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## Solution for Programming Exercise 2.6

This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.

### Exercise 2.6:

This exercise asks you to write a program that tests some of the built-in subroutines for working with Strings. The program should ask the user to enter their first name and their last name, separated by a space. Read the user's response using TextIO.getln(). Break the input string up into two strings, one containing the first name and one containing the last name. You can do that by using the indexOf() subroutine to find the position of the space, and then using substring() to extract each of the two names. Also output the number of characters in each name, and output the user's initials. (The initials are the first letter of the first name together with the first letter of the last name.) A sample run of the program should look something like this:

    Please enter your first name and last name, separated by a space.
    ? Mary Smith
    Your first name is Mary, which has 4 characters
    Your last name is Smith, which has 5 characters
    Your initials are MS

Discussion

Once you have the first name and the last name in separate String variables, it's easy to produce the desired output. The length of a string, str, can be obtained by calling the function str.length(). To get the first character of str, you can call str.charAt(0). (Remember that characters are numbered starting from zero.) If the first and last names are in the variables firstName and lastName, then the output of the program can be produced using

    System.out.println("Your first name is " + firstName + ", which has "
                              + firstName.length() + " characters.");
    System.out.println("Your last name is " + lastName + ", which has "
                              + lastName.length() + " characters.");
    System.out.println("Your initials are " + firstName.charAt(0) + lastName.charAt(0));

Or, using formatted output to make it easier to read:

    System.out.printf( "Your first name is %s, which has %d characters.%n",
                                       firstName, firstName.length() );
    System.out.printf( "Your last name is %s, which has %d characters.%n",
                                       lastName, lastName.length() );
    System.out.print( "Your initials are %s%s%n",
                                       firstName.charAt(0), lastName.charAt(0) );

Note that it is not necessary to put values like firstName.length() and firstName.charAt(0) into their own variables, if all that you want to do with them is print them out. (But some people might consider it better style to use variables.)

The harder part of the problem is breaking up the input string to get the first and last name. If the user's input is in a String named input, then input.indexOf(' ') is an int that gives the position of the space in the string. (Note that the space character is written as a literal that consists of a space between two left single quotation marks.)

Once you have the position of the space, you have to figure out how to use it to get the first and last name. Let's look at exactly how the characters in a string are numbered:

             M  a  r  y     S  m  i  t  h
             0  1  2  3  4  5  6  7  8  9

In this example, the space is at index 4. We have to extract the substring in positions 0 through 3 and the substring in positions 5 through 9. The value of input.substring(start,end) will be a string that starts in position start and ends in position end-1. Note that the value of the second parameter is the position after the end of the substring. So, the fist name in this case is input.substring(0,4) and the last name is input.substring(5,10). (Even though there is no character in position 10, that's the correct value for the position after the end of the substring that we want.) In fact, there's an easier way to get the last name, using another version of substring: The value of input.substring(start) is the substring starting at position start and extending to the end of the string, so the last name in this example is input.substring(4).

The important thing to note here is that the "4" in input.substring(0,4) is the index of the space character, and the "5" in input.substring(5) is that index plus 1. So we can get the first and last names using:

    space = input.indexOf(' ');
    firstName = input.substring(0,space);  // Everything before the space
    lastName = input.substring(space+1);   // Everything after the space

By the way, this program will only work correctly if the user follows the instructions. If the user's input does not contain a space, then the value of input.indexOf(' ') will be -1, and the -1 will cause the program to crash when used in the substring function. And if the input doens't have exactly one space, then the program output won't be correct. For now, you don't have any way to test the user's input to see whether it is correct. To do that, you need to know about if statements, which you will learn about very soon.

One more remark: Instead of using TextIO.getln() to read a line of text and then breaking that line into two words, you could simply use TextIO.getWord() twice to read the two words from the same line of input—except that the exercise says to use getln().

The Solution

    import textio.TextIO;

    /**
     * This program reads the user's first name and last name,
     * separated by a space.  It then prints the user's first and
     * last names separately, along with the number of characters
     * in each name.  It also prints the user's initials.  Note that
     * this program will crash if the user's input does not contain
     * a space.
     */
    public class FirstNameLastName {

        public static void main(String[] args) {

            String input;     // The input line entered by the user.
            int space;        // The location of the space in the input.
            String firstName; // The first name, extracted from the input.
            String lastName;  // The last name, extracted from the input.

            System.out.println();
            System.out.println("Please enter your first name and last name, separated by a space.");
            System.out.print("? ");
            input = TextIO.getln();

            space = input.indexOf(' ');
            firstName = input.substring(0, space);
            lastName = input.substring(space+1);

            System.out.println("Your first name is " + firstName + ", which has "
                                      + firstName.length() + " characters.");
            System.out.println("Your last name is " + lastName + ", which has "
                                      + lastName.length() + " characters.");
            System.out.println("Your initials are " + firstName.charAt(0) + lastName.charAt(0));

        }

    }

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