1. DCT1063 Programming 2 CHAPTER 3 STRINGS Mohd Nazri Bin Ibrahim Faculty of Computer, Media & Technology TATi University College nazri@tatiuc.edu.my

2. Strings In C++, a string is defined as a character array that is terminated by a null. A null character is specified using '\0', and is zero. For example, if you want to declare an array str that could hold a 10-character string, you would write: char str[11]=“ABCDEFGHIJ”; //Specifying the size as 11 makes room for the null at the end of the string.

3. Strings(2) C++ allows you to define a string literal. Here are some examples: 1."hello there" 2."I like C++" 3."#$%@@#$“ 4. "“ The last string contains only the null terminator, and no other characters. It is not necessary to manually add the null onto the end of string constants; the C++ compiler does this for you automatically.

4. Reading a String from the Keyboard The easiest way to read a string entered from the keyboard is to make the array that will receive the string the target of a cin statement. For example, the following program reads a string entered by the user: Although this program is technically correct, there is still a problem. To see what it is, examine the following sample run. Enter a string: This is a test Here is your string: This

5. // Using cin to read a string from the keyboard. #include using namespace std; int main() { char str[80]; cout << "Enter a string: "; cin >> str; // read string from keyboard cout << "Here is your string: "; cout << str; return 0; }

6. Reading a String from the Keyboard(2) The >> operator stops reading a string when the first whitespace character is encountered. Whitespace characters include spaces, tabs, and newlines. One way to solve the whitespace problem is to use another of C++’s library functions, gets( ). The general form of a gets( ) call is: gets(array-name);

7. Reading a String from the Keyboard(3) The gets( ) function will continue to read characters until you press ENTER. The header used by gets( ) is. This version of the preceding program uses gets( ) to allow the entry of strings containing spaces. Now, when you run the program and enter the string "This is a test", the entire sentence is read and then displayed, as this sample run shows. Enter a string: This is a test Here is your string: This is a test

8. // Using gets() to read a string from the keyboard. #include using namespace std; int main() { char str[80]; cout << "Enter a string: "; gets(str); // read a string from the keyboard cout << "Here is your string: "; cout << str; return 0; }

9. Some String Library Functions C++ supports a wide range of string- manipulation functions. The most common are:- 1.strcpy( ) 2.strcat( ) 3.strlen( ) 4.strcmp( ) The string functions all use the same header,. Let’s take a look at these functions now.

10. Some String Library Functions(2) strcpy –A call to strcpy( ) takes this general form: –strcpy(to, from); –The strcpy( ) function copies the contents of the string from into to. –The following program will copy "hello" into string str: #include using namespace std; int main() { char str[80]; strcpy(str, "hello"); cout << str; return 0; }

11. Some String Library Functions(3) strcat –A call to strcat( ) takes this form: –strcat(s1, s2); –The strcat( ) function appends s2 to the end of s1; s2 is unchanged. –Both strings must be null-terminated, and the result is null-terminated. –The following program will print hello there on the screen:

12. #include using namespace std; int main() { char s1[20], s2[10]; strcpy(s1, "hello"); strcpy(s2, " there"); strcat(s1, s2); cout << s1; return 0; }

13. Some String Library Functions(4) strcmp –A call to strcmp( ) takes this general form: –strcmp(s1, s2); –The strcmp( ) function compares two strings and returns 1.0 if they are equal. 2.Positive number if s1 is greater than s2 3.Negative number if s1 is less than s2 –The password( ) function, shown in the following program, is a password-verification routine. It uses strcmp( ) to check a user’s input against a password.

14. #include using namespace std; bool password(); int main() { if(password()) cout << "Logged on.\n"; else cout << "Access denied.\n"; return 0; } // Return true if password accepted; false otherwise. bool password() { char s[80]; cout << "Enter password: "; gets(s);

15. if(strcmp(s, "password")) { // strings differ cout << "Invalid password.\n"; return false; } // strings compared the same return true; }

16. Some String Library Functions(4) strlen –The general form of a call to strlen( ) is strlen(s); –where s is a string. –The strlen( ) function returns the length of the string pointed to by s. –Consider the following program.

17. #include using namespace std; int main() { char str[80]; cout << "Enter a string: "; gets(str); #include using namespace std; int main() { char str[80]; cout << "Enter a string: "; gets(str);

18. Strlen(cont) –If the user enters the string "Hi there", this program will display 8. The null terminator is not counted by strlen( ). –When the following program is run, the string entered at the keyboard is printed in reverse. For example, "hello" will be displayed as olleh.

19. // Print a string backwards. #include using namespace std; int main() { char str[80]; int i; cout << "Enter a string: "; gets(str); // Print the string in reverse. for(i=strlen(str)-1; i>=0; i--) cout << str[i]; return 0; }

20. Some String Library Functions(5) As a final example, the following program illustrates the use of all four string functions: #include using namespace std; int main() { char s1[80], s2[80]; cout << "Enter two strings: "; gets(s1); gets(s2); cout << "lengths: " << strlen(s1); cout << ' ' << strlen(s2) << '\n';

21. if(!strcmp(s1, s2)) cout << "The strings are equal\n"; else cout << "not equal\n"; strcat(s1, s2); cout << s1 << '\n'; strcpy(s1, s2); cout << s1 << " and " << s2 << ' '; cout << "are now the same\n"; return 0; }

22. Some String Library Functions(6) If this program is run and the strings "hello" and "there" are entered, then the output will be lengths: 5 5 not equal hellothere there and there are now the same Remember that strcmp( ) returns false if the strings are equal. This is why you must use the ! operator to reverse the condition, as shown in the preceding example, if you are testing for equality.

23. Using the Null Terminator All strings are null-terminated -can often be used to simplify various operations. Example-how little code is required to uppercase every character in a string. This program will print THIS IS A TEST. It uses the library function toupper( ). Notice that the test condition of the for loop is simply the array indexed by the control variable. This works because a true value is any non-zero value.

24. // Convert a string to uppercase. #include using namespace std; int main() { char str[80]; int i; strcpy(str, "this is a test"); for(i=0; str[i]; i++) str[i] = toupper(str[i]); cout << str; return 0; }

25. Summary The subscript operator, [], provides read/write access to any element of a string. string member function strcmp() compares two strings (or substrings) and returns 0 if the strings are equal, a positive number if the first string is lexicographically greater than the second or a negative number if the first string is lexicographically less than the second. string member functions strlen() return the size or length of a string (i.e., the number of characters currently stored in the string).