What’s Left in the Course Arrays and vectors (Chapter 8) including searching and sorting (partially Chapter 11) Recursion, scope rules, global and static variables partially Chapter 10

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

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 << stu.gpa; See structdemo.cpp (not in book)

What can and can’t 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 >> 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 struct see 7.4.2 for operator definitions for structs, but not responsible structs are useful mostly in vectors (arrays)

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. We’re using the class vector need #include <vector> vector is the standard C++ class vector

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

Example Previous example using vectors - see randnums.cpp randStats int num; int k; RandGen random; vector<int> randStats(7); // vector for counters int 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 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) vector<int> ivals(10); // ivals can store 10 ints ivals[0] = 3; // 0th element becomes 3 vector<string> svals(20); // svals can store 20 strings svals[4] = "cs201"; // 4th element contains "cs201"

Vector basics Syntax of vector declaration vector is a class, its declaration is construction 3 different methods vector<type> variable_name; empty vector (will see later) vector<type> variable_name (size_expression); vector with size_expression elements in it vector<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: vector <int> letters (int('Z')-int('A') + 1); creates a vector of 26 integer elements and name it letters cin >> num; vector <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 vector objects Can specify # elements in a vector, optionally an initial value vector<int> counts(300); //300 ints, values not initialized vector<int> nums(200, -1); // 200 ints, all -1 vector<double> d(10, 3.14); // 10 doubles, all pi vector<string> w(10, "cs"); // 10 strings, all "cs“ vector<string> 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 vector<Dice> cubes(10); since Dice doesn’t 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 vector definition)

Example vector definitions vector<int> counter(9, 0); each element is an integer (all initialized to 0) 0 1 2 3 4 5 6 7 8 counter 0 0 0 0 0 0 0 0 0 vector<char> letters(18); each element is a char (not initialized yet) 0 1 2 3 4 5 6 7 8 letters 9 10 11 12 13 14 15 16 17 vector<Date> holidays(6); each element is a date object that contains todays date holidays 12 2016 12 2016 12 2016 12 2016 12 2016 12 2016 4 0 1 2 3 4 5

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 vector<int> nums(9); nums[5] = 102; nums[0] = nums[5]*2-1; nums[nums[5]/20-3] = 55; nums[10] = 5; // error nums 203 0 1 2 3 4 5 6 7 8 55 102

Passing vectors to functions as parameters Vectors can be passed as parameters to functions Pass by reference (if function changes the vector) void Count (vector<int> & counts); Pass by const-reference (if no changes made). void Print(const vector<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 known vector has a member function, size, to return the size of a vector

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)

vector as a return type Vector can be return type of a function vector<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 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; }; vector<student> class(11); // a vector with 11 students class[1].gpa = 3.2; for (i = 0; i <= 10; i++) class[i].id = i + 1250; 1251 id 3.2 gpa name lastname 1 1260 id gpa name lastname 10

shuffle.cpp program

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”

Reading words into a vector (problematic version) vector<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

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

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 vector class utilities as in the following code vector<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 vector::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 adding half of the current capacity to the capacity 0, 1, 2, 3, 4, 6, 9, 13, 19, 28, ... n+(n/2) … If you want to use push_back mechanism, then the vector should be defined initially without specifying a size empty vector (zero size)

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 vector<string> names; // size is 0, capacity is 0 names.push_back("Ali"); // size is 1, capacity is 1 names.push_back("Husnu"); // size is 2, capacity is 2 names.push_back("Ayse"); // size is 3, capacity is 3 names.push_back("Cem"); // size is 4, capacity is 4 names.push_back("Jale"); // size is 5, capacity is 6 names.push_back("Hale"); // size is 6, capacity is 6 names.push_back("Veli"); // size is 7, capacity is 9 names.push_back("Gonca"); // size is 8, capacity is 9 names.push_back("Fatma"); // size is 9, capacity is 9 names.push_back("Yesim"); //size is 10, capacity is 13

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 increases 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

Vector Processing Examples – 1 (vectorproc.cpp – not in book) write a function that takes a vector 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 vector<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 vector 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 vector<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;