Develop Reference

Reversing strings in Java (loops) until "done"

this is a lab for class I'm trying to do. Here's the instructions:
Write a program that takes in a line of text as input, and outputs that line of text in reverse. The program repeats, ending when the user enters "Done", "done", or "d" for the line of text.
Ex: If the input is:
"Hello there
Hey
done"
the output is:
"ereht olleH
yeH"
And here's what I have right now:
public class LabProgram {
public static void main(String[] args) {
/* Type your code here. */
Scanner scnr = new Scanner(System.in);
String[] inputs = new String[100];
String input;
int i = 0;
while (true) {
input = scnr.nextLine();
if(input.equals("Done") || input.equals("done") || input.equals("d"))
break;
inputs[i] = input;
i++;
}
for (int j = 0; j < i; j++) {
int length = inputs[j].length();
String reverse = "";
for (int k = length - i; k >= 0; k--) {
reverse = reverse + inputs[j].charAt(k);
}
System.out.print("\n" + reverse);
}
}
}
Current output
What am I doing wrong??

Iterate through the array, and reverse elements at every index.
This solution is time consuming but does your job
for (int j = 0; j < inputs.lenght; j++) {
int length = inputs[j].length();
char a;
String rev = "";
for(int i =0; i< length; i++){
a = inputs[j].charAt(i);
rev = a + rev;
}
System.out.println(rev);
}

*Try to use StringBuilder And use method reverse -- #Artur Todeschini
To add to what Artur said, an ArrayList of StringBuilders could do the trick quite well:
for(StringBuilder nextEntry : stringBuilderList)
{
nextEntry.reverse();
}
The enhanced for-loop will go through each entry in the ArrayList, and the StringBuilder's reverse will change the order of the letters.
EDIT TO SHOW FORMATTING
ArrayList<StringBuilder> stringBuilderList= new ArrayList<>();
*note. given that this is for a lab, its probably for learning purposes and using built-in classes that does all the work for you are usually not the intended solution. -- #experiment unit 1998X

Try to use StringBuilder
And use method reverse

This is another "ArrayList and StringBuilder-less" version.
Create two Strings, one filled and one empty:
String nextString = stringArray[i],
template = new String();
Loop through the length of the String, adding the next character in from the end each time through.
int length = nextString.length() - 1;
for(int j = 0; j < length; j++)
{
template += nextString.charAt(length - j);
}
Add the whole String to the String array's index
stringArray[i] = template;
NOTE
This is an inner loop for a String array and is NOT complete code

How do I remove the last comma from the output in series using loop [duplicate]

So I made this to print primes between two numbers of my choice; however, it prints out a comma after the last number and I don't know how to take it off.
Example
in: 0 10
out: 2, 3, 5, 7,
I want 2,3,5,7
Scanner s = new Scanner(System.in);
int a = s.nextInt();
int b = s.nextInt();
for (int i = a; i <= b; i++){
int j;
for (j = 2; j<i; j++){
int p = i%j;
if(p==0){break;}
}
if(i == j){System.out.printf("%d,", i);}
}
}

Use a boolean to keep track of whether you've printed anything yet. Then your format string could be something like
anythingPrinted ? ",%d" : "%d"
That is, only include the comma in the format string if there's something printed.

Use a StringBuilder and write to the console at the end of your program.
StringBuilder sb = new StringBuilder();
for (int i = a; i <= b; i++){
int j;
for (j = 2; j<i; j++){
int p = i%j;
if(p==0){break;}
}
if(i == j){
// If the length of the StringBuilder is 0, no need for a comma
if(sb.length() != 0) {
sb.append(",");
}
sb.append(i);
}
}
System.out.println(sb);

This might seem like overkill, and for many cases it might be, but I have been writing a source code transcoder and I find this situation coming up a lot. Where I need commas in between values, or a prefix value which is only printed once. So I found it handy to create a class which simplifies things.
Again, you wouldn't probably want to use this if you code had one or two print loops in it, but maybe if you had more than a few. Perhaps you would remove in "on first" part if you were never going to use it.
public class FirstPrintOptions {
private PrintStream printStream;
private String onFirst;
private String remaining;
private boolean trip = false;
public FirstPrintOptions(PrintStream printStream, String onFirst, String remaining) {
this.printStream = printStream;
this.onFirst = onFirst;
this.remaining = remaining;
}
public void print() {
if (!trip) {
if (onFirst != null) {
printStream.print(onFirst);
}
trip = true;
} else {
if (remaining != null) {
printStream.print(remaining);
}
}
}
}
Then use it like this..
FirstPrintOptions firstPrintOptions = new FirstPrintOptions(System.out, null, ",");
for (int x=0;x<10;x++) {
firstPrintOptions.print();
System.out.print(x);
}
The results are..
0,1,2,3,4,5,6,7,8,9

I was testing and I came up with this. I was using compilejava.net so scanner doesn't work. I bypassed that part and just set a and b manually. Basically, it builds a string with the numbers and ends in a comma. Then it prints a substring including everything except the last comma.
import java.util.*;
public class HelloWorld {
public static void main(String[] args) {
//Scanner s = new Scanner(System.in);
int a = 2;
int b = 18;
String c = "Output = ";
for (int i = a; i <= b; i++){
int j;
for (j = 2; j<i; j++){
int p = i%j;
if(p==0){break;}
}
if(i == j){c=c+ Integer.toString(i) + ",";}
}
System.out.print(c.subSequence(0, c.length()-1));
}
}

this program for finding factors of a number
for(i=1;i<=number;i++)
{
if(number%i==0)
{
system.out.print(i);
if(i!=0)
{system.out.print(",");}
}
}
so i get the output for 10 as
1,2,5,10

Find common alphabets in two strings using for loop

I am trying to find the common characters in two strings just by using the for loop. The below code is working fine, if I provide two completely different strings ex.one and two but if I provide two strings with same input ex.teen and teen it doesn't work as expected.
import java.util.Scanner;
public class CommonAlphabets {
public static void main(String[] args) {
try(Scanner input = new Scanner(System.in)){
System.out.println("Enter String one ");
String stringOne = input.nextLine();
System.out.println("Enter String two ");
String StringTwo = input.nextLine();
StringBuffer sb = new StringBuffer();
for(int i=0;i<stringOne.length();i++){
for(int j=0;j<StringTwo.length();j++){
if(stringOne.charAt(i)== StringTwo.charAt(j)){
sb.append(stringOne.charAt(i));
}
}
}
System.out.println("Common characters are " +sb.toString());
}
}
}
Should I create another nested for loop to find duplicates in the StringBuffer or is there a better way to handle this scenario.

You do not need an inner for loop but use contains instead
String stringOne = "one";
String stringTwo = "one";
StringBuilder sb = new StringBuilder();
for(int i=0;i<stringOne.length() && i < stringTwo.length ();i++){
if(stringOne.contains(String.valueOf(stringTwo.charAt(i))) &&
!sb.toString().contains(String.valueOf(stringTwo.charAt(i)))){
// check already added
sb.append(stringTwo.charAt(i));
}
}
System.out.println (sb.toString());
edit
check to make sure char to be added does not already exist in StringBuilder -
Could use a Set instead
If using a Set
Set<Character> set = new HashSet<> ();
your logic could be simplified to
if(stringOne.contains(String.valueOf(stringTwo.charAt(i)))){
set.add(stringTwo.charAt(i));
}

You can use Set for it.
Set<Character> set = new HashSet<>();
for(int i = 0; i<stringOne.length(); i++) {
for(int j = 0; j < StringTwo.length(); j++) {
if(stringOne.charAt(i) == StringTwo.charAt(j)){
set.add(stringOne.charAt(i));
}
}
}
StringBuilder sb = new StringBuilder();
for (Character c : set) {
sb.append(c);
}
System.out.println("Common characters are " + sb);

well your approach is fine as the result is showing what you are expecting there fore that code is fine, but you need to stop the duplication , therefore you have to write the code for 'sb' variable so that it will remove duplicates or write code in loop so that it wont provide duplicate.
as your code is becoming complicated to read so i would prefer that you make a method to write code to remove duplicate it will go like
static void removeDuplicate(StringBuilder s){
for(int i=0,i<s.length-1,i++){
for(int j=i+1,j<s.length,j++){
if(s.charAt(i)==s.charAt(j)){
s.deleteCharAt(j);
}
}
}
call this method before printing

Another approach you could try is - combine the two input strings, iterate over the concatenated string and return the characters which exist in both the strings.
Using a Set will ensure you do not add characters which get repeated due to the concatenation of the strings.
Here's what I wrote -
import java.util.HashSet;
public class HelloWorld {
private static Character[] findCommonLetters(String combined, String w1, String w2) {
HashSet<Character> hash = new HashSet<>();
for(char c: combined.toCharArray()) {
if(w1.indexOf(c) != -1 && w2.indexOf(c) != -1) {
hash.add(c);
}
}
return hash.toArray(new Character[hash.size()]);
}
public static void main(String []args){
// System.out.println("Hello World");
String first = "flour";
String second = "four";
String combined = first.concat(second);
Character[] result = findCommonLetters(combined, first, second);
for(char c: result) {
System.out.print(c);
}
System.out.println();
}
}
Demo here.

This is the best way to do this because it's time complexity is n so that why this is the best you could do.
import java.util.Scanner;
public class CommonAlphabets
{
public static void main(String[] args)
{
try (Scanner input = new Scanner(System.in))
{
System.out.println("Enter String one ");
String stringOne = input.nextLine();
System.out.println("Enter String two ");
String StringTwo = input.nextLine();
StringBuffer sb = new StringBuffer();
/**
* Assuming char as index of array where A-Z is from index 0 to 25 and a-z is index 26-51
*/
int[] alphabetArray1 = new int[52];
for(int i = 0, len = stringOne.length(); i < len; i++)
alphabetArray1[stringOne.charAt(i) > 94 ? stringOne.charAt(i) - 71 : stringOne.charAt(i) - 65] = 1;
int[] alphabetArray2 = new int[52];
for(int i = 0, len = StringTwo.length(); i < len; i++)
alphabetArray2[StringTwo.charAt(i) > 94 ? StringTwo.charAt(i) - 71 : StringTwo.charAt(i) - 65] = 1;
// System.out.println(Arrays.toString(alphabetArray1));
// System.out.println(Arrays.toString(alphabetArray2));
for (int i = 0; i < 52; i++)
if (alphabetArray1[i] == 1 && alphabetArray2[i] == 1)
sb.append((char) (i < 26 ? i + 65 : i + 71));
System.out.println("Common characters are " + sb.toString());
}
}
}

For loop only runs through one time before finishing program

import java.util.Random;
import java.util.Scanner;
public class PassGen {
public static void main(String[] args) {
String[] characters = {"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","0","1","2","3","4","5","6","7","8","9"};
StringBuilder b = null;
Scanner scan = new Scanner(System.in);
System.out.println("Enter password length.");
int length = scan.nextInt();
while (length > 20 || length < 6) {
System.out.println("Password must be between 6 and 20 characters long.");
length = scan.nextInt();
}
Random rand = new Random();
for (int i = 0; i <= length; i++) {
int x = rand.nextInt(characters.length) + 1;
b = new StringBuilder(length + 1);
String s = characters[x];
b.append(s);
}
System.out.println("Your password is: " + b.toString());
}
}
For some reason when I run this program it only runs through the for loop once before displaying a single random character regardless of the length entered.

No, the loop is running multiple iterations - but on every iteration, you're creating a new StringBuilder:
// This is inside the loop, but should be outside.
b = new StringBuilder(length + 1);
Note that sometimes, I'd expect the loop to throw an exception - and if it doesn't, you'll end up with a string which is longer than you want anyway. Basically, you have three off-by-one errors... you should have:
b = new StringBuilder(length);
for (int i = 0; i < length; i++) {
int x = rand.nextInt(characters.length);
b.append(characters[x]);
}
Also note that it would be simpler if you just had a string instead of an array of strings, and used charAt:
String characters = "ABCDE...9";
...
int x = rand.nextInt(characters.length());
b.append(characters.charAt(x));

b = new StringBuilder(length + 1);
needs to be outside of the for loop. The way you have it, b is created each time the for loop is run.

You always recreate your b object inside your loop. It will always contain only one character.

Occurrences of each letter

this is the program that I have to write but I get this error,
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException:
50
Write a complete program using two arrays, upper and lower to keep the upper
And lower alphabet respectively.
Ask the user to enter string example:
This is a test from Jupiter. Soon you will see who is from
Jupiter!!! May be Dr. D.
Your program should parse the string and keep track of number of alphabet. Both arrays are indexed from 0 to 25. The logical way to do this is to use upper[0] to
Count the number of ‘A’, and upper[1] to count number of ‘B’ and so on. Likewise
For the lower array.
Output should look like:
A: 0 a:2
B: 0 b:0
.
.
.
Z:0 z:0
Code
import java.awt.*;
import javax.swing.*;
import java.io.*;
import java.util.*;
public class Letter {
public static void main(String[] args) {
// this is get results
char[] chars = userEnters();
System.out.println();
System.out.println("Occurrences of each letter are:");
PrintArray(countLow(chars), countUp(chars));
}
public static char[] userEnters() {
String inputX = JOptionPane.showInputDialog("Enter line of text: ");
char[] chars = inputX.toCharArray();
return chars;
}
public static int[] countLow(char[] input) {
int[] counts = new int[26];
for (int i = 0; i < input.length; i++) {
counts[input[i] - 'a']++;
}
return counts;
}
public static int[] countUp(char[] input2) {
int[] countsUp = new int[26];
for (int i = 0; i < input2.length; i++) {
countsUp[input2[i] - 'A']++;
}
return countsUp;
}
public static void PrintArray(int[] counts, int[] countsUp) {
for (int i = 0; i < counts.length; i++) {
System.out.print(counts[i] + " " + (char) ('a' + i) + " ");
System.out.print(countsUp[i] + " " + (char) ('A' + i) + "\n");
}
}
}

If you enter a character that is not a large cap, countUp will throw an exception and if you enter a character that is not a small cap, countLow will throw an exception.
Example: if you call countLow on a A, you calculate 'A' - 'a' which returns -32 and a negative index is not allowed.
You need to review your logic and call either countLow or countUp depending on the case of the letter and filter invalid characters out.
Or refactor the whole thing and use a char[52] for example where you hold both small and large caps.

I Hope you don't mind I did refactor your code a bit.
Please have a look at this alterantive solution to your problem and then read the comments at the bottom of the answer.
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import javax.swing.JOptionPane;
public class LetterCounter {
//Hash maps don't allow duplication.
//The letter will be the Key and the repetitions the value(Your goal!)
private Map<Character, Integer> resultsMap = new HashMap<Character, Integer>();
public static void main(String[] args) {
LetterCounter letterCounter = new LetterCounter();
letterCounter.fillMap();
letterCounter.showMapContents();
}
private void showMapContents() {
for (Entry<Character, Integer> entry : resultsMap.entrySet())
{
System.out.println("'" + entry.getKey() + "' - " + entry.getValue() + " times");
}
}
private void fillMap() {
char[] userInputAsArray = getUserInputAsLetterArray();
for (int currentLetter = 0; currentLetter < userInputAsArray.length; currentLetter++) {
int count = getOccurences(userInputAsArray[currentLetter],userInputAsArray);
resultsMap.put(userInputAsArray[currentLetter], count);
}
}
private int getOccurences(int letter, char[] userInputAsArray) {
int counter = 0;
for (int currentIndex = 0; currentIndex < userInputAsArray.length; currentIndex++) {
if(userInputAsArray[currentIndex] == letter)
counter++;
}
return counter;
}
public char[] getUserInputAsLetterArray() {
String userInput = JOptionPane.showInputDialog("Enter line of text: ");
char[] chars = userInput.toCharArray();
return chars;
}
}
Whenever you want to do an exercise where you need to manipulate data, you should pick the best data structure for the job. In your case, I think the hash map could be interesting because it avoids duplicates and will do a big part of the job for you. Find a very good cheat sheet in this link: http://www.janeve.me/articles/which-java-collection-to-use
I noticed that you used a lot static and that is not a very Object Oriented thing to do. As an alternative, when you want to just on the run do some quick examples like this one, you can just initialize the class inside itself.
I hope this was useful.

You should probably consider moving from arrays to a more complex and powerful data structure like Map<Character,Integer>.
With that data structure the code you need would look something like
public Map<Character,Integer> countOccurrencies(String inputString){
Map<Character,Integer> occurrencies = new HashMap<Character,Integer>();
for(Character c : inputString){
if(occurrencies.containsKey(c)){
occurrencies.put(c, occurrencies.containsKey(c) + 1);
} else {
occurrencies.put(c, 1);
}
}
return occurrencies;
}

Answer in java:
Here, countOfOccurances("pppggggkkkkpgaaaa") gives you count of occurrences of each character from a String
public static void countOfOccurances(String mainStr)
{
String temp = "";
for (int i = 0 ; i < mainStr.length();i++)
{
CharSequence ch = String.valueOf(mainStr.charAt(i));
temp=mainStr.replace(ch, "");
int count = (mainStr.length()-temp.length());
System.out.println(ch+" = "+count);
mainStr = temp;
i = -1;
}
}
Output of method:
p = 4
g = 5
k = 4
a = 4