We Keep Coding

Scanner input and array duplicates verification causes an infinite loop

I code a program that lists entered names. But if the entered name is already in the array, the user has to reenter a different name.
I left a picture below that shows what is happening when I run the code
import java.util.Scanner;
public class list {
public static void main(String[] args) {
int numberofhumans;
Scanner al = new Scanner(System.in);
System.out.print("Enter the number of person: ");
numberofhumans = al.nextInt();
String[] list = new String[numberofhumans];
for (int i = 0; i < numberofhumans; i++) {
System.out.print("Enter the name:");
list[i] = al.nextLine();
for (int j = 0; j < i; j++) {
if (list[i] == list[j]) {
System.out.println("Name you just typed is already in your list. Enter a different name.");
i--;
}
}
}
for (String string : list) {
System.out.println(string);
}
al.close();
}
}

Your issue is this line:
if (list[i] == list[j])
This checks if the 2 objects are equal, which they are not even if their content is the same. You need to use .equals method of String object to compare 2 strings. That should fix the problem:
if (list[i].equals(list[j]))
I would rather use a Set of String instead of an array for a better performance since you won't have to iterate over and over again for each new input. Something like:
Set<String> set = new HashSet<>();
for (int i = 0; i < numberofhumans; i++) {
System.out.print("Enter the name: ");
String name = al.nextLine();
while (set.contains(name)) {
System.out.println("Name you just typed is already in your list. Enter a different name.");
System.out.print("Enter the name: ");
name = al.nextLine();
}
set.add(name);
}

Looking into your code, i found that you are using al.nextLine() which can return an empty String if the previous element the Scanner gave you was not a whole line. This is why you get prompted twice to enter a name when you run your program. See this post for further details.
Solution: use al.next() instead.
You are also comparing two String with the == operator which is comparing their adresses in memory. This comparison will always return false
Here is as working program:
import java.util.Scanner;
public class main {
public static void main(String[] args) {
int numberofhumans;
Scanner al = new Scanner(System.in);
System.out.print("Enter the number of person: ");
numberofhumans = al.nextInt();
String[] list = new String[numberofhumans];
for (int i = 0; i < numberofhumans; i++) {
System.out.print("Enter the name:");
list[i] = al.next();
for (int j = 0; j < i ; j++) {
if (list[i].equals(list[j])) {
System.out.println("Name you just typed is already in your list. Enter a different name.");
i--;
}
}
}
for (String string : list) {
System.out.println(string);
}
al.close();
}

Java prints "null" while taking in array of strings as input from the user using Scanner and ends abruptly

import java.util.Scanner;
class ArrayofArrays {
public static void main(String[] args) {
String[][] ListofNames = {
{"barbie","cinderella","troomtroom"},
{"wonderwoman","captainmarvel","Cheetah"}
};
for(String[] i : ListofNames) {
for(String x: i) {
System.out.println(x);
}
}
int r,c;
Scanner obj = new Scanner(System.in);
System.out.println("Enter rows\n");
r = obj.nextInt();
System.out.println("Enter columns\n");
c = obj.nextInt();
String[][] Inputnames = new String[r][c];
for(int j = 0;j<r;j++) {
for(int l = 0;l<c;l++) {
System.out.println("Enter name\n");
Inputnames[j][l] = obj.nextLine();
}
for(int m = 0;m<r;m++) {
for(int n = 0;n<c;n++) {
System.out.println(Inputnames[m][n]);
}
}
}
}
}
I was learning java and when I tried to to take in Array of Arrays consisting of strings as user input, it printed out null and didnt take further inputs. What am I missing out?
The output is like this on the cmd:
C:\Users\dynam\Desktop\Java Files>java ArrayofArrays
barbie
cinderella
troomtroom
wonderwoman
captainmarvel
Cheetah
Enter rows
2
Enter columns
1
Enter name
null
Enter name
batman
batman

Because the Scanner.nextInt method does not read the newline character in your input created after pressing "Enter," and so the call to Scanner.nextLine returns after reading that new line.
Also Looks like your for loops are messed up, try the following code (cleaned up a bit)-
package com.example.demo;
import java.util.Scanner;
class ArrayofArrays {
public static void main(String[] args) {
String[][] ListofNames = {
{"barbie", "cinderella", "troomtroom"},
{"wonderwoman", "captainmarvel", "Cheetah"}
};
for (String[] i : ListofNames) {
for (String x : i) {
System.out.println(x);
}
}
int r, c;
Scanner obj = new Scanner(System.in);
System.out.println("Enter rows\n");
r = obj.nextInt();
System.out.println("Enter columns\n");
c = obj.nextInt();
obj.nextLine();
String[][] Inputnames = new String[r][c];
for (int j = 0; j < r; j++) {
for (int l = 0; l < c; l++) {
System.out.println("Enter name\n");
Inputnames[j][l] = obj.nextLine();
}
}
for (int m = 0; m < r; m++) {
for (int n = 0; n < c; n++) {
System.out.println(Inputnames[m][n]);
}
}
}
}
Hope this helps!

I've seen this happen a lot in several questions. The issue is described here more in detail.
To fix your issue, replace the obj.nextInt() with Integer.parseInt(obj.nextLine()). So basically your code should look like the following:
System.out.println("Enter rows\n");
r = Integer.parseInt(obj.nextLine());
System.out.println("Enter columns\n");
c = Integer.parseInt(obj.nextLine());
If by any chance your will enter something else than an integer here, you might have to use a try-catch block to make sure you deal with NumberFormatExceptions.

How to insert a value in 2D array in java

How can I fill a 3x3 matrix using a 2D array such that the user picks what
position of the array they want to input their String value?
The position format is: (Row Number, Column Number)
For example:
Person 1, please pick where to place an X: (0,1)
Person 2, please pick where to place an O: (0,2)
This is what I have:
import java.util.Scanner;
public class idk {
public static void main(String[] args)
{
int i;
int j;
int arr[][] = new int[3][3];
// Getting user input
Scanner input = new Scanner(System.in);
System.out.println("Enter a number: ");
for (i = 0; i < 3; i++) {
for (j = 0; j < 3; j++) {
arr[i][j] = input.nextInt();
}
}
// Outputting the user input
System.out.println("The output is: ");
for (i = 0; i < 3; i++) {
for (j = 0; j < 3; j++) {
System.out.printf("%d ", arr[i][j]);
}
}
}
}

Something like the following has most of the parts you would want, except for error handling. Input is number, space (or any whitespace), finally followed by another number. Input numbers are 1 to 3 inclusive, which is what a normal person would expect.
import java.util.Scanner;
public class TicTacToe {
char board[][] = new char[][]{{'-','-','-'},{'-','-','-'},{'-','-','-'}};
public static void main(String[] args) {
TicTacToe ttt = new TicTacToe();
ttt.run();
}
public void run() {
Scanner input = new Scanner(System.in);
int row = -1, col = -1;//just to initialize
char symbol = 'o';
while (true) {
symbol = (symbol == 'x')?'o':'x';
boolean error = false;
System.out.println("Enter a number: ");
if (input.hasNext()) {
row = input.nextInt();
System.out.println("row: " + row);
} else {
error = true;
}
if (input.hasNext()) {
col = input.nextInt();
System.out.println("col: " + col);
} else {
error = true;
}
if (!error) {
board[row - 1][col - 1] = symbol;
}else{
System.out.println("an error has occurred");
}
input.reset();
this.drawBoard();
}
}
public void drawBoard() {
System.out.println("The output is: ");
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
System.out.printf("%c ", board[i][j]);
}
System.out.println();
}
System.out.println();
}
}
If you look up Scanner you will see an example to parse using a regex, I like that method better since with a regex you can validate the whole string at once but since that was not in the questions code I didn't use that method.

Simply
arr[Row Number][Column Number]=X;
eg
arr[0][1]=X;
or
arr[1][0]=O;
but because it is an int array you cannot place String i.e "O" and "X" in it.
Try making it an String array

How to print even and odd position characters of an array of strings in Java?

Question
Given a string S of length N, that is indexed from 0 to N-1, print it's even indexed and odd indexed characters as 2 space separated strings on a single line.
Assume input starts at index position 0(which is considered even)
Input
The first line contains an integer, T (the number of test cases).
Each line i of the T subsequent lines contain a String, S.
Output
For each string S, print it's even-indexed characters, followed by space, followed by odd-indexed characters.
Sample Input
2
Hacker
Rank
Sample Output
Hce akr
Rn ak
The Code I Wrote
public static void main(String[] args)
{
Scanner scan = new Scanner(System.in);
int T = scan.nextInt();
scan.nextLine();
for(int i=0 ; i<T ; i++)
{
String myString = scan.nextLine();
int evn = 0,
odd = 0,
len = myString.length();
char strE[] = new char[50],
strO[] = new char[50];
for(int j=0 ; j<len ; j++)
{
if(j%2 == 0)
{
strE[evn] = myString.charAt(j);
evn++;
}
if(j%2 == 1)
{
strO[odd] = myString.charAt(j);
odd++;
}
}
System.out.print(strE);
System.out.print(" ");
System.out.println(strO);
}
}
My Output
Hce akr
Rn ak
The Problem
As you can see, my program successfully meets the test case, and other test cases (using custom input) but every time the HackerRank compiler tells me that my program did not meet the test case.
Clearly, my program is producing the required output but every time the HackerRank compiler tells me that I got the solution wrong.
Could anyone please tell me where I am making a mistake?
Further Modifications
I then decided to change the last 3 lines of print statements into one statement as follows:
System.out.println(strE + " " + strO);
However, this time the program did not produce the desired output and rather printed some garbage values as follows:
[C#5c3f3b9b [C#3b626c6d
[C#3abc8690 [C#2f267610
My Doubts
1. In the first case, when I was printing the two strings separately using 2 print statements, I was getting a correct output everytime but the HackerRank compiler rejected it. Why?
2. In the second case, when I modified the program by using one print statement instead of 3 to get the desired result, the program gave a completely different output and rather printed garbage values! Why?
Here is a link to the HackerRank problem for more info:
hackerrank.com/challenges/30-review-loop
All help and guidance is greatly appreciated and thanks a lot in advance!

Try to submit this:
Scanner scan = new Scanner(System.in);
int T = scan.nextInt();
scan.nextLine();
for (int i = 0; i < T; i++) {
String myString = scan.nextLine();
String even = "";
String odd = "";
for (int j = 0; j < myString.length(); j++) {
if (j % 2 == 0) {
even += myString.charAt(j);
} else {
odd += myString.charAt(j);
}
}
System.out.println(even + " " + odd);
}
i get the right output and it should meet all the requirements. i think your code fails because its not a real string you print in the end and you have empty spots in your arrays

Scanner scanner = new Scanner(System.in);
System.out.println("Enter the no.of test-cases:");
int t = scanner.nextInt();
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter the String(s)");
for (int i = 0; i < t; i++) {
String myString = br.readLine();
String even = "";
String odd = "";
for (int j = 0; j < myString.length(); j++) {
if (j % 2 == 0) {
even += myString.charAt(j);
} else {
odd += myString.charAt(j);
}
}
System.out.println(even);
System.out.println(odd);
}
scanner.close();

int T = scan.nextInt();
This reads quantity of test cases, which we're going to process.
String string[] = new String[T];
for(int i = 0; i<T; i++){
string[i] = scan.next();
}
Next we're creating an array named "string" (BTW, this a bad name for variables/objects) which has size T and in the for loop reading test cases from the input T times and saving them in the array.
for(int temp = 0; temp<T; temp++){
Now, for each of test cases we do the following...
for(int j = 0; j<string[temp].length(); j = j+2)
{
System.out.print(string[temp].charAt(j));
}
We create a local variable j, which is visible only in this for loop. j holds index of the string (=string[temp]), which we're processing. So, we're printing a character on position j (by using standard method "charAt" of String class, which returns character of given index of the string) and then increasing it by 2. So, this code will print every even character. For string "example", it will print "eape" (j=0, j=2, j=4, j=6).
System.out.print(" ");
Separating sequences with a space.
for(int j = 1; j<string[temp].length(); j = j+2){
System.out.print(string[temp].charAt(j));
}
System.out.println();
We're doing the same (creating index j, running though all characters of the string), but starting from "1", so it will print all odd characters of the string. For string "example", it will give you "xml" (j=1, j=3, j=5). and After this, it will end the string. I hope, it will help you to understand. :)

I can solve your the second question:
---> System.out.print(strE);-->At the bottom, the method is called( public void print(char s[]));
-->System.out.println(strE + " " + strO);-->At the bottom, the method is called (public void println(String x) )

For your first answer I am unable to answer you as I have no idea about how the compiler works, but I can answer your second question.
The reason why System.out.print(strE); System.out.print(" "); System.out.println(strO); works is because System.out.print(char[]) and System.out.println(char[]) automatically turn the char arrays into a readable string before printing it.
However, in the second case System.out.println(strE + " " + strO);, what you are doing is directly turning the char array into strings, which just prints the class and the hash code of the array object because the toString() method is not overriden in the array class. What you want to do is System.out.println(new String(strE) + " " + new String(strO));. It will give you the result you want.

import java.io.*;
import java.util.*;
public class Solution {
private static void f(String s) {
// TODO Auto-generated method stub
char c[]=s.toCharArray();
int i,j;
for (i = 0; i <c.length;i++){
System.out.print(c[i]);
i+=1;
}
System.out.print(" ");
for (j = 1; j<c.length;j++){
System.out.print(c[j]);
j+=1;
}
}
public static void main(String[] args){
// TODO Auto-generated method stub
Scanner sc=new Scanner(System.in);
int s=sc.nextInt();
while(hasNext()){
//for loop for multiple strings as per the input
for(int m=0;m<= s;m++){
String s1=sc.next();
f(s1);
System.out.println();
}
}
}
}

I've solved this question in 2 ways & both are producing correct output.
Have a look & let me know if you've any problem.
Instead of using char array, you can use String
//char[] even = new char[10000];
String even = "";
Let's look at the code
private static Scanner scanner = new Scanner(System.in);
public static void main(String[] args) {
String s = scanner.next();
char[] array = s.toCharArray();
int count=0;
//char[] even = new char[10000];
//char[] odd = new char[10000];
String even = "";
String odd = "";
for(char ch : array){
if(count%2 == 0){
even = even + ch;
}else{
odd = odd + ch;
}
count++;
}
count = 0;
System.out.println(even + " " + odd);
}
Output:
hacker
hce akr
No need of extra char[] or String to store even & odd position characters, we can directly print them using appropriate condition.
private static Scanner scanner = new Scanner(System.in);
public static void main(String[] args){
String s = scanner.next();
char[] array = s.toCharArray();
int count=0;
for(char ch : array){
if(count%2 == 0){
System.out.print(ch);
}
count++;
}
count = 0;
System.out.print(" ");
for(char ch : array){
if(count%2 != 0){
System.out.print(ch);
}
count++;
}
count = 0;
}
Output:
hacker
hce akr

Try this:
public static void main(String[] args) {
System.out.println("Enter string to check:");
Scanner scan = new Scanner(System.in);
String T = scan.nextLine();
String even = "";
String odd = "";
for (int j = 0; j < T.length(); j++) {
if (j % 2 == 0) { //check the position of the alphabet by dividing it by 0
even += T.charAt(j);
} else {
odd += T.charAt(j);
}
}
System.out.println(even + " " + odd);
scan.close();
}

** JavaScript version **
function processData(input) {
for (let i = 1; i < input.length; i++) {
printOutput(input[i]);
}
}
function printOutput(input) {
var result = [];
input.length % 2 == 0 ? result[input.length / 2] = ' ': result[Math.ceil(input.length / 2)] = ' ';
for (let i = 0; i < input.length; i++) {
if (i % 2 == 0) {
result[i / 2] = input[i];
}
else {
result[Math.ceil(input.length / 2) + Math.ceil(i / 2)] = input[i];
}
}
console.log(result.join(''));
}
process.stdin.on("end", function () {
processData(_input.split('\n'));
});

import java.io. * ;
import java.util. * ;
public class Solution {
String myString;
public Solution(String myString) {
this.myString = myString;
int len = myString.length();
for (int j = 0; j < len; j++) {
if (j % 2 == 0) {
System.out.print(myString.charAt(j));
}
}
System.out.print(" ");
for (int j = 0; j < len; j++) {
if (j % 2 == 1) {
System.out.print(myString.charAt(j));
}
}
}
public static void main(String[] args) {
/* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
Scanner sc = new Scanner(System. in );
int T = sc.nextInt();
for (int i = 0; i < T; i++) {
String word = sc.next();
Solution sol = new Solution(word);
System.out.println();
}
sc.close();
}
}

import java.io.*;
import java.util.*;
public class Solution {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
int T;
T = s.nextInt();
String[] str = new String[T];
int i;
for(i=0;i<T;i++) {
str[i] = s.next();
}
for(i=0;i<T;i++) {
char[] even = new char[5000];
char[] odd = new char[5000];
int ev =0,od=0;
for(int j= 0;j< str[i].length();j++) {
if(j%2== 0) {
even[ev] = str[i].charAt(j);
ev++;
}else {
odd[od] = str[i].charAt(j);
od++;
}
}
String strEven = new String(even);
String strOdd = new String(odd);
System.out.print(strEven.trim());
System.out.print(" ");
System.out.println(strOdd.trim());
}
s.close();
}
}
I am sure that this will work You've forgotten to convert it to a string and also increase the size of the character array

import java.io.*;
import java.util.*;
public class Solution {
public static void main(String[] args) {
Scanner scan= new Scanner(System.in);
int n= scan.nextInt();
for(int i=0;i<n;i++){
String s= scan.next();
int len= s.length();
StringBuffer str_e= new StringBuffer();
StringBuffer str_o= new StringBuffer();
for(int j=0;j<len;j++){
if(j%2==0)
str_e= str_e.append(s.charAt(j));
if(j%2==1)
str_o= str_o.append(s.charAt(j));
}
System.out.println(str_e+" "+str_o);
}
}
}

Try this:
import java.io.*;
import java.util.*;
public class Solution {
public static void main(String[] args) {
/* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
Scanner pp=new Scanner(System.in);
int n=pp.nextInt();
for(int i=0; i<n; i++)
{
String ip=pp.next();
String re1="",
re2="";
for(int j=0; j<ip.length(); j++)
{
if(j%2 == 0)
{
re1+= ip.charAt(j);
}
if(j%2 == 1)
{
re2+= ip.charAt(j);
}
}
System.out.print(re1+" "+re2);
System.out.println("");
}
}
}

public class PrintCharacters{
public static void main(String args[]) {
Scanner sc = new Scanner(System.in);
int noOfTestCases = sc.nextInt();
sc.nextLine();
String []inputStrings= new String[noOfTestCases];
for(int i=0;i<noOfTestCases;i++) {
inputStrings[i]=sc.nextLine();
}
for(String str: inputStrings) {
String even ="";
String odd ="";
for(int i=0;i<str.length();i++) {
if(i%2==0) {
even+=str.charAt(i);
}else {
odd+=str.charAt(i);
}
}
System.out.println(even+" "+odd);
}
sc.close();
}
}
Input:
2
Hacker
Rank
Output:
Hce akr
Rn ak

import java.util.*;
public class Solution {
public static void main(String[] args) {
/* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
Scanner scan=new Scanner(System.in);
int n=scan.nextInt();
while(n>0) {
String str=scan.next();
for(int i=0;i<str.length();i++) {
if(i%2==0) {
System.out.print(str.charAt(i));
}
}
System.out.print(" ");
for(int i=0;i<str.length();i++) {
if(i%2==1) {
System.out.print(str.charAt(i));
}
}
n--;
System.out.println();
}
}
}

Not sure where to insert a for loop

I need to code a program that asks the user for the number of spaces between symbols.For eg,
& & &
& & &
& & &
The user will enter an integer and the spacing between the symbols should change.
I have the following code uptil now:
import java.util.Scanner;
public class Spacing
{
public static void main(String[]args)
{
Scanner c=new Scanner(System.in);
System.out.println("Enter spaces between stars: ");
int l=c.nextInt();
String a="*";
String b=" ";
for(int i=0;i<5;i++)
{
for(int j=0;j<l;j++)
{
System.out.print(a+b);
}
System.out.println();
}
}
}
I know how to change the number of symbols and the number of lines.But the problem is how to change the number of spaces.I feel as if there's going to be a for loop involved but I have no clue how to put one in.
Any help would be appreciated.

Here's something that improves readability and introduces some modularity in your code:
import java.util.Scanner;
public class Spacing {
public static void main(String[]args) {
Scanner scanner = new Scanner(System.in);
System.out.println("Enter spaces between stars: ");
int numSpaces =scanner.nextInt();
String charToDisplay = "*";
String spaces = buildNSpaces(numSpaces);
int numberOfRows = 3;
int numberOfCharactersPerRow = 3;
for(int i = 0; i < numberOfRows; i++) {
for(int j=0; j < numberOfCharactersPerRow; j++) {
System.out.print(charToDisplay+spaces);
}
System.out.println();
}
}
private static String buildNSpaces(final int numSpaces) {
StringBuilder builder = new StringBuilder();
for(int i = 0; i < numSpaces; i++) {
builder.append(" ");
}
return builder.toString();
}
}

for(int i = 0;i < l;i++)
{
System.out.print(" ");
}
code above will print the number of spaces you input.
so you can use it like:
for(int i = 0; i < 3; i++)
{
System.out.print("&");
for(int j = 0; j < l; j++)
System.out.print(" ");
}
this will print out
& & & (there's 3 spaces after the last & also)
if you input 3 in l.

you need another variable and another for loop
try this one
public class Spacing
{
public static void main(String[]args)
{
Scanner c=new Scanner(System.in);
System.out.println("Enter spaces between stars: ");
int l=c.nextInt();
String a="*";
String b=" ";
for(int i=0;i<5;i++)
{
for(int j=0;j<3;j++)
{
System.out.print(a);
for(int k=0;k<l;k++)
{
System.out.print(b);
}
}
System.out.println();
}
}
}