort using call by reference

• The program in the following slides is a bubble
  sort using call by reference
• The call by reference occurs in the function
  swap, see these statements
• int holdelement1Ptr
• element1Ptrelement2Ptr
• element2Ptrhold

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7.6 Bubble sort using call by reference

• The prototype of the function swap is defined
  inside the function bubbleSort because only
  bubbleSort uses swap. This restricts proper call

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The sizeof operator (pp. 267-269)

• The sizeof operator returns the number of bytes
  used by a variable, that of a data type, or that
  of a constant
• See the programs in the following slides for some
  examples

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7.7 Pointer expressions and pointer arithmetic

• If a pointer points to an array element, pointer
  expression and arithmetic are meaningful
• Pointers can be increased, decreased, added by a
  value, subtracted by a value, or subtracted by
  another pointer

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7.7 Pointer expressions and pointer arithmetic

• Example
• int vPtr, v1,2,3,4,5,6,7,8,9,10,i
• vPtrv / this statement is the same as
  vPtrv0 /
• for(i0ilt10vPtr,i)
• printf(d\n,vPtr) / what will be
  outputted? ?
• The vPtr advances the pointer to the next array
  element. Therefore, vPtr in the above example
  causes the value of vPtr to be increased by
  sizeof(int)

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v0
v1
v2
v9
vPtr vPtr1 vPtr2
vPtr9
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7.7 Pointer expressions and pointer arithmetic

• Example
• float vPtr, v10
• vPtrv / suppose that 7000 is assigned to
  vPtr/
• vPtr2
• Suppose that sizeof(float) is 4. Then, what value
  will be assigned to vPtr by the statement vPtr2?

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7.7 Pointer expressions and pointer arithmetic

• Example
• float ptr1,ptr2,v10
• int i
• ptr1v3/ suppose ptr1 is set 700C/
• ptr2v1/ suppose ptr2 is set 7004/
• iptr2-ptr1 / i will be set the number of array
  elements between the two pointers /
• What will i gets in the last statement above?

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Simple exercise

• Use pointer arithmetic to assign the summation of
  two arrays to the third array, element by element

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7.7 Pointer expressions and pointer arithmetic

• A pointer can be assigned to another pointer in
  case that they have the same type
• Cast operators should be used to convert pointer
  type for the assignment between different pointer
  types. For example
• ptr1(int)ptr2

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7.7 Pointer expressions and pointer arithmetic

• void pointer can used in assignment
• Example
• void vptr1,vptr2
• int iptr1
• float fptr1
• vptr1vptr2
• vptr1iptr1
• fptr1vptr2

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7.7 Pointer expressions and pointer arithmetic

• NOTE a void pointer cannot be dereferenced
  because the compiler does not know what is the
  type pointed by the pointer

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7.8 The relationship between pointers and arrays

• An array name can be though as a constant pointer
  (pointing to which array element?)
• Equivalent expressions (suppose that b is an
  integer array and bPtr is an integer pointer)
• bPtrb bPtrb0
• b3 (bPtr3)
• b3 bPtr3
• b3 (b3)

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7.8 The relationship between pointers and arrays

• If a pointer points to an array, the pointer can
  be increased or decreased (what do the increment
  and decrement mean?)
• On the other hand, array name cannot be increased
  or decreased because it is actually a constant
  pointer. See the next slide for an example

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int ptr,a101,2,3,4,5 ptra ptr3 /what
does this statement mean?/ printf(d,ptr) a
3 / a compiler error occur here /
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7.8 The relationship between pointers and arrays

• The programs in the next slides show the
  reference of an array using different methods

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7.9 Arrays of pointers

• The same type of pointer can be structured as an
  array
• The following declaration results in the figure
  in the next slide
• const char suit4 Hearts, Diamonds,
  Clubs, Spades

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???

• Write a program to
• 1. input 10 different integers from the keyboard,
  in which five integers are stored in array a and
  five are in array b, and
• 2. use a function other than main to place the
  largest five numbers in array a and the others in
  array b. Please use pointers in this function.
• Due the day after 2 weeks