2nd Midterm Exam December 26th, 2008, Friday

17:40 – 19:20 Places:

FENS G032    Acar - Avcı FENS G035    Aydeniz - Büyükkıdık FASS G022    Büyüknacar - Demir FASS G049    Demirci - Eroğlu FASS G052    Erol - Gürsel FENS L045    Gürtunca - Konukoğlu 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 won’t 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.

Extra Recitations for Review Purposes

by Tolga ErenSeveral recitations will be held until MT2First Meeting to determine the coverage and time

Wednesday December 17, 19:40 in FASS G022Exact times will be determined in this first meetingActually this is done and two recitations were held

previous weekRemaining ones

December 23, Tuesday, 19:40 – 22:30 (may last longer) in FENS G035

December 25, Thursday, 8:40 – 10:30 in FENS L045

Announcements about HW6You 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), orthat 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 homeworkAttempting to read from a file that has not been opened yetAttempting to write to a file that has not been opened yetRange and index problems while trying to access characters of a

string using find, substr and at member functions.

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.

7th and last homework

7th and last homework will be assigned this weekDue Jan 7 Wednesday, 19:00About vectorsThis week recitations will be about vectors and this

homework

structs (7.4)Used as data aggregates for an entity

can be different types of datae.g. for student

id, name, GPA, address, ...

Similar to classes, but everything is publicstructs can have constructorsstructs can have member functionswe will not deal with constructors and member

functions for structs unless they are necessarymostly we will use structs for combining data for an

entity into a single structure

StructsExample: struct for studentFirst 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 structstring 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 << stu.gpa;

See structdemo.cpp (not in book)

What can and can’t be done with structsStructs 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 changingstruct fields behave as variables/objects of field type

id is an integername is a stringYou may read, write, use as operands in operations, etc.

However, processing the entire struct variable is restrictive

cannot read or write (using >> and <<) structs unless those operators are specially defined for that struct

cannot use operators between two structs unless those operators are specially defined for that structsee 7.4.2 for operator definitions for structs, but not responsible

structs are useful mostly in vectors (arrays)

Vectors and ArraysArrays are collections of several elements of the

same typeE.g. 100 integers, 20 strings, 125 students, 12 dates, etc.Single name is given to the entire arrayBut 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 typeWe will see it, but later

Vectors are a class-based version of arraysFirst we will see vectors.

VectorsWe’re 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.hIf 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

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 counters7 declarations7 initializations7 conditions to increment after each occurrence7 cout statements to display the result

Fortunately, we have shorter way: ARRAYS/VECTORSWe can use vectors to store counters for all possible

outcomes of the random numbers under a single nameeasier processing in loopssee next slide for the program

ExamplePrevious example using vectors - see randnums.cpp

int num;int k;RandGen random;tvector<int> randStats(7); // vector for countersint n = PromptRange("how many random numbers",1,20000); for(k=0; k <= 6; k++) // initialize counters to zero{ randStats[k] = 0;} for(k=0; k < n; k++) // pick all random numbers{ num = random.RandInt(7); // between 0 and 6 randStats[num] = randStats[num] + 1; // and increment // corresponding counter} cout << "number\t\t# of occurrences" << endl;for(k=0; k <= 6; k++){ cout << k << "\t\t" << randStats[k] << endl;}

0 1 2 3 4 5 6

randStats

Vector/Array basicsVectors/Arrays are homogeneous

each item (sometimes called element) has the same typethat type must be specified at declaration

Items in a vector/array are numbered (e.g. 1st, 3rd, or 105th)those are called index or subscriptnumbering starts with 0we 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)tvector<int> ivals(10); // ivals can store 10 intsivals[0] = 3; // 0th element becomes 3tvector<string> svals(20); // svals can store 20 stringssvals[4] = "cs201"; // 4th element contains "cs201"

Vector basics Syntax of vector declaration

tvector is a class, its declaration is construction 3 different methods

tvector<type> variable_name; empty vector (will see later)

tvector<type> variable_name (size_expression); vector with size_expression elements in it

tvector<type> variable_name (size_expression, init_value);

vector with all size_expression elements initialized to init_value

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 <int> letters (int('Z')-int('A') + 1); creates a vector of 26 integer elements and name it letterscin >> num;tvector <double> 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

Defining tvector objectsCan specify # elements in a vector, optionally an initial value tvector<int> counts(300); //300 ints, values not initializedtvector<int> nums(200, 0); // 200 ints, all zerotvector<double> d(10, 3.14); // 10 doubles, all pitvector<string> w(10, "cs"); // 10 strings, all "cs"tvector<string> words(10); // 10 strings, all ""

If the vector type is a class, then this class must have a default constructorDefault constructor is the one without parametersCannot define tvector<Dice> cubes(10); since Dice doesn’t

have default constructor

Vectors of classes are initialized with the default constructorthat 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)

Example tvector definitions tvector<int> counter(9, 0);each element is an integer

(all initialized to 0)0 1 2 3 4 5 6 7 8

counter

0 1 2 3 4 5 6 7 8

letters

9 10 11 12 13 14 15 16 17

tvector<char> letters(18);each element is a char (not initialized yet)

0 1 2 3 4 5

holidaystvector<Date> holidays(6);each element is a date object

that contains todays date

17122007

17122007

17122007

17122007

17122007

17122007

0 0 0 0 0 0 0 0 0

How to reach a single vector/array elementspecify the index value within square brackets after the

vector/array namevar_name [index_expr]the value of index expression must be between 0 and (vector size – 1)

Examplestvector<int> nums(9);

nums[5] = 102;nums[0] = nums[5]*2-1;nums[nums[5]/20-3] = 55;

nums[10] = 5; // error

0 1 2 3 4 5 6 7 8

nums

102203 55

Passing vectors to functions as parametersVectors can be passed as parameters to functions

Pass by reference (if function changes the vector)void Count (tvector<int> & counts);

Pass by const-reference (if no changes made). void Print(const tvector<int> & 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 knowntvector has a member function, size, to return the size of a vector

ExampleCounting letters of a file

display number of occurrences of each letter at the end

counting is case insensitivesee letters.cpp (the one in book is a bit different)

tvector as a return typeVector can be return type of a function

tvector<int> Count (istream & input, int & total);

Example: modify letters.cpp such that count returns the vector (not as reference parameter)see letters2.cpp

Vectors of structsWe can define vectors of structs

struct student{unsigned int id; string name, lastname;double gpa;

};

tvector<student> class(11); // a vector with 11 students

class[1].gpa = 3.2;

for (i = 0; i <= 10; i++) class[i].id = i + 1250;

1250

id gpa

name

lastname

1251

id3.2

gpa

name

lastname

1260

id gpa

name

lastname

0

1

10

Vector of structExample

define a struct for a track on a CDtrack number and title are fields

define a vector for 10 tracksshuffle these 10 tracks at randomsee shuffle.cpp (in book, but this version is slightly

modified)

Vectors as listsThe “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 elementsConsider reading words from a text file, storing them

in a vectorHow big should we define vector initially? What are

potential problems?When a vector is used as a list, we’ll use a different

method for adding elements to the vector so that the vector can “grow”

Reading words into a vector (problematic version) tvector<string> words(1000); string w; int i = 0; string filename = PromptString("enter file name: "); ifstream input(filename.c_str());

while (input >> w) { words[i]=w;

i++; } cout << "read " << i << " words" << endl;

What is the problem?there might be more than 1000 words in the filein this case index runs out of range

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

tvector<string> words(1000); string w; int i = 0; string filename = PromptString("enter file name: "); ifstream input(filename.c_str());

while ((input >> w) && (i < 1000)) { words[i]=w;

i++; } cout << "read " << i << " words" << endl;

What is the problem?works fine if there are no more than 1000 wordsbut if there are more than 1000 words, the rest is not read

Reading words into a vector (no problems)One method would be to pass over the file two times

one to find out number of wordssecond to read the words into array

Another method is to benefit from tvector class utilities as in the following code

tvector<string> words; //create empty vector string w; string filename = PromptString("enter file name: "); ifstream input(filename.c_str());

while (input >> w) { words.push_back(w); //adds the next word to the vector //also increases the size if necessary

} cout << "read " << words.size() << " words" << endl;

Using tvector::push_backThe method push_back adds new objects to the “end” of

a vector,Internally, the vector keeps track of its capacityIf there is capacity, then there is no problem; the new item is added

to the end of the vectorWhen 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 sizeempty vector (zero size)

Size versus CapacityCapacity is the allocated size of the vectorSize is how many elements are in the vector so farThey are not the same concepts, but related as

described in the previous slide and illustrated below

tvector<string> 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

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 vectorThis is the number of calls of push_back() if no elements are deletedIf 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 onereturns the size specified during declaration if no push_back() is

usedreturns the size specified during declaration + the number push_back()s, if push_back() is used

capacity()and reserve()The capacity of vector is accessible using capacity() member functionprogrammers don’t often need this value

An initial capacity of N elements can be specified using reserve(N) member function

Demo ExampleRead some strings from keyboard and store in a

tvector of strings. At the end display the vector.version 1: no reserveversion 2: (decomment the reserve lines): with

reserveversion 3: vector is not created empty (decomment

second definition and comment out first one and reserve lines)

See tvectordemo.cpp (not in the book)

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 vector’s size - 1

int max (const tvector<int> & v)//pre: vector v is not empty//post: return max of elements in v{

int i, max_so_far = INT_MIN;

for (i=0; i < v.size(); i++){

if (v[i] > max_so_far){

max_so_far = v[i];}

}return max_so_far;

}

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 tvector<int> & 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 assumptionint i =1;

while (i < v.size() && s == true) { //check until the end of array or until a counterexample is found

if (v[i-1] > v[i]) // if not sorted s = false; // counterexample is foundi++;

}return s;

}

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”

Counting searchint countmatches(const tvector<string> & a, const string& s)// post: returns # occurrences of s in a{ int count = 0; int k; for(k=0; k < a.size(); k++) { if (a[k] == s) { count++; } } return count;}

How can we change this code to return the index of the first occurrence? see next slide

One occurrence searchint firstmatch(const tvector<string> & a, const string& s)// post: returns the index of occurrence of s in a, -1 // otherwise{ int k; for(k=0; k < a.size(); k++) { if (a[k] == 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?

Collecting searchCollect the occurrences in another vector

void collect(const tvector<string> & a, tvector<string> & matches)// pre: matches is empty// post: matches contains all elements of a with// first letter 'A'{ int k; for(k=0; k < a.size(); k++) { if (a[k].substr(0,1) == "A") { matches.push_back(a[k]); } }}

Binary searchAlternative to sequential search for sorted vectorsIf a vector is sorted we can use the sorted property to

eliminate half of the vector elements with one comparisonWhat number (between 1 and 100) do we guess first in

number guessing game?

Idea of creating program to do binary searchCheck the middle element

If it has the searched value, then you’re done! If not, eliminate half of the elements of the vector

search the rest using the same ideacontinue until match is found or there is no match

how could you understand that there is no match?let’s develop the algorithm on an example

we need two index values, low and high, for the search space

Binary Search (search for 62)

10 24 34 52 55 62 67 75 80 81 90 92 100 101 111

low=0 mid=7 high=14

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

low = 0 mid=3 high=6

low=4 high=6

mid=5 => FOUND

Binary Search (search for 60)

10 24 34 52 55 62 67 75 80 81 90 92 100 101 111

low = 0 mid=7 high =14

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

low=0 mid=3 high=6

low=4 high=6mid=5

low=4 high=4mid=4

low=5 high=4 => NO MATCH FOUND – STOP

Binary search codeint bsearch(const tvector<string>& 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 <= high) { mid = (low + high)/2; if (list[mid] == key) // found key, exit search { return mid; } else if (list[mid] < key) // key in upper half { low = mid + 1; } else // key in lower half { high = mid - 1; } } return -1; // not in list}

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)