CSci 160 Lecture 40

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CSci 160Lecture 40

• Martin van Bommel

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lvalues

• In C, any expression that refers to an internal
  memory location is called an lvalue
• Appear on left side of assignment statement
• e.g. x 1.0 intarray2 17
• Constants expressions are not lvalues

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Memory and Data Representation

• Every lvalue is stored somewhere in memory and
  therefore has an address
• Once declared, the address of an lvalue never
  changes, even though its contents may change
• Depending on type of data, different lvalues
  require different amounts of memory
• The address of an lvalue is itself data that can
  be manipulated and stored in memory

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Pointer Declaration

• A pointer is a variable storing the address of
  another variable
• Declaration type ptr
• Example int p1, p2
• p1 and p2 are referred to as pointers-to-int, and
  may contain the addresses of integers

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Pointer Operations

• C defines two operators that manipulate pointer
  values address-of value-pointed-to
  (dereference)
• Operand of must be lvalue, and returns the
  address of the lvalue
• Operand of must be a pointer, and returns the
  value pointed to by the pointer

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Special pointer NULL

• NULL is the value of a pointer that does not
  currently point to any data
• Can not dereference NULL pointer
• Exists for checking file pointers, and for
  applications we will see later
• infile fopen(file.txt, r)if (infile
  NULL)
• printf(File not open)

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Pass by Reference

• To modify value of argument, use pointer
• void SetToZero(int ip)
• ip 0
• which makes ip a pointer to the location of
  the value of the argument of the call
• SetToZero(x)

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Advantage of use of operator

• When calling a function using
• func(x)
• you know the value of the variable x will not
  change, whereas calling a function using
• func(x)
• is allowed to change the value of x
• Easier to predict effects of function call

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SwapInteger function

• When sorting arrays, wanted to use
• SwapIntegers(arraylow, arrayhigh)
• Using pointers allows the following
• void SwapIntegers(int p1, int p2)
• int temp
• temp p1
• p1 p2
• p2 temp

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Returning Multiple Results

• Convert time (minutes) to hours and minutes
• void ConvertTime(int time,
• int hours,
• int mins)
• hours time / 60
• mins time 60

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Better to use pure functions

• int Hours(int time)
• return (time / 60)
• int Minutes(int time)
• return (time 60)
• printf(d02d\n,Hours(time),Minutes(time))