1. Announcements HW6 due this Wednesday
Output of Sample Run 1:
Konya Turkey
Tuzla Turkey
Istanbul Turkey
Name of output file in Sample Run 3:
2000_2002_Turkey.txt
HW7 will be assigned this week and will be due January 6th Wednesday (last week)
You may see your Midterm2 papers on Thursday, 12:30 – 14:30 in FENS L068 (Office Hour Room).
Please complete the Course Evaluations
BAGEM should have sent a message about this

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. 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)

5. tvector as a return type
Vector 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

6. Vectors of structs We can define vectors of structs struct student {
1250 id
gpa
name
lastname
We 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 ;
1251 id
3.2
gpa
name
lastname 1
1260 id
gpa
name
lastname 10

7. 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)

8. 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, we’ll use a different method for adding elements to the vector so that the vector can “grow”

9. 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 file
in this case index runs out of range

10. 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 words
but if there are more than 1000 words, the rest is not read

11. 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
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;

12. Using tvector::push_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)

13. 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
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

14. 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

15. capacity()and reserve()
The capacity of vector is accessible using capacity() member function
programmers don’t often need this value
An initial capacity of N elements can be specified using reserve(N) member function

16. 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)

17. 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;

18. 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 assumption
int 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 found
i++; }
return s;

19. 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”

20. Counting search
int 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

21. One occurrence search
int 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?

22. Collecting search Collect 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]); }

23. 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 you’re 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?
let’s develop the algorithm on an example
we need two index values, low and high, for the search space

24. Binary Search (search for 62)
low= mid= high=14
low = 0 mid= high=6
low=4 high=6
mid=5 => FOUND

25. Binary Search (search for 60)
low = mid= high =14
low= mid= high=6
low=4 high=6
mid=5
low=4 high=4
mid=4
low=5 high=4 => NO MATCH FOUND – STOP

26. Binary search code
int 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

27. 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)