2nd Midterm Exam

1
2nd Midterm Exam

• December 26th, 2008, Friday
• 1740 1920
• Places
• FENS G032    Acar - Avci FENS G035   
  Aydeniz - Büyükkidik FASS G022    Büyüknacar -
  Demir FASS G049    Demirci - Eroglu FASS
  G052    Erol - Gürsel FENS L045    Gürtunca -
  Konukoglu FASS G062    Korudu - Pek FENS
  G077    Pelen - Zilan
• Sample question set and solutions are posted
• Close everything except two A4-size cheat notes
• In the final exam, two sheet limit wont change!
• Exam covers the topics until the beginning of
  structs and vectors!
• Details are sent to SUCourse and as an email to
  the class email list.

2
Extra Recitations for Review Purposes

• by Tolga Eren
• Several recitations will be held until MT2
• First Meeting to determine the coverage and time
• Wednesday December 17, 1940 in FASS G022
• Exact times will be determined in this first
  meeting
• Actually this is done and two recitations were
  held previous week
• Remaining ones
• December 23, Tuesday, 1940 2230 (may last
  longer) in FENS G035
• December 25, Thursday, 840 1030 in FENS L045

3
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.

4
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.

5
7th and last homework

• 7th and last homework will be assigned this week
• Due Jan 7 Wednesday, 1900
• About vectors
• This week recitations will be about vectors and
  this homework

6
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

7
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)

8
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)

9
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.

10
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

11
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

12
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

13
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"

14
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

15
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

16
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)

17
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
18
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
19
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

20
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)

21
tvector as a return type

• Vector can be return type of a function
• tvectorltintgt Count (istream input, int
  total)
• Example modify letters.cpp such that count
  returns the vector (not as reference parameter)
• see letters2.cpp

22
Vectors of structs

• We can define vectors of structs
• struct student
• unsigned int id
• string name, lastname
• double gpa
• tvectorltstudentgt class(11)
• // a vector with 11 students
• class1.gpa 3.2
• for (i 0 i lt 10 i)
• classi.id i 1250

0
1
10
23
Vector of struct

• Example
• define a struct for a track on a CD
• track number and title are fields
• define a vector for 10 tracks
• shuffle these 10 tracks at random
• see shuffle.cpp (in book, but this version is
  slightly modified)

24
Vectors as lists

• The vector as counters example constructs and
  initializes a vector with a specific number of
  elements
• Other uses of vector require the vector to grow
  to accommodate new elements
• Consider reading words from a text file, storing
  them in a vector
• How big should we define vector initially? What
  are potential problems?
• When a vector is used as a list, well use a
  different method for adding elements to the
  vector so that the vector can grow

25
Reading words into a vector (problematic version)

• tvectorltstringgt words(1000)
• string w
• int i 0
• string filename PromptString("enter file name
  ")
• ifstream input(filename.c_str())
• while (input gtgt w)
• wordsiw
• i
• cout ltlt "read " ltlt i ltlt " words" ltlt endl
• What is the problem?
• there might be more than 1000 words in the file
• in this case index runs out of range

26
Reading words into a vector (with index range
control but still problematic)

• tvectorltstringgt words(1000)
• string w
• int i 0
• string filename PromptString("enter file name
  ")
• ifstream input(filename.c_str())
• while ((input gtgt w) (i lt 1000))
• wordsiw
• i
• cout ltlt "read " ltlt i ltlt " words" ltlt endl
• What is the problem?
• works fine if there are no more than 1000 words
• but if there are more than 1000 words, the rest
  is not read

27
Reading words into a vector (no problems)

• One method would be to pass over the file two
  times
• one to find out number of words
• second to read the words into array
• Another method is to benefit from tvector class
  utilities as in the following code
• tvectorltstringgt words //create empty vector
• string w
• string filename PromptString("enter file name
  ")
• ifstream input(filename.c_str())
• while (input gtgt w)
• words.push_back(w) //adds the next word to the
  vector
• //also increases the size if
  necessary
• cout ltlt "read " ltlt words.size() ltlt " words" ltlt
  endl

28
Using tvectorpush_back

• The method push_back adds new objects to the
  end of a vector,
• Internally, the vector keeps track of its
  capacity
• If there is capacity, then there is no problem
  the new item is added to the end of the vector
• When the capacity is reached and push_back
  attempts to add a new element to the vector, then
  the vector automatically grows by doubling the
  capacity
• 0, 2, 4, 8, 16, 32, ...
• If you want to use push_back mechanism, then the
  vector should be defined initially without
  specifying a size
• empty vector (zero size)

29
Size versus Capacity

• Capacity is the allocated size of the vector
• Size is how many elements are in the vector so
  far
• They are not the same concepts, but related as
  described in the previous slide and illustrated
  below
• tvectorltstringgt names // size is 0, capacity
  is 0
• names.push_back("Ali") // size is 1, capacity
  is 2
• names.push_back("Husnu") // size is 2, capacity
  is 2
• names.push_back("Ayse") // size is 3, capacity
  is 4
• names.push_back("Cem") // size is 4, capacity
  is 4
• names.push_back("Jale") // size is 5, capacity
  is 8

30
size()member function

• size() member function basically returns the
  number of elements in the vector
• When a vector is defined with no initial
  capacity, and push_back is used to add elements,
  size() member function returns the number of
  elements exist in the vector
• This is the number of calls of push_back() if no
  elements are deleted
• If elements deleted using pop_back(), size
  updated too (decremented)
• If a non-empty vector is created, then the
  capacity and the size is set to the number of
  elements of the vector. This capacity is
  considered full, so the first push_back causes to
  double the capacity.
• What about size() in case the vector is created
  as a non-empty one
• returns the size specified during declaration if
  no push_back() is used
• returns the size specified during declaration
  the number push_back()s, if push_back() is used

31
capacity()and reserve()

• The capacity of vector is accessible using
  capacity() member function
• programmers dont often need this value
• An initial capacity of N elements can be
  specified using reserve(N) member function

32
Demo Example

• Read some strings from keyboard and store in a
  tvector of strings. At the end display the
  vector.
• version 1 no reserve
• version 2 (decomment the reserve lines) with
  reserve
• version 3 vector is not created empty (decomment
  second definition and comment out first one and
  reserve lines)
• See tvectordemo.cpp (not in the book)

33
Vector Processing Examples 1 (vectorproc.cpp
not in book)

• write a function that takes a tvector of integers
  as parameter and returns the maximum of numbers
  in it
• process all array elements for loop from 0 to
  vectors size - 1

int max (const tvectorltintgt v) //pre vector v
is not empty //post return max of elements in
v int i, max_so_far INT_MIN for (i0 i lt
v.size() i) if (vi gt max_so_far)
max_so_far vi return max_so_far
34
Vector Processing Examples 2 (vectorproc.cpp
not in book)

• Write a function that takes a tvector of integers
  as parameter and returns true if the vector is
  sorted in ascending manner, false otherwise
• may not process all vector elements
• In this type of rule-checking applications, a
  possible method is to assume that the rule is
  satisfied before the loop and find a
  counterexample in the loop

bool issorted (const tvectorltintgt v) //post
returns true if the array is acsending sorted
bool s true // initially assume that
array is sorted //in the function
try to break this assumption int i 1 while
(i lt v.size() s true) //check
until the end of array or until a counterexample
is found if (vi-1 gt vi) // if not
sorted s false //
counterexample is found i return s
35
Searching a vector

• We can search for one occurrence, return
  true/false or the index of occurrence
• Search the vector starting from the beginning
• Stop searching when match is found
• We can search and count the number of occurrences
  and return count
• Search entire vector
• Similar to one occurrence search, but do not stop
  after first occurrence
• We can search for many occurrences, but return
  occurrences in another vector rather than count
• In all these cases, we search the vector
  sequentially starting from the beginning
• This type of search is called sequential search

36
Counting search

• int countmatches(const tvectorltstringgt a, const
  string s)
• // post returns occurrences of s in a
• int count 0
• int k
• for(k0 k lt a.size() k)
• if (ak s)
• count
• return count
• How can we change this code to return the index
  of the first occurrence?
• see next slide

37
One occurrence search

• int firstmatch(const tvectorltstringgt a, const
  string s)
• // post returns the index of occurrence of s in
  a, -1
• // otherwise
• int k
• for(k0 k lt a.size() k)
• if (ak s)
• return k
• return -1
• Does not search the entire array if one match is
  found
• good for efficiency purposes
• How could you modify this to return true/false?

38
Collecting search

• Collect the occurrences in another vector
• void collect(const tvectorltstringgt a,
• tvectorltstringgt matches)
• // pre matches is empty
• // post matches contains all elements of a with
• // first letter 'A'
• int k
• for(k0 k lt a.size() k)
• if (ak.substr(0,1) "A")
• matches.push_back(ak)

39
Binary search

• Alternative to sequential search for sorted
  vectors
• If a vector is sorted we can use the sorted
  property to eliminate half of the vector elements
  with one comparison
• What number (between 1 and 100) do we guess first
  in number guessing game?
• Idea of creating program to do binary search
• Check the middle element
• If it has the searched value, then youre done!
• If not, eliminate half of the elements of the
  vector
• search the rest using the same idea
• continue until match is found or there is no
  match
• how could you understand that there is no match?
• lets develop the algorithm on an example
• we need two index values, low and high, for the
  search space

40
Binary Search (search for 62)
0 1 2 3 4 5 6 7 8 9 10 11 12
13 14
10 24 34 52 55 62 67 75 80 81
90 92 100 101 111
low0 mid7
high14
low 0 mid3 high6
low4 high6
mid5 gt FOUND
41
Binary Search (search for 60)
0 1 2 3 4 5 6 7 8 9 10 11 12
13 14
10 24 34 52 55 62 67 75 80 81
90 92 100 101 111
low 0 mid7
high 14
low0 mid3 high6
low4 high6
mid5
low4 high4
mid4
low5 high4 gt NO MATCH FOUND STOP
42
Binary search code

• int bsearch(const tvectorltstringgt list, const
  string key)
• // pre list.size() elements in list
• // post returns index of key in list, -1 if key
  not found
• int low 0 // leftmost
  possible entry
• int high list.size()-1 // rightmost
  possible entry
• int mid // middle of
  current range
• while (low lt high)
• mid (low high)/2
• if (listmid key) // found key,
  exit search
• return mid
• else if (listmid lt key) // key in
  upper half
• low mid 1
• else // key in
  lower half
• high mid - 1

43
Comparing Sequential and Binary Search

• Given a list of N elements
• Binary search makes on the order of log N
  operation
• O(log N)
• Linear (sequential) search takes on the order of
  N operations
• O(N)