Tests your ability to use charAt and to write loops.
Write a Java program that repeatedly asks a user for a line of text until
the user enters the string done. The program
counts the number of pairs of repeated characters; for example the string:
running sunning paddle waffle woof
has 5 pairs of repeated characters (shown in red). The program should find
all pairs of repeated characters (not just letters) including repeated spaces.
A character can only belong to one pair; for example the string
aaa has only one pair of repeated a's (the first pair)
and the string aaaa has two pairs of repeated a's.
Hint: Try to solve the counting part of the problem first (ie. only ask
the user for a string once). Once you solve the counting part of
the problem, put it inside of a sentinel guarded loop that repeatedly asks for a
line until the user enters done.
Hint: The counting part can be solved
by using a loop that processes the string from left to right using
charAt.
My solution:
import java.io.PrintStream;
import java.util.Scanner;
public class Practice1
{
public static void main(String[] args)
{
PrintStream output = System.out;
Scanner input = new Scanner(System.in);
final String DONE = "done";
output.println("Enter a line of text");
String line = input.nextLine().trim();
for (; !line.equals(DONE); )
{
int numRepeats = 0;
// The tricky part is getting this loop correct.
// Notice that you look at the character at
// index i and index i + 1 using charAt. You have
// to be make sure that i + 1 is less than the
// length of line, otherwise you will get an
// exception which is why the conditional-part is:
//
// i < line.length() - 1;
//
// Also, notice that you have to skip over 2
// characters if you find a repeated pair which
// is why i is updated inside the body of the
// loop instead of in the update part of the loop.
for (int i = 0; i < line.length() - 1; )
{
char ci = line.charAt(i);
char cj = line.charAt(i + 1);
if (ci == cj)
{
numRepeats++;
i = i + 2;
}
else
{
i = i + 1;
}
}
output.printf("%d repeated characters%n%n", numRepeats);
output.println("Enter a line of text");
line = input.nextLine().trim();
}
}
}
Tests your ability to use indexOf and substring
in a variable length code problem, and to write a loop.
Question 6.17 from the textbook. Write your program so that it repeatedly
asks the user for a string until the user enters the string done.
Hint: Use the variable length code technique.
Consider trying the inverse problem: given a string such as
SevenTwoFour output the equivalent string using
digits (ie. 724). Ultimately, you still want to
use the variable length code technique. The problem you need
to solve is how do you isolate the numbers from the input
string given that they are not separated by a separator
character. Note that you determine if a charater is in
uppercase by using the static method in the Character
class named isUpperCase.
My solution:
import java.io.PrintStream;
import java.util.Scanner;
public class Practice2
{
public static void main(String[] args)
{
PrintStream output = System.out;
Scanner input = new Scanner(System.in);
final String WORDS = "One " +
"Two " +
"Three" +
"Four " +
"Five " +
"Six " +
"Seven" +
"Eight" +
"Nine " +
"Zero ";
final String NUMS = "1 " +
"2 " +
"3 " +
"4 " +
"5 " +
"6 " +
"7 " +
"8 " +
"9 " +
"0 ";
final int SIZE = 5;
final String DONE = "done";
output.println("Enter a string of digits");
String line = input.nextLine().trim();
for (; !line.equals(DONE); )
{
// not part of the question
String regex = "\\d+";
if (line.matches(regex))
{
for (int i = 0; i < line.length(); i++)
{
char ci = line.charAt(i);
int index = NUMS.indexOf(ci);
String word = WORDS.substring(index, index + SIZE).trim();
output.print(word);
}
output.println();
}
else
{
output.println("Not a string of digits");
}
output.println("Enter a string of digits");
line = input.nextLine().trim();
}
}
}
Tests your ability to use indexOf,
charAt, substring, toLowerCase,
string buffer, to write loops, and to use an if statement.
Question 6.18 from the textbook. Write your program so that it repeatedly asks the user for a string until there is no more input (ie. the user hits Ctrl-D in Linux or Ctrl-Z in Windows).
Write your program so that it first makes an all lowercase version
of the string entered by the user. Your program should
repeatedly use indexOf to
find all of the ei substrings in the lowercase version of
the string; when such a
substring is found, you would use charAt to check if it
is preceded by the letter c. You should use a string buffer
to build the corrected string.
Hint: Try to solve the correction part of the problem first (ie. only ask the user for a string once). Once you solve the correction part of the problem, put it inside a loop that repeatedly asks the user for a string.
Hint: The correction part of the problem needs a loop to repeatedly
look for ei substrings in the string.
My solution:
import java.io.PrintStream;
import java.util.Scanner;
public class Practice3
{
public static void main(String[] args)
{
PrintStream output = System.out;
Scanner input = new Scanner(System.in);
final char C = 'c';
final String EI = "ei";
output.println("Enter a line of text:");
for (; input.hasNext();)
{
String line = input.nextLine();
// Make a lowercase version of line so that you don't
// have to worry about case
String lowerCase = line.toLowerCase();
// You need to modify characters in the user-input string
// so it's easier to use a string buffer
StringBuffer fixed = new StringBuffer(line);
// A loop to look for all of the "ei" substrings in lowerCase
int prevIndex = 0;
for (int index = lowerCase.indexOf(EI);
index != -1;
index = lowerCase.indexOf(EI, prevIndex))
{
// Only fix "ei" if it's not preceded by 'c'
char charBefore = ' ';
int before = index - 1;
if (before > 0)
{
// if statement is needed in case "ei" happens
// to be at the start of the line (in which
// case before == -1 and charAt(before) will
// throw an exception
charBefore = lowerCase.charAt(before);
}
if (charBefore != C)
{
// ok, go ahead and swap the 'i' and 'e'
char theE = fixed.charAt(index);
char theI = fixed.charAt(index + 1);
fixed.setCharAt(index, theI);
fixed.setCharAt(index + 1, theE);
}
prevIndex = index + 1;
}
output.println(fixed);
}
}
}
Question 6.21 from the textbook.
This question is a lot easier than it looks. It can be solved using
only the methods substring (or charAt, but
substring saves you from having to convert a character
to a string), replace, and length inside
of a single loop.
Note: Question 6.23 from the textbook is almost identical to this problem but it cannot be solved so easily.
Question 6.22 from the textbook.
Hint: First use length to make sure the length of the
string is a multiple of 3.
Hint: Use substring and equals to make
sure the string starts with ATG. Alternatively, use
startsWith.
Hint: Use a regular expression and matches to make
sure the string is made up exclusively of the letters
A, C, G, T.
The midterm question from last year.
The first thing you need to do is to read in the split letter as a string split
and then trim the string split (to be safe). You can convert split
to uppercase if you want.
The steps needed to process each full name are:
namenameindexOf to find the space in the namesubstring twice to extract the first
and last names as first and lastcompareToIgnoreCase with
split and last to figure out which
room to send the person toWrite a program that asks the user for a string and replaces
all instances of a date with today's date. You can assume that
a date is always the 3-letter abbreviated name of the month
followed by the day followed by a comma followed by the 4-digit
year; for example Jan 12, 2003.
Use a regular expression and replaceAll. Your
regular expression will probably match strings that are not
necessarily dates, but that's ok. You do not need to use the
Date class; just use a hard coded string for
today's date.