More Arrays

1
More Arrays

• Arrays and classes
• Multi-dimensional Arrays
• Dynamic arrays

2
Arrays and classes

• You can have arrays of objects
• element class - use array of elements to store
  data needed for amino acid program
• Money class - use an array of Money objects to
  store all the prices for a customer
• You can have arrays inside classes
• a Molecule class could have an array of all the
  elements in the molecule
• an Inventory class could have an array of all the
  items stocked by a store

3
Arrays vs classes

• Sometimes you can accomplish the same thing
  easier with classes
• nX3 array of numbers to represent a series of
  cartesian points
• 1-D array of 3D_point objects
• Parallel arrays vs array of objects
• arrays of objects easier to keep data todgether

4
4/15/02

• Exam 3

5
Why multiple dimensions

• Sometimes it is useful to have arrays of more
  than one dimension.
• a game program that used a checkerboard
  represent the squares as elememts of a 2D array
• array of words, you could use a 2-D array of char

6
Multi-D arrays

• Think of 2-D arrays as arrays of a particular
  kind of 1-D array.
• The first index tells you how many of these 1-D
  arrays you are declaring,
• the second index gives the the size these 1-D
  arrays.
• 3-D arrays are arrays of 2_D arrays, etc.
• more than two or three dimensions not common.

7
Declaring 2-D arrays

• similar to one dimensional arrays.
• int matrix86
• declares an 8 by 6 array of ints, i.e. an array
  of 8 6-element arrays
• char wordList1020
• declares an array of 10 cstrings, each of which
  can have 19 characters

8
Declaring 2 Dimensional Arrays

• Declaration
• char ticTacToe 33
• Storage location
• ticTacToe 12
• Name Row Column

9
Example

• const int Num_Rows 11
• const int Seats_In_Row 9
• string seatPlan Num_RowsSeats_In_Row

10
Initialization

• const int Num_Rows 2
• const int Num_Cols 3
• float matrixNum_RowsNum_Cols 5.0, 4.5,
  3.0, -16.0, -5.9, 0.0
• Nested loops with two-dimensional arrays
• SumMatrx.cpp example

11
Bigger Arrays

• const int people 10
• const int years 5
• money salespeopleyears12

12
Accessing elements

• access single elements with indices just the same
  way as you do those of 1-D arrays,
• matrix23
• wordList15
• access a single "row" with a single index
• matrix2
• wordList1

13
4/17/02
14
Dynamic Arrays

• Arrays we have looked at so far are static - size
  is fixed
• sometimes you don't know how many elements you
  will need
• To understand dynamic arrays, we need to
  understand pointers

15
Pointer Variables

• int i
• declares an int variable and sets aside a named
  memory location to store the int
• int iptr
• declares a variable that holds the address of an
  int memory is allocated for the address
  (pointer) but not the int

16
Pointing to an existing int

• int i 25
• Allocates memory for an int and stores the valkue
  25
• int iptr
• Allocates memory for a pointer to an int
• iptr i
• sets the value of iptr to the address of i
• i and iptr are the same
• changing either one will change the other

17
Creating a new int

• int j 15
• Allocates momory for an int and stores the value
  15
• int jptr new int
• sets aside memory for an int and puts the
  address int j
• j j
• stores value of j in memory pointed to by jptr

18
Pointers and Dynamic arrays

• A dynamic array is declared as a pointer
• int iArray
• use new to allocate appropriate amount of memory
• iArray new int desiredSize
• use iArray just like you use any array
• iArrayindex someValue

19
Changing the size

• Allocate a new block of memory with new
• int temp new intnewSize
• Copy the elements in the current memory
• for (int j0 jltcurrentSize j)
• tempj iArrayi
• delete the old memory
• delete iArray
• reassign the pointer
• iarray temp

20
4/19

• Design Exercise
• dynamic array class (section 1)
• command line arguments
• getline
• more on pointers

21
Command line arguments

• g -o prog prog.cpp

Your programs can do this too
22
use a different prototype for main

• int main( int argc, char argv)
• ...
• argc is the number of "words" in the command line
  (space delimited) including the name of the
  program
• argv is a 2-D array of char
• uses pointer notation

23
reading text

• Think of a filestream as a sequence of characters
• When you open the stream, the file pointer is at
  the beginning

24
reading with gtgt

• cin gtgt word gt word now
• cin gtgt word gt word is

25
reading with getline

• getline( cin, line) gt line " the time "
• getline( cin, line) gt
• line "for all good men"

26
Using operatorgtgt and getline

• Behavior is basically the same for C-style
  strings (char arrays) and string objects
• there are two overloaded versions of getline
• void istreamgetline( char, int)
• void getline(istream , string)

27
string Class

• used to store character strings
• include ltstringgt
• string text
• text "a literal string"
• cin gtgt text // gets one word of text
• getline( cin, text) // gets to next newline

28
reading C-style strings

• char a10
• cin gtgt a
• whitespace delimited
• to read multiple words, use getline
• cin.getline( a, 10)
• the number is the number of elements in the array

29
getline with delimiters

• You may sometimes have data separated by other
  characters than the newline
• spreadsheet and database programs will usually
  print out tab or comma separated text
• two versions of getline allow you to specify the
  delimiter
• void istreamgetline( char, int, char)
• void getline(istream , string, char)

30
char input

• reading into a char with gtgt skips whitespaces
• if you need to get the space characters
• istreamget( ch)

31
Pointers and the new Operator

• Pointer Declarations
• pointer variable of type pointer to double
• can store the address of a double t in p
• double p
• The new operator creates a variable of type
  double puts the address of the variable in
  pointer p
• p new double
• Dynamic allocation - memory is allocated while
  the program is running instead of before it
  starts

32
Pointers

• Actual address has no meaning
• Form type variable
• Example double p

P
?
33
new Operator

• Actually allocates storage
• Form new type // one memory location
• new type n // n memory locations
• Example new double

34
Pointer Variables

• If they aren't used for arrays, you have to use
  them differently

35
Accessing Data with Pointers

• - indirection operator
• p 15.5
• Stores floating value 15.5 in memory location p
  - the location pointed to by p

p
15.5
36
Pointer Statements

• double p
• p new doublet
• p 15.5
• cout ltlt The contents of the memory cell pointed
  to by p is ltlt p ltlt endl
• Output
• The contents of memory cell pointed to by p is
  15.5

37
Pointer Operations

• Pointers can only contain addresses
• So the following are errors
• p 1000
• p 15.5
• You need to assign an address to p
• p varOfAppropriateType

38
Pointer Operations

• Assignment of pointers if q p are the same
  pointer type
• q p
• p and q both refer to the same memeory location -
  tha same variable
• relational operations and ! compare
  addresses not the values stored at those addresses

39
Pointers to Objects

• class electric
• public
• string current
• int volts
• electric p, q
• p and q are pointers to objects of type electric

40
Pointers to Structs

• p new electric
• Allocates storage for struct of type electric and
  places address into pointer p

current
volts
p
?
?
41
Assignments

• p.current AC
• p.volts 115
• Statements above can also be written
• p -gtcurrent AC
• p -gtvolts 115

current
volts
p
AC
115
42
Struct Member Access via Pointers

• From p -gtm
• Example p -gtvolts
• Example
• cout ltlt p-gtcurrent ltlt p-gtvolts ltlt endl
• Output
• AC115

43
Pointers to Objects

• q new electric
• Allocates storage for object of type electric and
  places address into pointer q
• Copy contents of p struct to q struct
• q p

q-gtcurrent
q-gtvolts
q
AC
115
44
Pointers to Objects

• q-gtvolts 220
• q p

q-gtcurrent
q-gtvolts
q
AC
220
p-gtcurrent
p-gtvolts
p
q-gtcurrent
q-gtvolts
115
AC
q
AC
220
45
13.2 Manipulating the Heap

• When new executes where is struct stored ?
• Heap
• C storage pool available to new operator
• Effect of p new node
• Figure 14.1 shows Heap before and after executing
  new operator

46
Effect on new on the Heap
47
Returning Cells to the Heap

• Operation
• delete p
• Returns cells back to heap for re-use
• When finished with a pointer delete it
• Watch dual assignments and initialization
• Form delete variable
• Example delete p