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Using Classes

Learn about classes, function members, and the iostream library in C++. Understand how to create and use objects, perform operations on data, and work with strings.

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Using Classes

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  1. Using Classes Classes and Function Members —An Introduction to OOP (Object-Oriented Programming) Chapter 7 The "++" in C++ 1

  2. Classes The iostream library provides the objectscin, cout and cerr. These objects were not originally provided in C++, but were added to the language using its _________ mechanism — a major modification of C's struct. This mechanism allows any programmer to add new_______ to the language. They are necessary to model real-world objects that have ________________________; e.g., ____________ 2

  3. Classname identifier; _________ An _________ is a program entity whose type is a class. Their main difference from other things we've been calling "program objects" is that in addition to storing data values they also have _______________________ for operating on this data. anObject operations data 3

  4. anObject operations internalFunction(...) data Although objects can be processed by "shipping them off" to functions for processing, externalFunction(anObject) they can also operate on themselves using their built-in operations, which are functions. These functions are called by means of the "push-button" ___________________: _____________________________________ We say that ______________________ to anObject. 4

  5. I/O Classes Bell Labs’ Jerry Schwarz used the class mechanismto create: • an istream class, to define the object cin; and • an ostream class, to define cout and cerr. The resulting I/O system was so powerful and yet easy to use that it was incorporated into the language. We will study these classes and the operations they provide later after we look at another class provided in C++. 5

  6. Read §7.4carefully String Class C has a library of basic functions that can be used to process strings, which are simply chararrays (see slide #9). C++ added a new __________________that provides: • an easy way to store strings, and • a large assortment of useful built-in string-processing operations. • To use the string type, we must _________________________ Lab 7 Warning: #include <string> NOT#include <string.h>which is C's string-processing library <cstring> 6

  7. First one used in Lab 7 Skip leadingwhite space;read until nextwhite space; leave it in stream Some string Operations(others in §7.4) Operation string function read a word from input (e.g., cin) input >> str; read an entire line from input getline(instream, str); find the length of string strstr.size() check if a str is empty str.empty() access the char in str at index istr[i] concatenate str1 and str2str1 + str2 compare str1 and str2str1 == str2 (or !=, <, >, <=, >=) access a substring strstr.substr(pos, numChars) insert a substring into a strstr.insert(pos, subStr); remove a substring from strstr.remove(pos, numChars); find first occurrence of string aStrin str starting at position posstr.find(aStr, pos) find first occurrence of any char of str.find_first_of(aStr, pos) string aStr in str starting at pos Read all chars up to but not including the next newlinecharacter; remove itfrom stream Constant string::npos is returned for unsuccessful searches 7 Print out handy reference sheet in Lab 7

  8. getline(cin, aString); They are external agents that act on objects. Other string operations are internal agents— built-in function members that determine how the object is to respond to messages they receive. These messages are sent using the ("push button") dot operator. • Note that some string operations are "normal" functions: aString.size(); For example, aString "knows" how big it is, so when it receives the size() message via the dot operator, it responds with the appropriate answer. In a sense, class objects are "smarter" than regular char, int, double, ... objects because they can do things for themselves. The "I can do it myself" principle of OOP 8

  9. name 0 1 2 3 4 5 6 7 8 9 10 String Objects Variables, such as string variables, whose data is stored in an ____________ (a sequence of items) are called _______________________ because each individual item can be accessed by attaching an __________ (also called a subscript), enclosed in square brackets, to the variable's name: var[index]. For example, suppose name is declared by string name = "John Q. Doe"; name's value is an array of 11 characters: Note that indexes are numbered beginning with 0. We can use the subscript operator[] to access individual chars: char firstInitial = ___________; // firstInitial = _____ _________________________________// last name -> Roe 9

  10. J o h n Q . D o e name 0 1 2 3 4 5 6 7 8 9 10 M a r y M . S m i t h name 0 1 2 3 4 5 6 7 8 9 10 11 12 Dynamic string Objects Objects of type string can grow and shrink as necessary to store their contents(unlike C-style strings): name = "Mary M. Smith"; // name.size() = string name = "John Q. Doe"; // name.size() = ___ ___ More examples: myName.size() string myName; myName = "John Calvin"; 10 myName = "Susie Doe";

  11. name A large number of built-in string operations like those described on later slides — e.g., size()empty()insert()find() J o h n Q . D o e 10 0 1 2 3 4 5 6 7 8 9 Note: The diagram for the string object name on the preceding slide is really not correct. It shows only the data part of this object and not the built-in operations. But to save space, we will usually show only the string of characters that it stores. 11

  12. The I/O Classes As we noted earlier, C++ provides an istream class for processing input and an ostream class for processing output and that • cin is an object of type istream • cout and cerr are objects of type ostream To use these classes effectively, you must be aware of the large collections of operations provided by them (although like the string class, it really isn't feasible to memorize all of them and how they are used.) Read § 7.3 carefully; note the diagrams of streams; note how I/O actually takes place. 12

  13. Some ostream Operations ostream function Description cout << expr Insert expr into cout. cout.put(ch); Tell cout, "Insert ch into yourself." cout << flush Write contents of cout to screen. cout << endl Write a newline to cout and flush it. cout << fixed Display reals in fixed-point notation. cout << scientific Display reals in scientific notation. cout << showpoint Display decimal point and trailing zeros for real whole numbers. cout << noshowpoint Hide decimal point and trailing zeros for real whole numbers. format manipulators Read §7.3carefully  cout is buffered; cerr is not.    = default Once used, stay in effect(except for setw()) 13

  14. Read §7.3carefully More ostream Operations ostream function Description cout << showpos Display sign for positive values cout << noshowpos Hide sign for positive values cout << dec Display numbers in decimal format cout << oct " " " octal " cout << hex " " " hexadecimal " cout << boolalpha Display true, false as "true", "false" cout << noboolalpha Display true, false as 1, 0 cout << setprecision(n) Display n decimal places for reals cout << setw(w) Display next value in field of width w cout << left Left-justify subsequent values cout << right Right-justify subsequent values cout << setfill(ch) Fill leading/trailing blanks with ch   #include <iomanip> for these 14

  15. Example: #include <iostream> #include <iomanip> using namespace std; int main() { int n1 = 111, n2 = 22; double d1 = 3.0 , d2 = 4.5678; char c1 = 'A', c2 = 'B'; cout << "1. " << n1 << n2 << d1 << d2 << c1 << c2 << endl; cout << "2. " << n1 << " " << n2 << " " << d1 << " " << d2 << " "<< c1 << " " << c2 << endl; cout << fixed << showpoint << setprecision(2); cout << "3. " << n1 << " " << n2 << " " << d1 << " " << d2 << " " << c1 << " " << c2 << endl; cout << "4. " << setw(5) << n1 << " " << n2 << " " << setw(8) << d1 << " " << setw(2) << d2 << " " << c1 << " " << c2 << endl; Output ------------------------------------------------------------------------------------------------------------------------ 15

  16. Some istream Operations istream function Description cin >> var; Skip white space and extract characters from cin up to the first one that can't be in a value for var; convert and store it in var. cin.get(ch); Tell cin, "Put your next character; (whitespace or not) into ch." Tabs, spaces, end-of-lines Both are used in Lab 7 16

  17. Examples (cont. from earlier) cout << "> "; cin >> n1 >> d1 >> n2 >> d2 >> c1 >> c2; cout << "Output:\n" << n1 << " " << n2 << " " << d1 << " " << d2 << " " << c1 << " " << c2 << endl;        Input/Output: cin: > 1 2.2 3 4.4 A B Output: --------------------- Input/Output: cin: > 1 2.2 3 4.4 A B Output: --------------------- 17

  18. cout << "> "; cin >> d1 >> c1 >> n1 >> d2 >> c2 >> n2; cout << "Output:\n" << n1 << " " << n2 << " " << d1 << " " << d2 << " " << c1 << " " << c2 << endl;  B  1 2 . 2 3 4 . 4 A 5 cout << "> "; cin >> n1 >> d1 >> n2 >> d2 >> c1 >> c2; cout << "Output:\n" << n1 << " " << n2 << " " << d1 << " " << d2 << " " << c1 << " " << c2 << endl; Input/Output: cin: > 12.2 34.4AB Output: --------------------- Input/Output: cin: > 12.2 34.4A5B Output: --------------------- The character B is left in cin for the next input statement. 18

  19. In the last example, a character was left in cin for the next input statement. To see how this can cause problems, suppose the following code came after the preceding example: for (int i = 1; i <= 5; i++) { cout << "> "; cin >> d1 >> c1 >> n1 >> d2 >> c2 >> n2; cout << "Output:\n" << n1 << " " << n2 << " " << d1 << " " << d2 << " " << c1 << " " << c2 << endl; } When executed, the following output would be produced. Execution would not pause to allow input of new values for the variables. They retain their old values. > Output: 4 5 12.20 0.40 3 A > Output: 4 5 12.20 0.40 3 A > Output: 4 5 12.20 0.40 3 A > Output: 4 5 12.20 0.40 3 A > Output: 4 5 12.20 0.40 3 A The following operations on istreams like cin show how we can recover from bad input. 19

  20. More istream Operations istream function Description cin.good() Ask cin, "Are you in good shape?" cin.bad() Ask cin, "Is something wrong?" cin.fail() Ask cin, "Did the last operation fail?" cin.clear(); Tell cin, "Reset yourself to be good." cin.ignore(n, ch); Tell cin, ignore the next n characters, or until ch occurs, whichever comes first.    20

  21. Example showing how to read a valid real number: Infinite Loop double number; cout << "Enter a real number: "; cin >> number; while (true) // or for(;;) { } if (cin.fail()) // input failure? { cerr << "\n** Non-numeric input!\n"; cin.clear(); // reset all I/O status flags cin.ignore(80, '\n'); // skip next 80 input chars } // or until end-of-line char else break; 21

  22. Another C++ Class (Template) Another C++ class you may find useful is for ____________ numbers:____________________, where T may be float, double, or long double. Mathematically: a + bi C++: complex<T>(a,b) 1.5 + 3.2i complex<double>(1.5, 3.2) i complex<double>(0, 1) Inputs Outputs (1.5, 3.2)(1.5,3.2) (0, 1)(0, 1) 3.14 (3.14,0) 22

  23. Figure 7.2 Quadratic Equation Solver — Complex Roots /* This program solves quadratic equations using the quadratic formula. Input: the three coefficients of a quadratic equation Output: the complex roots of the equation. -----------------------------------------------------------*/ #include <iostream> // cout, cin, <<, >> #include <complex> // complex types using namespace std; int main() { complex<double> a, b, c; cout << "Enter the coefficients of a quadratic equation: "; cin >> a >> b >> c; complex<double> discriminant = b*b - 4.0*a*c, root1, root2; root1 = (-b + sqrt(discriminant)) / (2.0*a); root2 = (-b - sqrt(discriminant)) / (2.0*a); cout << "Roots are " << root1 << " and " << root2 << endl; } 23

  24. Sample runs: Enter the coefficients of a quadratic equation: 1 4 3 Roots are (-1,0) and (-3,0) Enter the coefficients of a quadratic equation: 2 0 -8 Roots are (2,0) and (-2,0) Enter the coefficients of a quadratic equation: 2 0 8 Roots are (0,2) and (-0,-2) Enter the coefficients of a quadratic equation: 1 2 3 Roots are (-1,1.41421) and (-1,-1.41421) Enter the coefficients of a quadratic equation: (1,2) (3,4) (5,6) Roots are (-0.22822,0.63589) and (-1.97178,-0.23589) 24

  25. More info in §7.5 Random Numbers Slides 25-30are optional The text provides a RandomInt class. Objects of this class are integers with "random" values, which can be used to simulate all sorts of "random" occurrences. #include "RandomInt.h" ... RandomInt die1(1,6), die2(1,6); // two dice die1.generate(); // roll the dice die2.generate(); cout << "dice roll = " // display results << die1 + die2 << endl; 25

  26. RandomInt Objects The range of random values is specified when an object is declared: #include "RandomInt.h" ... const int HEADS = 0, TAILS = 1; RandomInt coin(HEADS,TAILS); coin.generate(); // flip coin cout << coin << endl; // display result 26

  27. RandomInt Operations Operation RandomInt function Display a RandomInt ostream << randInt Declare a RandomInt RandomInt name; Declare a RandomInt within range first..last RandomInt name(first, last); Generate new random value randInt.generate(); Generate new random value from range first..lastrandInt.generate(first, last); Add two RandomInt values randInt1 + randInt2 (also -, *, /) Compare two RandomInt values randInt1 == randInt2 (also !=, <, >, <=, >=) 27

  28. Figure 7.4 Simulate Shielding of a Nuclear Reactor /* This program simulates particles entering the shield described in the text and determines what percentage of them reaches the outside. Input: thickness of the shield, limit on the number of direction changes, number of neutrons, current direction a neutron traveled Output: the percentage of neutrons reaching the outside --------------------------------------------------------------------*/ #include <iostream> // cin, cout, <<, >> using namespace std; #include "RandomInt.h" // random integer generator int main() { int thickness, collisionLimit, neutrons; cout << "\nEnter the thickness of the shield, the limit on the \n" << "number of collisions, and the number of neutrons:\n"; cin >> thickness >> collisionLimit >> neutrons; 28

  29. RandomInt direction(1,4); int forward, collisions, oldDirection, escaped = 0; for (int i = 1; i <= neutrons; i++) { // Next neutron forward = oldDirection = collisions = 0; while (forward < thickness && forward >= 0 && collisions < collisionLimit) { direction.generate(); if (direction != oldDirection) collisions++; oldDirection = direction; if (direction == 1) forward++; else if (direction == 2) forward--; } if (forward >= thickness) escaped++; } cout << '\n' << 100 * double(escaped) / double(neutrons) << "% of the particles escaped.\n"; } 29

  30. Sample runs: Enter the thickness of the shield, the limit on the number of collisions, and the number of neutrons: 1 1 100 26% of the particles escaped Enter the thickness of the shield, the limit on the number of collisions, and the number of neutrons: 100 5 1000 0% of the particles escaped Enter the thickness of the shield, the limit on the number of collisions, and the number of neutrons: 4 5 100 3% of the particles escaped Enter the thickness of the shield, the limit on the number of collisions, and the number of neutrons: 8 10 500 0.2% of the particles escaped 30

  31. Some Final Notes about Classes Well-designed classes provide a rich set of operations that make them useful for many problems. Operations can be external (normal) functions to which objects are passed for processing; or they may be internal function members of the class. Function members receive messages to class objects and determine the response. 31

  32. Be aware of the functionality a class provides (but don’t memorize the nitty-gritty details). • Know where (in a reference book) to look up the operations a class supports. • Then, when a problem involves an operation on a class object, scan the list of operations looking for one that you can use — don’t reinvent the wheel! To use a class effectively, you must know what capabilities the class provides; and how to use those capabilities. 32

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