1 / 27

Intro to Classes via the C++ String Class

Intro to Classes via the C++ String Class. November 18, 2002 CSE103 - Penn State University Online at http://www.personal.psu.edu/djh300/cse103/string.ppt Prepared by Doug Hogan. Announcements. Late drop deadline is Wednesday, 11/20 Review scores

palti
Download Presentation

Intro to Classes via the C++ String Class

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Intro to Classes via the C++ String Class November 18, 2002 CSE103 - Penn State University Online at http://www.personal.psu.edu/djh300/cse103/string.ppt Prepared by Doug Hogan

  2. Announcements • Late drop deadline is Wednesday, 11/20 • Review scores • If you do not earn a C or better, you’ll need to retake the class for the major. • 4-credit D – GPA impact? • Dr. Quick has normal office hours this week. • HW5.dat was modified on Wednesday, 11/13 (leading 0 in an ID) • HW5 due next Monday

  3. Overview • String class • Headers • Creating strings • Manipulating and comparing strings • Motivation for Object Oriented Programming • Strings as objects • Terminology and theory • Problems

  4. The string type • Alternative to character arrays • Hides many details • Easier to manipulate • Required headers • #include <string> • using namespace std; • string is part of the C++ standard library

  5. Declaring strings • Uninitialized: • Like primitive data types • e.g. string myString; • Can then use assignment operator • e.g. myString = “this is a string”; • Initialized: • Use string keyword, name, and initial value in parentheses • e.g. string myString(“a string”);

  6. Input/Output • cin and cout • cin stops at whitespace • getline can be used for reading in strings with spaces included: • getline(stream, receivingString); • example: • cout << “Enter a string”;getline(cin, str1);

  7. Manipulating Characters • Exactly the same as with arrays of characters! • Use an index in brackets to get or manipulate that character. • string myString(“a string”); • cout << myString[0]; • prints “a” • myString[0] = “A”; • changes myString to “A string”

  8. Exercises • Create a string called testString that is initially “It’s 8 a.m. and he’s expecting me to think” • string testString(“It’s 8 a.m. and he’s expecting me to think”); • Change the “8” to a “9.” • testString[5] = ‘9’; • Output the string • cout << testString;

  9. Operators and strings • The string class lets you use the following operators: • Assignment: = • Comparison: >=, >, <, <= • Equality: ==, != • Concatenation: + • Example: • if(string1 < string2){ cout << string1 << “ is before “ << string2 << endl;}

  10. Problem (should be familiar…) • Suppose you have these declarations: • string str1 = “I love ”; • string str2 = “computer programming!”; • Create a string called str3 from these two strings that reads “I love computer programming!” • string str3 = str1 + str2;

  11. A bit of terminology before the fun part… • We’ll call string variables objects. • We can operate on strings with functions • use dot notation • e.g. objectName.operation(); • said to be sending a message to the string object

  12. length() message • length( ) returns the length of the string it’s called on • ex: • string hello(“Hello”); • cout << hello.length() << endl; • prints 5 • Don’t forget the parentheses!! • Must give the string object, then the dot operator!!

  13. Practice • Given • string noun; • cin >> noun; • Output the length of noun. • cout << noun.length();

  14. find() message • find( ) takes a string as an argument • returns the index where the argument is found in the object it’s called on • ex: • string hello(“Hello”); • cout << hello.find(“ll”); << endl; • prints 2 • if the string isn’t found, find( ) returns -1

  15. Substrings: substr() message • Takes two integer arguments: • first is starting character • second is length • returns a substring of the given length • string hello(“Hello World”); • cout << hello.substr(6, 5); << endl; • prints “World” • goes up to string’s length if 2nd argument is too short

  16. Problems • Givenstring s1(“abcdefghi”); • string s2(s1.substr(4, 3));What is stored in s2? • Answer: efg • Write a line of code to store the location of the letter “d” from s1 in the following int: • int d; • Answer: d = s1.find(“d”);

  17. length(s) s.length s(length) s.length() find(“Any”) s.find(“ “) s.substr(2) s.substr(2, 5) s.substr(“tri”) s.find(“tri”) Givenstring s(“Any string”);Give the result of each message or what is wrong with it. Modified Self-Check 4-8 from: Mercer, Rick. Computing Fundamentals with C++. Wilsonville, OR: Franklin, 1999.

  18. length(s) no dot notation length takes no argument s.length no parentheses s(length) parentheses misplaced s.length() 10 find(“Any”) no object s.find(“ “) 3 s.substr(2) not enough arguments s.substr(2, 5) y str s.substr(“tri”) wrong arguments s.find(“tri”) 5 Givenstring s(“Any string”);Give the result of each message or what is wrong with it. Modified Self-Check 4-8 from: Mercer, Rick. Computing Fundamentals with C++. Wilsonville, OR: Franklin, 1999.

  19. More string Messages • The ones we’ve discussed are the most useful • See page 593 of your book for more

  20. Motivation for classes • Object-Oriented Programming (OOP) • Package together a set of related data and operations (encapsulation) • Define a class (abstract data type), or a new data type with its operations • One instance of a class is called an object • The data and operations of a class are called its members. • string is an example of a class

  21. Access rights in OOP • Classes are similar to structs • Add the notion of access rights • class member data and operations can be • public – accessible to anyone • private – accessible only to the object • usually • data are private • operations are public

  22. Information Hiding • Client (user) has only the information needed to use the software. • Implementer has only the information needed to implement the software. • Communication through pre/post. Graphic from: Headington, Mark A. and David Riley. Data Abstraction and Structures using C++. Lexington, MA:Heath, 1994.

  23. Information Hiding Applied • Client can access the public methods of an object • Sending a message • Methods can access private data

  24. An example of a class: bankAccount • Data: • name • balance • Operations: • create an account • withdraw • deposit • check balance

  25. Summary • string class • created with string keyword • operators =, >, <, == defined • messages length(), find(str), substr(start, length) defined • Object Oriented Programming • classes and objects • information hiding

  26. Homework for next time • Another motivation for classes is the notion of abstraction. • Find out what the word means. • Come up with an example of how we use abstraction in everyday life. • Write down an example of a bankAccount object.

  27. Homework for next time • Implement the following nonmember function: • string returnedExpression(string inputString) • // PRE: inputString is a line of valid C++ code // (<=80 chars) containing the “return”// keyword and ending with a semicolon • // POST: FCTVAL == the expression that follows // the return keyword, not including the// semicolon • Hint: you’ll need to (and should) use all of the string member functions we discussed today. • Ex: returnedExpression(“ return a+foo(a-1);”); • returns “a+foo(a-1)” • This PowerPoint is online at http://www.personal.psu.edu/djh300/cse103/string.ppt

More Related