C-strings and C++ string Class

1. C-strings and C++ string Class

2. Topics • C-Strings • Library Functions for Working with C-Strings • Conversions Between Numbers and Strings • Character Testing • Character Case Conversion • Writing Your Own C-String Handling Functions • C++ string Class

3. Strings • C-string: array of characters terminated by NULL character • The C-string "Hi there!" would be stored in memory as shown: r e ! H i t h e \0

4. Array of char • Array of char can be defined and initialized to a C-string char str1[20] = "hi there!"; • Array of char can be defined and later have a string copied into it char str2[20]; strcpy(str2, "hi there!");

5. Pointer to char • Pointer to char can refer to C-strings defined as arrays of char char str[20] = "hi"; char *pStr = str; cout << pStr; // prints hi • Can dynamically allocate memory to be used for C-string using new

6. Library Functions for C-Strings (Requires #include <cstring>) • int strlen(char *str) // Returns length of a C-string:cout << strlen("hello"); // Prints: 5

7. const int LENGTH = 30; char line[LENGTH]; cout << "Enter a string (no more than " << LENGTH - 1 << " characters.)\n"; cin.getline(line, LENGTH); cout << "The C-string read was " << "\"" << line << "\"" << " whose length was " << strlen(line) << endl;

8. strcpy • char name[20] = “Mary” // This is OKchar name2[20];name2 = “Nancy”; // Now allowed char name3[20];strcpy(name3, “Nancy”); //Copycout << name3; // Prints Nancy

9. strcpy • strcpy(char *dest, char *source) // Copies a string from a source address // to a destination address char name[15]; strcpy(name, "Deborah"); cout << name; // prints Deborah

10. strcat • strcat(char *dest, char *source)//Takes two C-strings as input. // Adds the contents of the second string to the // end of the first string: char str1[15] = "Good "; char str2[30] = "Morning!"; strcat(str2, str1); cout << str1; // prints: Good Morning! • No automatic bounds checking: programmer must ensure that str1 has enough room for result

11. strcmp • intstrcmp(char*str1,char*str2)//Compares strings stored at two addresses • Returns a value: less than 0 if str1 precedes str2 equal to 0 if str1 equals str2 greater than 0 if str1 succeeds str2

12. strcmp • Often used to test for equality if(strcmp(str1, str2) == 0) cout << "equal"; else cout << "not equal"; • Also used to determine ordering of C-strings in sorting applications • Note that C-strings cannot be compared using == or > if (str1 > str2) -- this is not allowed(compares addresses of C-strings, not contents) • If

13. Swapping const int NAME_LENGTH = 20;char name1[NAME_LENGTH];char name2[NAME_LENGTH];char temp[NAME_LENGTH];strcpy(name1, "William"); strcpy(name2, "Catherine"); cout << "Originally...\n";cout << "Name1: " << name1 << endl;cout << "Name2: " << name2 << endl;

14. Swapping (cont.) if (strcmp(name1, name2) > 0){ strcpy(temp, name1); strcpy(name1, name2); strcpy(name2, temp); }cout << "\nAfter switch...\n";cout << "Name1: " << name1 << endl;cout << "Name2: " << name2 << endl;

15. Character Testing Requires #include <cctype>

16. Character Testing Require #include<cctype>

17. C++ String Class

18. The C++ string Class • The string class offers several advantages over C-style strings: • large body of member functions • overloaded operators to simplify expressions • Requires #include <string>

19. string class constructors • string() • string(string str) initializes string object with str • string(char *cstr) initializes string ojbect with C-string • Various other constructorswww.cplusplus.com

20. String Class Member Functions • int find(string str) // returns the position of str in the string object • int find (char ch) // returns the position of ch in the string object • int find (string str, int x) // returns the position of str beyond x • int find (chch, intx) // returns the position of ch beyond

21. String Class Member Functions (cont.) • void insert (int x, string str) // inserts str at position x • void insert (int x, char ch) // inserts char at position x • string replace(int pos, int n, string str) // replaces n characters starting at pos with // substring str • Other member functions www.cplusplus.com

22. Overloaded string Operators

23. Overloaded string Operators (continued)

24. Overloaded string Operators string word1, phrase; string word2 = " Dog"; cin >> word1; // user enters "Hot" // word1 has "Hot" phrase = word1 + word2; // phrase has // "Hot Dog" phrase += " on a bun"; for (inti = 0; i < 16; i++) cout << phrase[i]; // displays // "Hot Dog on a bun"

25. Swapping Two Strings string name1 = "William"; string name2 = "Catherine"; string temp; cout << "Originally...\n"; cout << "Name 1: " << name1 << endl; cout << "Name 2: " << name2 << endl; if (name1 > name2){ temp = name1; name1 = name2; name2 = temp; } cout << "\nAfter swap...\n"; cout << "Name 1: " << name1 << endl; cout << "Name 2: " << name2 << endl;

26. Conversion to C-strings • data() and c_str() both return the C-string equivalent of a string object • Useful when using a string object with a function that is expecting a C-string char greeting[20] = "Have a "; string str("nice day"); strcat(greeting, str.data());

27. Modification of string objects • str.insert(intpos, string s) inserts s at position pos in str • Convert constructor for string allows a C-string to be passed in place of s string str("Have a day"); str.insert(7, "nice "); • insert is overloaded for flexibility

28. Modification of string objects • str.append(string s) appends contents of s to end of str • Convert constructor for string allows a C-string to be passed in place of s string str("Have a "); str.append("nice day"); • append is overloaded for flexibility

29. Your Turn(Formatting Number) • Problem: Design a function which formats a string like “1234567.89” to “$1,234,567.89”. • Plan: • Find the position dp of the decimal point • Starting at dp, count backwards and insert a comma every 3 places. • At position 0, insert ‘$’.

30. Refinement dp • dp  position of decimal point in stringpos  dp • Loop (while dp is greater than 3) go back 3 places in the string insert ‘,’ thereEnd Loop • Insert “$” at front of string

31. Your Turn • Write a program which inputs a long text and counts the frequency of occurrence of each letter in the text. letters frequency

32. In Main() // Declarations const int MAX = 26; int frequency[MAX]; string text; cout << "Please enter a “ + “substantially long text.\n"; getline(cin, text); string input(text); // convert to // c-string letterFrequency(frequency, MAX, input);

33. Letter Frequency • The quick brown fox jumps over the lazy dog. • Pack my box with five dozen liquor jugs. • Now is the time for all good people to come to the aid of their country.

34. letterFrequency() void letterFrequency(int frequency[], int count, string input) { // Initialize requency for (int i = 0; i < count; i++){ frequency[i] = 0; } // Count frequency int length = input.length(); for (int i = 0; i < length; i++) { switch(input[i]) { case 'a': case 'A': frequency[0]++; break; case 'b': case 'B': frequency[1]++; break;

35. letterFrequency() (cont.) case 'c': case 'C': frequency[2]++; break; case 'd': case 'D': frequency[3]++; break; case 'e': case 'E': frequency[4]++; break; . . . . . .; case 'x': case 'X': frequency[23]++; break; case 'y': case 'Y': frequency[24]++; break; case 'z': case 'Z': frequency[25]++; break; default:; } // switch } }

36. String I/O • string str1;cin >> str1; // delimitted by // white-spacecout << str1; • string str2;getline(cin, str2); // delimitted by // by eol

37. Your Turn • Problem: • Input a long sentence and count the number of words. • Assumption: • All words are delimitted by a single space.

38. In Main() • // Input text • // wordCount = countWords(text) • // Output results

39. countWords() • int countWords(string text) Loop (while length > 0 && !done) ps  position of ‘ ‘ in text If (ps = 0) done  true End If Add one to count Replace first part of text with “” • End Loop