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Cs1123 9 strings

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Cs1123 9 strings

  1. 1. Strings (CS1123) By Dr. Muhammad Aleem, Department of Computer Science, Mohammad Ali Jinnah University, Islamabad
  2. 2. Reading strings (C-String) with spaces • getline function of input-stream (cin) can be used to get input in a character array including spaces. • Syntax: cin.getline(char[ ] arr, int size); Character Array Size of Array
  3. 3. Reading strings (C-String) with spaces - OR, the programmer can specify its own terminating character • Syntax: cin.getline(char[ ] arr, int size, char term); Character Array Array size Terminating character
  4. 4. Reading strings (C-String) with spaces • Examples char line[25]; cout << "Type a line terminated by enter key”; cin.getline(line,25); cout << “You entered: ” << line; • Reads up to 24 characters (char type values) and inserts ‘0’ (Null character) at the end. If extra characters (> 25 in this example) entered by user, C++ ignore the remaining characters.
  5. 5. Reading strings (C-String) with spaces char name[60]; char university[100]; cout << "Enter your name: "; cin.getline(name,60); cout << "Enter your university name: "; cin.getline(university,100); cout << name << “ study in ”<< university;
  6. 6. Reading strings (C-String) with spaces char line[100]; cout << "Type a line terminated by t: “; cin.getline( line, 100, 't' ); cout << line;
  7. 7. Strings - Introduction • A string is a sequence of characters. • We have used null terminated <char> arrays (C- strings) to store and manipulate strings. • C++ also provides a class called string • We must include <string> in our program. – More convenient than the C-String
  8. 8. Operations • Creating strings. • Reading strings from keyboard. • Displaying strings to the screen. • Finding a substring from a string. • Modifying string. • Adding strings. • Accessing characters in a string. • Obtaining the size of string. • And many more…
  9. 9. Declaration of strings • Following instructions are all equivalent. • They declare country to be an variable of type string, and assign the string “Pakistan” to it: –string country(“Pakistan”); –string country = “Pakistan”; –string country; x=“Pakistan”;
  10. 10. Operations: Concatenation • Concatenation: combining two or ore strings into a single string • Let x and y be two strings • To concatenate x and y, write: x+y string x= “high”; string y= “school”; string z; z = x+y; cout<< “z=“ << z <<endl; z = z + “ was fun”; cout<< “z=“ << z <<endl; Output: z=highschool z= highschool was fun
  11. 11. concat-assign Operator += • Assume x is a string. • The statement x += y; is equivalent to x = x + y; where y can be a string OR a C-style (character string), a char variable, a double-quoted string literal, or a single-quoted char.
  12. 12. Example of Mixed-Style Concat. string x= “high”; char y[ ]= “school”; char z[ ]= {‘w’, ’a’, ’s’, ‘0’}; string p = “good”; string s = x + y + ’ ‘+ z + ” very” + ” “ + p +’ ! ’; cout<<“s= “<<s<<endl; cout<<“s= “+s<<endl; Output: s= highschool was very good! s= highschool was very good!
  13. 13. Example of concat-assign Operator += string x = “high”; string y = “school”; x+=y; cout<<“x= “<<x<<endl; Output: x= highschool
  14. 14. Comparison Operators for string Objects • We can compare two strings x and y using the following operators: ==, !=, <, <=, >, >= • Comparison is alphabetical, the outcome of each comparison is: true or false • Comparison works as long as at least x or y is a string. • The other string can be a string Or a C-String variable, or a double-quoted string (string literal).
  15. 15. Example of String Comparisons string x = “high”; char y[ ] = “school”; if (x<y) cout<<“x<y”<<endl; if (x<“tree”) cout<<“x<tree”<,endl; if (“low” != x) cout<<“low != x”<<endl; Output: x<y x<tree low != x
  16. 16. Using Index Operator [ ] • If x is a string, and you wish to obtain the value of the k-th character in the string, you may write: x[k]; • This feature makes string variables similar to arrays of char string x = “high”; char c = x[0]; // c is ‘h’ c = x[1]; // c is ‘i’ c = x[2]; // c is ‘g’
  17. 17. Getting a string size, checking for Emptiness • To obtain the size (number of bytes) of a string variable, call the method length() or size() functions: • To check of x is empty (that is, has no characters in it): int len = x.length( ); // OR int len = x.size( ); If (x.empty() ) cout<<“String x is empty…”;
  18. 18. Obtaining sub-strings of Strings • A substring of a string x is a subsequence of consecutive characters in x • Example: “duc” is a substring of “product” • If x is a string variable, and we want the substring that begins at position pos and has len characters (where pos and len are of type int): • The default value of len is x.length( ) • The default value for pos is 0 string y = x.substr(pos, len); string y = x.substr( );
  19. 19. Inserting a String Inside Another • Suppose x is a string, and let y be another string to be inserted at position pos of the string of x: • The argument y can be: a string variable, a C-style string variable, or a double-quoted string: string str = "to be question"; string str2 = "the "; str.insert(6,str2); // to be the question x.insert( pos, y);
  20. 20. Replacing a Substring by Another • Suppose x is a string variable, and suppose you want to replace the characters in range [pos, len] in x by a string y. To do so, write: • Aargument y can be: a string variable, a C-style string variable, or a double-quoted string x.replace(pos, len, y);
  21. 21. Replacing a Substring by Another • Example: string base="this is a test string."; string str2="n example"; string str=base; // "this is a test string." str.replace(9,5,str2); cout<<str; // "this is an example string."
  22. 22. Deleting (Erasing) a Substring of a string • Suppose x is a string variable, and suppose you want to delete/erase the characters in the range [pos, len] in x. • To do so, write: • The default value of len is the x.length( ): • To erase the whole string of x, do: x.erase(pos, len); x.clear( ); x.erase(pos);
  23. 23. Deleting (Erasing) a Substring of a string • Example: string str("This is an example phrase."); str.erase(11,8); cout << str << endl; Output: “This is an phrase.”
  24. 24. Searching for (and Finding) Patterns in Strings • Suppose x is a string variable, and suppose you want to search for a string y in x. • To do so, write: • Method returns the starting index of the leftmost occurrence of y in x, if any occurrence exits; otherwise, the method returns the length of x. • To search starting from a position pos, do: int startLoc = x.find(y); int startLoc = x.find(y, pos);
  25. 25. An Example (find and substr) string x =“FROM:ayoussef@abc.edu”; int colonPos=x.find(‘:’); string prefix=x.substr(0,colonPos); // FROM string suffix = x. substr(colonPos+1); cout<<“This message is from ”<<suffix<<endl; Output: This message is from ayoussef@abc.edu
  26. 26. Class Exercise • Write a C++ program that finds number of occurrences of a string in another string. The program takes as an input a string str1 (based on string data- type). Then it ask the user to enter another string str2. In the end program display the count or number value stating the occurrence of str2 in str1.

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