C for Java Programmers

1
C for Java Programmers

• Chapter 2. Fundamental Daty Types
• Timothy Budd

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Integers

• Java Integer Internal Representation
• long - 32 bit
• integer -32 bit
• long - 64 bit
• short int x // declare x as a small integer
• long y // declare y as long integer
• C Integer Internal Representation
• long and/or short may have the same size as
  integer

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C Integer

• An unsigned integer can only hold nonnegative
  values
• int i -3
• unsigned int j i
• cout ltlt j ltlt endl // will print very large
  positive integer
• Assigning a negaitve value to an unsigned
  variable is confusing
• Integer division involving negative numbers is
  platform dependent, but following equality must
  be preserved a (a / b) b a b

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Integers

• Never use the remainder operator with negative
  values.
• unsigned long a // can hold largest integer
  value
• signed short int b
• C does not recognize the Byte data type in
  Java. Instead signed char is often used to
  represent byte-sized quantities.

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Characters

• 8 bit quatity -
• Legal to perform arithmatic on characters
• Character can be signed or unsigned.
• w_char - recent addition wide character alias for
  another interger type such as short.

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Booleans

• Recent addtion - bool
• Historical boolean representation
• nonzero - true
• zero - false
• Integer and pointer types can be used as boolean
  values.
• Cannot be signed or unsigned.

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Examples of Booleans
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Booleans

• Even pointer value can be used as booleans. False
  if it is null, true otherwise.
• aClass aPtr // declare a pointer variable
• ...
• if (aPtr) // will be true if aPtr is not null
• Legacy code can contain different boolean
  abstractions.

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Bit Fields

• Seldome used feature
• Programmer can specify explicitly the number of
  bits to be used.
• struct infoByte
• int on1 // one-bit value, 0 or 1
• int 4 // four bit padding, not named
• int type 3 // three bit value, 0 to 7

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Floating Point Values

• float, double, long double
• int i
• double d 3.14
• i d // may generate a warning
• Never use float use double instead.
• math rountines will not throw an exception on
  error

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Floating Point Values

• Always check errno
• double d sqrt(-1) // should generate error
• if (errno EDOM)
• ... // but only caught if checked
• Java Nan, NEGATIVE INFINITY, POSITIVE INFINITY

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Enumerated Values

• Nothing in commonwith Enumeration calss in Java
• enum declaration in C
• enum color red, orange, yellow
• enum fruit apple, pear, orange
• // error orange redefined

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Enumeration Values

• Can be converted into integers and can even have
  their own internal integer values explicitly
  specified.
• enum shape circle12, square3, triangle
• Can be assigned to an integer and incremented,
  but the resulting value must then be cast back
  into the enumrated data type before
• fruit aFruit pear
• int i aFruit // legal conversion
• i // legal increment
• aFruit fruit(i) // fruit is probably now
  orange
• i
• aFruit fruit(i) // fruit value is now
  undefined

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Enumeration Values

• Cast operation can be written by type(value) or
  older (type)value syntax.
• Not legal to change a pointer type.
• int i
• char c
• c char (i) // error not legal syntax
• static_cast would be even better.

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The void type

• In Java, used to represent a method or function
  that does not yield a result.
• In C, type can also be uses as a pointer type
  to describe a universal pointer that can hold a
  pointer to any type of value.

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Arrays

• An array need not be allocated by using new
  directive as in Java.
• The number of element determined at compile time.
• int data100 // create an array of 100
  elements
• The number of element can be omitted.
• char text "an array of characters"
• int limits 10, 12, 14, 17, 0

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Arrays

• Not legal to place the square brackets after type
  as in Java
• double limits 10, 12, 14, 17, 0 //
  legal Java, not C
• The limit can be omitted when arrays are passed
  as arguments to a function.
• // compute average of an array of data values
• double average (int n, double data )
• double sum 0
• for (int i 0 i lt n i)
• sum datai
• return sum / n

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Structure

• The major differences in C between a strunct
  and a class is that the access is by default
  public rather than private as in classes.
• // holds an int, a double, AND a pointer
• struct myStruct
• int i
• double d
• anObject p

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Unions

• Similar to a structure, but the different data
  fields all sharre the same location in memory.
• // can hold an int, a double, OR a pointer
• union myUnion
• int i
• double d
• anObject p
• Object-oriented languages made unions unnecessary
  by introducing polymorphic variables

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Object Values

• Java uses reference semantics for assignment
• class box // Java box
• public int value
• box a new box()
• box b
• a.value 7 // set variable a
• b a // assign b from a
• a.value 12 // change variable a
• System.out.println("a value " a.value)
• System.out.println("b value " b.value)

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Object Values

• C uses copy semantics.
• class box // C box
• public
• int value
• box a // note, explicit allocation not required
• box b
• a.value 7
• b a
• a.value 12
• cout ltlt "a value " ltlt a.value ltlt endl
• cout ltlt "b value " ltlt b.value ltlt endl

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Object Values

• The concept of reference variable in C, which
  is a variable declared as a direct alias.
• box a new box() // java reference assignment
• box b a
• b new box() // reassignment of reference
• box a // C example
• box b a // reference assignment
• box c
• b c // error not permitted to reassign
  reference

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Functions

• C permits the definition of function that are
  not member of any class.
• // define a function for the maximum
• // of two integer values
• int max (int i, int j)
• if (i lt j) return j
• return i
• int x ...
• int y ...
• int z max(x, y)

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Functions

• Prototypes are necessary in C as every function
  name with its associated parameter types must be
  known to the compiler.
• // declare function max defined elsewhere
• int max(int, int)

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Order of Argument Evaluation

• In Java, argument is evaluated from left to
  right.
• String s "going, "
• printTest (s, s, s "gone ")
• void printTest (String a, String b, String c)
• System.out.println(a b c)
• In C, order of argument evaluation is undefined
  and implement dependent.

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The function main

• In C, main is a function outside any class.
• Always return zero on successful completion of
  the main program.
• int main (int argc, char argv )
• cout ltlt "executing program " ltlt argv0 ltlt
  '\n'
• return 0 // execution successful
• The first command line argument in C is always
  the application name.

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Altenative main Entry points

• Individual libraries may provide threir own
  version of main and then require a different
  entry point.
• Many Windows graphical systems come with their
  own main routine already written, which will
  perform certain initializations before invoking a
  different function such as WinMain.