C for Java Programmers

1
C for Java Programmers

• Lecture 3

2
Last Lecture

• Last Lecture we had a look at
• Single Dimensional Arrays
• Multi Dimensional Arrays
• First Mention of Pointers
• C-Style Strings
• The Null Terminator
• String Manipulation

3
Todays Lecture

• This lecture we will be looking at
• The ? command
• Structs
• Typedef Command
• Functions
• Unfortunately theres no pictures of Bjarney
  today.
• Nonetheless Bjarney is with us in spirit.

4
The ? Operator

• You can use the ? operator to replace if-else
  statements
• However both the if and else parts must be part
  of the single expression not another statement.
• The ? is called a ternary operator because it
  requires three operands.
• Just like regular expressions in perl, this is
  the main contributor for making C/C unreadable

5
? operators General Form

• Exp1 ? Exp2 Exp3
• The value of a ? expression is determined as
  follows
• Exp1 is evaluated.
• If it is true Exp2 is evaluated and becomes the
  value of the entire ? expression
• If Exp1 is false, Exp3 is evaluated and then that
  value becomes the value of the ? expression.

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Example

• y x 9 ? 100 200
• This can be basically translated from Creek to
  mean
• If x is greater than nine, then assign 100 to y,
  and if x is not greater than 9 assign 200 to y.
• The point to note is that when using the ?
  operator, is that it will ultimately evaluate an
  expression - the following statement alone would
  do nothing
• x 9 ? 100 200

7
When should you use it?

• Only use this operator when it is the right
  situation!
• The following statement shows how you might use
  the operator to return the square of a value i,
  but still preserving its sign
• isquare i0 ? ii -(ii)
• However the next bit of code which returns a
  certain prime number dependent on i, would be
  better written and much easier to understand for
  any reader using a switch statement
• n i1?1i2?3i3?5i4?711

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? evaluate expressions

• You can use any valid expressions after the ?
  such as
• int magic 10
• int guess 17
• if (guess magic) cout Blaine
• else guess magic ? cout low

In this code fragment, having guessed wrong we
use the ? operator to help us display on screen
whether the guess was too high or too low.
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Expressions in Conditionals

• It can be confusing that that you are allowed to
  use any valid expression to control the if or the
  ? operator.You are not restricted to expressions
  which use relational operators (as you are in
  basic or pascal)
• You can use any expression that resolves to a
  true (zero) or false (non-zero) value. For
  example
• if (b) cout
• else cout muppet

10
Structures

• A structure is a collection of related data
  identified as a single storage unit.
• Each element in a structure is called a data
  member.
• After it has been declared, an instance of that
  structure can be created for use in your program.
• It is almost an unwritten law in Computer Science
  lectures that any examples of structures must,
  and I mean must, be boring. Im talking about
  stuff like address books or bus timetables.

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Jims Chauvinistic Struct

• struct vital_Statistics
• char modelsName50
• char hairColour20
• long int age
• int nrOfPublications

Here our struct has two char arrays and 4 ints,
but we could have used any data types, including
other structs.
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And the code would be

• vital_statistics jordan
• strcpy( jordan.modelsName, Jordan)
• strcpy( jordan.hairColour, Brunette)
• jordan.age 24
• jordan.nrOfPubs 33

Note the use of the dot operator, sometimes
called the direct membership operator.
integers and other fundamental types can be
directly accessed. For the char arrays we use the
strcpy function.
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You can have lots of instances

• Once you have defined the struct you can create
  as many instance as you want
• vital_Statistics brad
• strcpy( brad.modelsName, Brad Pitt)
• strcpy( brad.hairColour, Blonde-ish)
• brad.nrOfPubs 13

Example for the girls so I dont get in trouble.
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Quick Initialisation

• Similar to the way you can initialise arrays when
  they are declared, you can also instantiate an
  object and supply all of its members at once
• vital_Statistics anotherPerson
• whoever,
• ginger,
• 87

You have to remember what order your data
elements were declared
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Whats happening in memory?
modelsName (50 Bytes)
hairColour (20 Bytes)
age (32 Bytes)
nrOfPubs(16 Bytes)
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Manipulating Structures

• Accessing Structure Members
• cout
• Assigning to Structure Members
• jordan.age 24
• Assigning whole structures
• vital_Statistics a,b
• a.age 24
• b a

Assigning one struct to the other.
17
Arrays of Structures

• Perhaps the most common use of structures is in
  arrays of structures. To declare an array of
  structures, you must first define a structure,
  and then declare an array variable of that type
• vital_Statistics topTrumps52
• To access the structs variables simply index the
  the structure name as you would with a normal
  array
• someString topTrumps7.name

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Complex Structures

• Members of structures can either be of type
• simple or compound
• Code fragment Example of a struct with compound
  members
• struct phone_book
• char address50
• int number
• vital_statistics info

Any of the built in data types such as an integer
or a character
Arrays of these fundamentals, or other structures
themselves.
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Structs the origin of Classes

• Structures are part of the C-subset and were
  inherited in C from the C Language.
• But C Structs are very closely related to C
  Classes.
• In C the role of a structure was expanded,
  allowing you to have both private and public
  variables and even methods. Its only difference
  from a class is that by default all members of a
  struct are public.
• Struct and Classes are so similar why keep
  structs at all?
• Tradition, but also because it allows the
  definition of a class to be free to evolve.
  Furthermore it allows C to remain compatible
  with C.

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User Defined Types

• You can define a new data type name using the
  keyword typedef.
• It doesnt actually create a new type, but rather
  defines a new name for one.
• General Form typedef type newName
• Example typedef float balance

21
FUNCTIONS

• Functions are the fundamentals of Procedural
  Programming. C is a language of this type.
• Before OO this was how everything was written.
• In many instances this is how things are still
  written.
• Functions are the building blocks of C and C

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Learning Progression
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General Form

• Functions compartmentalise a repetitive task.
• General Form is
• returnType functionName (parameter list)
• Example
• float bowel(int i, char c)

Functions can return any type of data except an
array. This one returns a float.
body
name of the function
parameters
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Example An Addition Function

• include
• int foo(int a, int b)
• int r
• rab
• return (r)
• int main ()
• int z
• z foo(5,3)
• cout

Our function foo, that sums numbers
statement that calls the function and puts it
into z.
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Formal Parameters

• If a function takes in arguments as in the above
  example you must declare variables for them to go
  in.
• int foo(int a, int b)
• z foo( 5, 3)
• Even if you declare these the same name as those
  used in your main program, they are not the same
  variables.

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Sending Parameters

• You can pass variables into a function by two
  methods - value or by reference.
• When you pass a variable by value into a
  function, the variable passed in is only changed
  locally within the function. Essentially it is
  copied across.
• Passing by reference makes global changes to a
  variable passed in. To pass by reference we use
  the address operator when we receive the
  argument.

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Passing by Value

• void cube(int x)
• x xxx
• cout
• // MAIN PROGRAM
  
• int main()
• int number 3
• cube(number)
• cout
• OUTPUT The answer is 27 and the number is 3

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Passing by Reference

• void cube(int x)
• x xxx
• cout
• // MAIN PROGRAM
  
• int main()
• int number 3
• cube(number)
• cout
• OUTPUT The answer is 27 and the number is
  27

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Scoping

• As you know scope rules govern whether a piece of
  code knows about or has access to another piece
  of code or data.
• Each function is a discrete piece of code
  everything inside it is essentially hidden from
  the rest of the program.
• So Variables defined within the function are
  local to it.
• The function can only see its own data apart
  from any global variables that you use. Strive
  against global variables however.

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Having Arrays as Parameters

• We discussed arrays last lecture and I mentioned
  that sending arrays to functions was not
  straightforward.
• Passing arrays is an exception to the normal
  send-by-value parameter passing.
• There are several ways of passing arrays to
  functions but in each case we only pass the
  arrays address. This is very similar what we just
  considered when we were passing variables by
  reference.
• Because of this we are not sending a copy - any
  changes in the function will affect our original
  array!

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3 methods for passing Arrays

• void foo1 (int x10) //sized array
• void foo2 (int x) //unsized array
• void foo3 (int x) //pointer

Here we take in an address and use x to remember
it as an array with 10 elements
but the length of the array doesnt matter as
far as C is concerned
final method uses a pointer.
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Passing Multidimensional Arrays

• Be careful when sending multi-dimensional arrays
  to functions

void mal (int x45) int
main() int hipHip333 mal(hipHip)

For a multi-dimension array we must declare all
but the first of the dimensions
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The main function

• A C program must have a main function of some
  form, to act as an entry point.
• N.B. This is a function and not a method.
• In Java everything is a class not so in C.
• main is a function like any other function, and
  acts in exactly the same way

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Arguments to main()

• Just as other functions can take arguments, so
  can your main function.
• Generally you pass information into the main via
  command line arguments, that follow the programs
  name when you are executing it.
• There are two special built in arguments argv
  and argc, that are used to receive information
  from the command line.
• argc holds the number of arguments entered
• argv is an array of these arguments themselves

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Example of command line info

• int main (int argc, char argv)
• if (argc!2)
• cout
• cout

check on the number of arguments
statement that ouputs the info to the screen.
example of use if we had compiled the code as
myProgram.
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Return Statement

• The return statement can be used to leave a
  function at any point
• It is essentially a jump statement (like break
  and continue) because it jumps back to the point
  in your program where the function was called.
• A return statement may or may not have a value
  associated with it.
• However if your definition indicated that it
  would send back a value, any return statement
  must send back a value of that type.

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multiple returns

• a function may contain several return statements.
  For example, here is some code for a function
  that checks the sign of any integer passed into
  it

int completeDys(int number) if (numberreturn 1 if (number0) return 1 return 0
If true returns to the main program before
reaching the next statement
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So what does main() return?

• You dont have to use a return statement the
  block of code will return itself when it hits its
  final curly bracket.
• When you use the command in the main function it
  returns an integer to the calling process, which
  is generally the operating system.
• If you dont send a return from main() what
  happens is technically undefined. Better safe
  than sorry and always put it in.
• The Exit(0) statement is very similar to return.

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Function Prototypes

• Until now, we have defined the functions before
  the first appearance of a call to it, leaving the
  function main for the end.
• If you try to repeat some of the examples of
  functions described until now but placing the
  function main before any other function that is
  called from within it, you will most likely
  obtain an error.
• The reason is that to be able to call to a
  function it must have been declared previously -
  it must be known.

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Prototyping functions

• We can avoid this by prototyping functions.
• This consists of making a previous shorter
  declaration of the complete definition, so that
  the compiler can know the arguments and the
  return type needed.
• returnType functionName (parameter types)

ends with a semicolon
It is enough to list the parameter types alone if
you wish
Almost identical to a function header, except it
has no code body and
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Example of Prototyping

• include
• void odd (int a)
• void even (int a)
• int main ()
• int i
• cout
• cin i
• odd (i)
• void odd (int a)
• if ((a2)!0) cout
• else even (a)

prototypes
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Recursion

• Functions can, and often do, call other
  functions.
• Recursivity is the property that functions have
  to be called by themselves.
• Recursion is the process of defining something in
  terms of itself, sometimes called circular
  definition.
• Recursion is useful for all sorts of tasks
  examples are certain sorting methods or to
  calculate the factorial of a number.

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Example Factorial Calculator

• include
• long factorial (long a)
• if (a 1) return (a factorial (a-1))
• else return (1)
• int main ()
• long l
• cout
• cin l
• cout
• return 0

A Recursive function, calling itself, but
stopping when a reaches 1
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Being recursive

• Most recursive routines dont significantly
  reduce code size or improve memory utilisation.
• Recursive routines may also run slower because of
  the repeated function calls, or could eventually
  cause the stack to potentially overrun.
• However recursive solutions can be very elegant
  and they lend themselves to certain problems
  especially well (in AI for example).
• They also tend to make you feel pretty damn
  clever.

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Exercise of the day

• Rewrite the step in your 3N1 program using the ?
  operator.
• Incorporate the 3N1 algorithm into a function
  T(t,n,s), accepting a table t, a dimension n and
  a seed s as parameters. The function should fill
  t with the first n numbers in the 3N1 row
  starting from s and return the number of steps
  until a loop is detected.
• Put the table t and the dimension n into a
  structure and use it as a parameter for the
  function. Should it be passed by value or by
  reference? Experiment with both.

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C Extras

• There are quite a few difference in what a
  function in C is capable of in comparison to
  functions in C.
• Function Overloading
• Default Parameters
• Class Member Functions
• Inline Functions
• Well look at these next lecture