Extra Recitations

1
Extra Recitations

• Wednesday 1940-2230 FENS L055 (tomorrow!)
• Friday 1340-1630  FENS L063
• Friday   1740-2030  FENS L045
• Friday 1940-2230  FENS G032

2
Announcements about HW6

• You may need to use clear() member function
  before you try to reopen an input file stream
  object
• that you failed to open previously (for example
  due to wrong file name), or
• that you opened and processed but for some reason
  if the error flags are set (for example due to
  reaching the end of the file).
• Possible reasons for run time errors in this
  homework
• Attempting to read from a file that has not been
  opened yet
• Attempting to write to a file that has not been
  opened yet
• Range and index problems while trying to access
  characters of a string using find, substr and at
  member functions.

3
Announcements about HW6

• Should we check if the output file is opened
  successfully or not?
• Not required, but advised
• There might be some cases that the output files
  are not opened successfully
• If you check and the output file is not opened,
  then do not continue with the program.
• What happens if the files are opened but the
  content is irrelevant?
• What happens if the file names are entered in the
  wrong order?
• What happens if both file names are the same?
• All of these questions are reduced into the same
  main question should we make the content check
  for the files that are opened successfully?
• NO. As mentioned in the HW document, the content
  of the files are assumed to be correct. What you
  have to do is only to check if the files are
  opened successfully or not and continue to read
  file names until opened. Once opened, we assume
  that the first file is the business database, and
  the second one is the distance database files.

4
structs (7.4)

• Used as data aggregates for an entity
• can be different types of data
• e.g. for student
• id, name, GPA, address, ...
• Similar to classes, but everything is public
• structs can have constructors
• structs can have member functions
• we will not deal with constructors and member
  functions for structs unless they are necessary
• mostly we will use structs for combining data for
  an entity into a single structure

5
Structs

• Example struct for student
• First struct must be defined by giving it a name
  and its fields (data)
• struct student // student is struct
  name
• unsigned int id // fields of student
  struct
• string name, lastname
• double gpa
• // dont forget at the
  end
• Then variables of that type are declared and
  used.
• dot operator is used to refer fields of a struct
  variable
• student stu
• stu.name "Ali"
• cout ltlt stu.gpa
• See structdemo.cpp (not in book)

6
What can and cant be done with structs

• Structs can be passed to functions as parameters
• use const-reference if not changing (using value
  parameter is syntactically OK, but not preferred
  due to performance reasons)
• use reference parameter if changing
• struct fields behave as variables/objects of
  field type
• id is an integer
• name is a string
• You may read, write, use as operands in
  operations, etc.
• However, processing the entire struct variable is
  restrictive
• cannot read or write (using gtgt and ltlt) structs
  unless those operators are specially defined for
  that struct
• cannot use operators between two structs unless
  those operators are specially defined for that
  struct
• see 7.4.2 for operator definitions for structs,
  but not responsible
• structs are useful mostly in vectors (arrays)

7
Vectors and Arrays

• Arrays are collections of several elements of the
  same type
• E.g. 100 integers, 20 strings, 125 students, 12
  dates, etc.
• Single name is given to the entire array
• But each element is accessed separately
• Any element of an array can be accessed just as
  quickly as any other element (this is called
  random access but do not get confused with
  RandGen type of randomness)
• In C/C there is a built-in array type
• We will see it, but later
• Vectors are a class-based version of arrays
• First we will see vectors.

8
Vectors

• Were using the class tvector
• Copy tvector.h and tvector.cpp to your project
  folder
• need include "tvector.h"
• But do NOT add tvector.cpp to your project
• It is already included within tvector.h
• If you mistakenly add tvector.cpp to your
  project, then you get tons of errors.
• tvector is a tapestry class that has the same
  functionality and based on the standard C class
  vector, but safer
• Safe means programming errors are caught rather
  than ignored

9
Why do we need arrays/vectors?

• Consider the following example (not in the book)
• pick n random numbers between 0 and 6 and count
  total number of occurrences of all outcomes (0,
  1, 2, 3, 4, 5, 6)
• n is an input
• we need 7 counters
• 7 declarations
• 7 initializations
• 7 conditions to increment after each occurrence
• 7 cout statements to display the result
• Fortunately, we have shorter way ARRAYS/VECTORS
• We can use vectors to store counters for all
  possible outcomes of the random numbers under a
  single name
• easier processing in loops
• see next slide for the program

10
Example

• Previous example using vectors - see randnums.cpp
• int num
• int k
• RandGen random
• tvectorltintgt randStats(7) // vector for
  counters
• int n PromptRange("how many random
  numbers",1,20000)
•  
• for(k0 k lt 6 k) // initialize
  counters to zero
• randStatsk 0
•  
• for(k0 k lt n k) // pick all random
  numbers
• num random.RandInt(7) // between 0 and 6
• randStatsnum randStatsnum 1 // and
  increment

11
Vector/Array basics

• Vectors/Arrays are homogeneous
• each item (sometimes called element) has the same
  type
• that type must be specified at declaration
• Items in a vector/array are numbered (e.g. 1st,
  3rd, or 105th)
• those are called index or subscript
• numbering starts with 0
• we have to use the index value to refer an
  element in a vector/array
• Example definition and use of vectors (array
  definition is a bit different)
• tvectorltintgt ivals(10) // ivals can store 10
  ints
• ivals0 3 // 0th element becomes
  3
• tvectorltstringgt svals(20) // svals can store 20
  strings
• svals4 "cs201" // 4th element
  contains "cs201"

12
Vector basics

• Syntax of vector declaration
• tvector is a class, its declaration is
  construction
• 3 different methods
• tvectorlttypegt variable_name
• empty vector (will see later)
• tvectorlttypegt variable_name (size_expression)
• vector with size_expression elements in it
• tvectorlttypegt variable_name (size_expression,
  init_value)
• vector with all size_expression elements
  initialized to init_value

13
Vector basics

• size_expression can be any expression of type
  integer (or cast into integer)
• not necessarily a constant value (this is
  actually a very important flexibility as compared
  to built-in arrays)
• examples
• tvector ltintgt letters (int('Z')-int('A') 1)
• creates a vector of 26 integer elements and name
  it letters
• cin gtgt num
• tvector ltdoublegt counters (num)
• creates a vector of doubles total number of
  elements is input
• Index value starts with 0 and ends with size-1
• type is type of the vector elements
• can be built-in types (int, double, ...) or user
  defined types or classes or structs (string and
  date are class examples student is struct
  example)
• classes must have default constructors to be used
  in vector definition as element type

14
Defining tvector objects

• Can specify elements in a vector, optionally an
  initial value
• tvectorltintgt counts(300) //300 ints, values not
  initialized
• tvectorltintgt nums(200, 0) // 200 ints, all
  zero
• tvectorltdoublegt d(10, 3.14) // 10 doubles, all
  pi
• tvectorltstringgt w(10, "cs") // 10 strings, all
  "cs"
• tvectorltstringgt words(10) // 10 strings, all
  ""
• If the vector type is a class, then this class
  must have a default constructor
• Default constructor is the one without parameters
• Cannot define tvectorltDicegt cubes(10) since Dice
  doesnt have default constructor
• Vectors of classes are initialized with the
  default constructor
• that is why all words are "" (empty string)
• Vectors with built-in types are not initialized
  (unless explicitly initialized with the second
  argument of tvector definition)

15
Example tvector definitions

• tvectorltintgt counter(9, 0)
• each element is an integer(all initialized to 0)

0 0 0 0 0 0 0 0 0
tvectorltchargt letters(18) each element is a char
(not initialized yet)
9 10 11 12 13 14 15 16 17
tvectorltDategt holidays(6) each element is a date
object that contains todays date
holidays
17 12 2007
17 12 2007
17 12 2007
17 12 2007
17 12 2007
17 12 2007
0 1 2 3 4 5
16
How to reach a single vector/array element

• specify the index value within square brackets
  after the vector/array name
• var_name index_expr
• the value of index expression must be between 0
  and (vector size 1)
• Examples
• tvectorltintgt nums(9)
• nums5 102
• nums0 nums52-1
• numsnums5/20-3 55
• nums10 5 // error

nums
203
55
102
17
Passing vectors to functions as parameters

• Vectors can be passed as parameters to functions
• Pass by reference (if function changes the
  vector)
• void Count (tvectorltintgt counts)
• Pass by const-reference (if no changes made).
• void Print(const tvectorltintgt counts)
• Passing by value makes a copy, requires time and
  space, so not preferred
• IMPORTANT!!! Vector size cannot be given in
  parameter definition. Three solutions to this
  problem
• the size may be passed as another parameter
• the size may be fixed and known
• tvector has a member function, size, to return
  the size of a vector

18
Example

• Counting letters of a file
• display number of occurrences of each letter at
  the end
• counting is case insensitive
• see letters.cpp (the one in book is a bit
  different)