Strings Continued

Review: Strings Versus Characters

• Remember that a string is a series of characters enclosed in double quotes such as:

  "Hello"  "b"  "3.14159"  "$3.95"  "My address is 378 Eastbrook Dr"

• We can store text in a variable of type string, like:

  string firstName;             // declaration
  firstName = "Jennifer";       // assignment
  string lastName = "Parrish";  // declaration + assignment
  string fullName = firstName + " " + lastName; // concatenation (+) of 2 stings
  

• On the other hand, a character is a single letter, number or special symbol
• We enclose characters in a single quote, rather than a double quote, like:

  'a'   'b'   'Z'   '3'   'q'   '$'   '*'

• Also, we can store a a single character using a variable of type char, such as:

  char letterA = 'A';
  char letterB = 'B';
  

• Each character is stored as a number, using its ASCII Table value
• By declaring a char variable or using single quotes, C++ knows to treat the number as a character
• Thus, when we print a character, we see a letter rather than a number:

  char letter = 'A';
  cout << letter << 'B' << endl;
  

• As we can see, a string is made up of characters and characters are numerical codes
• We can use this information to work with characters and strings

Indexing a String

• Strings have a built-in indexing system with each stored in a character sequence starting at 0 (zero)

• We can access any individual character of a string variable using square brackets [ ]
• The general syntax is:

  stringVariable[index];

• Where:
  • stringVariable: the name of your string variable
  • index: the number of the character position
• For example:

  string str = "abcdef";
  char firstLetter = str[0];
  cout << firstLetter << str[1] << endl;
  

• The above code displays:

  ab

• Notice that the square bracket notation returns a char data type

• What will the following print?

#include <iostream>

using namespace std;

int main() {

      string sport = "Basketball";

      cout << "The league: " << sport[1] << sport[7] << endl;

      cout << "Abbreviation for this sport: " << sport[0] << sport[6] << endl;

     return 0;

}

Using a For Loop to Iterate Strings

• Recall that member function length() returns the number of characters in a string variable:

  string s = "abcdef";
  int n = s.length();

• After we know the length, it is easy to iterate through the individual characters of a string using a counting loop:

  cout << "Enter a message: ";
  string msg;
  cin >> msg;
  for (int i = 0; i < msg.length(); i++) {
      cout << "Char[" << i << "]: " << msg[i] << endl;
  }
  

Group Activity: Iterating Strings

• Copy the following program into a text editor, save it as test.cpp, and then compile and run the starter program to make sure you copied it correctly.

  #include <iostream>
  using namespace std;
  
  int main() {
      // Enter your code here
  
      return 0;
  }
  

• Add the following for-loop code to the main() function.

  string msg = "Hello, world!";
  for (int i = 0; i < msg.length(); i++) {
      cout << i << ": " << msg[i] << endl;
  }
  

• Compile and run your code. What do you see when you compile?

String Input With Spaces

• We have been using the >> operator to enter data into a string variable:

  string something;
  cout << "Enter something: ";
  cin >> something;
  cout << "You entered: " << something << "END OF OUTPUT\n";
  

• However, there are some complications
• >> skips whitespace and stops on encountering more whitespace
• Thus, we only get a single word for each input variable
• If a user types in "Hello Mom!", we would only read "Hello" and not " Mom!"
• This is because cin >> s1 works as follows:
  1. Skips whitespace
  2. Reads non-whitespace characters into the variable
  3. Stops reading when whitespace is found

Input Using getline()

• To read an entire line we use function getline()
• Syntax:

  getline(cin, stringVariable);
  

• Where:
  • stringVariable: the name of the string variable
• For example:

  string line;
  cout << "Enter a line of input:\n";
  getline(cin, line);
  cout << line << "END OF OUTPUT\n";
  

• Note that getline() stops reading when it encounters a '\n'

Activity 14.2: How many words in your sentence? (10 pts)

• Let's write a program that counts the number of words in a sentence input by the user.
• Find a partner and open up a new C++ file called numWords.cpp.
• Copy and paste the following starter code into your file:

#include <iostream>

using namespace std;

int main() {

      //Your statements go here

      return 0;

}

• Add the appropriate block comments with your name and section information.
• Next declare a string variable at the top of your program named sentence:

string sentence;

• Welcome the user to the program with the following message:

Give me a sentence, and I will count the number of words.

• Prompt the user to input a sentence and store the user input as the sentence variable.

Please enter your sentence:_

• Don't forget to use getline() here.
• How can we determine how many words are in a sentence?
• We need to look at the whitespace.
  
• Next, we will use a for loop to scroll through the sentence looking for blank spaces. 
  
• Each time we encounter a new blank space, we will add one to our total for the number of words in the sentence.
• Create a new variable at the top of main to store our counter for the number of words in the sentence. Assign it a value of 1. Why do we want to give it an initial value of 1 not 0?
  

int numWords = 1;

• Now, create a for loop to iterate through the sentence. Don't forget to use the length() function here.

for (int i = 0; i < sentence.length(); i++) {

      cout << sentence[i] << endl;

}

• Now, run your program and verify that you get the following output.
  

• However, this is not the purpose of our program.
• We want to count the number of words.
• Remove the cout statement from your for loop.
• Repalce the cout with an if statement to check the value of the the s[i] variable to determine if it is a blank space.

if (sentence[i] == ' ') 

• When we encounter a blank space, we need to add one to the numWords variable.
• Your for loop should now look like this:
  

for (int i = 0; i < sentence.length(); i++) {
    if (sentence[i] == ' ') {

        numWords++; 
    }

}

• Finally, outside of your for loop, add a statement to print out the number of words.

cout << "There are " << numWords << " words in \"" << sentence << "\"" << endl;

• Run your program to verify it works correctly. Then, upload to Catalyst.
  
• Your program should now look like this:

Wrap Up

• With your partner, answer the questions from today's learning objectives

Upcoming Assignments

• Assignment 14 due Tuesday at 3:20pm on Catalyst
• Midterm 2 one week from today
  
• No Lab on Friday - Veteran's Day Holiday!

~Have a Great Weekend!~