METU Dept. of Computer Eng. Summer 2002

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Lecture 07
Mon July 22, 2002

• METU Dept. of Computer Eng. Summer 2002
• Ceng230 - Section 01
• Introduction To C Programming
• by Ahmet Sacan

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Pointers

• A pointer is variable which points to a place in
  computer's memory.
• pointer address
• pointer variable a variable dedicated to hold
  addresses.

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and unary operators

• , when applied to a variable, yields its address
  (pointer to the variable)
• , when applied to an address (pointer), fetches
  the value at that address.

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contents .... x107 3 ......
address .... x103 x104 x105 x106 x107 ...
variable name ap a
ip ip a a
x107 3 3 x107
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Pointer declaration

• lttypegt ltvarNamegt
• int iVar / declares an int. /
• int iVarPtr / declares a variable that
• will POINT to an integer /
• int ptr2
• float f
• float myFloatPointer

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Pointer Assignment

• ltvarNamegt / address of ltvarNamegt /
• iVarPtr iVar
• ptr2 iVarPtr
• myFloatPointer f
• iVarPtr iVar / BAD... /
• myFloatPointer iVarPtr / INVALID /

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NULL

• a pointer value of 0 is known as NULL pointer.
• int x
• x NULL

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Dereferencing pointers

• ltptrVarNamegt / value inside the
• memory
  pointed at /
• iVar iVarPtr
• f iVarPtr
• f myFloatPtr

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• int x3, y4, p1NULL
• int p20
• p1 x
• printf("d xd xd d", x, x, p1, p1)
• x5
• printf("d xd xd d", x, x, p1, p1)
• p1 6
• printf("d xd xd d", x, x, p1, p1)
• p2 p1
• printf("xd xd d xd xd d",
• p1, p1, p1, p2, p2, p2 )

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Functions Pointers- Call by reference -

• int square(int x) return xx
• void main()
• ...
• a square(a)

int a void square( ) a aa void main()
... square()
void square(int x) x (x)(x) void
main() ... square(a)
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int Add(int x, int y) return xy void
main() ... c Add(a, b) ...
void Add(int x, int y, int z) z xy
void main() ... Add(a, b, c) ...
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• void OrderSwap ( int x, int y )
• int t
• if(x gt y)
• t x x y y t
• void main()
• int a, b, c
• scanf("d d d", a, b, c)
• OrderSwap(a,b)
• OrderSwap(b,c)
• OrderSwap(a,b)
• printf("d d d", a, b, c)

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• float maxPtr(float xp, float yp)
• return xp gt yp ? xp yp
• void main()
• float a, b, c
• c maxPtr(a, b)
• ...

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Goal

• First read 10 numbers.
• Then read numbers 1lt x lt 10 and tell what the
  xth number was.

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• int a, b, c, d, e, f, g, h, i, j int x
• scanf("d d d d d d d d d d d", a, b,
  c, d, e, f, g, h, i, j)
• scanf("d", x)
• if(x1) printf("d", a)
• else if(x2) printf("d", b)
• ....

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Arrays

• int arr 10
• int i, x
• for(i0 ilt10 i)
• scanf("d", arri)
• scanf("d", x)
• printf("d", arrx)

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Arrays

• array a finite, ordered collection of the
  same-typed data items.
• element individual data items in an array
• subscript (index) Only one name is assigned to
  an entire array. Individual elements are
  referenced by specifying an index. Subscripts
  start at ZERO.

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Array Declaration

• lttypegt ltarrayNamegt expr-1...expr-n
• expr-i must be a constant integral type.
• int midtermScores50
• define NUM_EXAMS 3
• float exams3
• int x 10
• float pricesx / INVALID!
• array-expr must be
  constant /

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