Develop Reference

How to check if the System.in contains only numbers

Have a task to get N numbers from console, find the longest and the shortest ones and their length. The task is not difficult and works correctly, but I decided to make a check, if the console input corresponds the conditions of the task:
Are there only Integer numbers.
Are the exactly N numbers, not more/less.
I decided to write a boolean method isInputCorrect(), which would take the Scanner and check if the input is correct, but it doesn't work correctly.
public static void main(String[] args) {
int n = 5;
Scanner sc = new Scanner(System.in);
do {
System.out.println("Hello, please enter " + n + " integer numbers:");
while (!isInputCorrect(sc,n)){
System.out.println("Wrong! Try again:");
sc.next();
}
} while (!isInputCorrect(sc, 5));
String scLine = sc.nextLine();
String[] arr = scLine.split("\\s+");
String maxLengthNum = arr[0];
String minLengthNum = arr[0];
for (int i = 1; i < arr.length; i++){
if (maxLengthNum.length() < arr[i].length()){
maxLengthNum = arr[i];
}
if (minLengthNum.length() > arr[i].length()){
minLengthNum = arr[i];
}
}
String equalMaxNum = "";
String equalMinNum = "";
int countMax = 0;
int countMin = 0;
for (String s : arr){
if (maxLengthNum.length() == s.length()){
countMax += 1;
equalMaxNum += s + " ";
}
if (minLengthNum.length() == s.length()){
countMin += 1;
equalMinNum += s + " ";
}
}
if (countMax > 1){
System.out.println("The longest numbers are: " + equalMaxNum + " Their length is: " + maxLengthNum.length());
}
else {
System.out.println("The longest number is: " + maxLengthNum + " Its length is: " + maxLengthNum.length());
}
if (countMin > 1){
System.out.println("The shortest numbers are: " + equalMinNum + " Their length is: " + minLengthNum.length());
}
else {
System.out.println("The shortest number is: " + minLengthNum + " Its length is: " + minLengthNum.length());
}
}
public static boolean isInputCorrect(Scanner sc, int n){
boolean flag = true;
for (int i = 0; i < n; i++){
if (sc.hasNextInt()){
sc.next();
}else {
flag = false;
break;
}
}
return flag;
}
EDIT
This code still doesn't work. I realize, that the problem is in isDigit(). And exactly in regular conditions in last if statement. It is something like this:
public static boolean isDigit (String input, int n){
String[] arr = input.split("\\s+");
boolean flag = false;
if (arr.length != n){
flag = false;
}
else {
for (String s : arr) {
if (s.startsWith("-")) {
if (s.substring(1).matches("[0-9]*")) {
flag = true;
}
} else if (s.matches("[0-9]*")) {
flag = true;
} else {
flag = false;
}
}
}
return flag;
}
This method takes the console input as a string, then it checks, how many numbers(strings) does it contain, are there any negative numbers and so on. But it can be applied only for substrings(words without whitespace). In my case it can be applied to arr[i].
So I modified it to split String into array[] and tried to check every single element. I've got:
public static boolean isDigit (String input, int n){
String[] arr = input.split("\\s+");
boolean flag = false;
if (arr.length != n){
flag = false;
}
else {
for (String s : arr) {
if (s.startsWith("-")) {
if (s.substring(1).matches("[0-9]*")) {
flag = true;
}
} else if (s.matches("[0-9]*")) {
flag = true;
} else {
flag = false;
}
}
}
return flag;
}
but it returnes true even when input is:
1 3213 w 15 3
I can't understand, what is the problem? The full code is:
public static void main(String[] args) {
int n = 5;
boolean validInput = false;
String input;
do {
System.out.println("Please enter " + n + " integer numbers:");
Scanner sc = new Scanner(System.in);
input = sc.nextLine();
if (isDigit(input, n)) {
validInput = true;
} else {
System.out.println("Wrong input! Try again: ");
}
}
while (!validInput);
String[] arr = input.split("\\s+");
String maxLengthNum = arr[0];
String minLengthNum = arr[0];
for (int i = 1; i < arr.length; i++){
if (maxLengthNum.length() < arr[i].length()){
maxLengthNum = arr[i];
}
if (minLengthNum.length() > arr[i].length()){
minLengthNum = arr[i];
}
}
String equalMaxNum = "";
String equalMinNum = "";
int countMax = 0;
int countMin = 0;
for (String s : arr){
if (maxLengthNum.length() == s.length()){
countMax += 1;
equalMaxNum += s + " ";
}
if (minLengthNum.length() == s.length()){
countMin += 1;
equalMinNum += s + " ";
}
}
if (countMax > 1){
System.out.println("The longest numbers are: " + equalMaxNum + " Their length is: " + maxLengthNum.length());
}
else {
System.out.println("The longest number is: " + maxLengthNum + " Its length is: " + maxLengthNum.length());
}
if (countMin > 1){
System.out.println("The shortest numbers are: " + equalMinNum + " Their length is: " + minLengthNum.length());
}
else {
System.out.println("The shortest number is: " + minLengthNum + " Its length is: " + minLengthNum.length());
}
}
public static boolean isDigit (String input, int n){
String[] arr = input.split("\\s+");
boolean flag = false;
if (arr.length != n){
flag = false;
}
else {
for (String s : arr) {
if (s.startsWith("-")) {
if (s.substring(1).matches("[0-9]*")) {
flag = true;
}
} else if (s.matches("[0-9]*")) {
flag = true;
} else {
flag = false;
}
}
}
return flag;
}
SOLVED
Thanks to everybody, your help was really usefull. I finally found the problem
It was in isDigit() method. It was checking out every element of array, and switched a flag according to the last result. I wrote "break" to stop the further checking if there was at least one false flag in loop.
public static boolean isDigit (String input, int n){
String[] arr = input.split("\\s+");
boolean flag = false;
if (arr.length != n){
flag = false;
}
else{
for (String s: arr){
if (s.startsWith("-")) {
if (s.substring(1).matches("[0-9]*")){
flag = true;
}
else {
flag = false;
break;
}
}
else {
if (s.matches("[0-9]*")){
flag = true;
}
else {
flag = false;
break;
}
}
}
}
return flag;
}

You may use regular expressions to verify that the input contains only integer numbers:
int n = 5;
// ... your current code
String scLine = sc.nextLine();
if (!scLine.matches("\\d+(?:\\s+\\d+)*")) {
throw new IllegalArgumentException("input contained non-integers");
}
String[] arr = scLine.split("\\s+");
if (arr.length != n) {
throw new IllegalArgumentException("found " + arr.length + " number inputs but expected " + n + ".");
}

you can check if input string has only numbers as below
public boolean isDigit(String input) {
if (input == null || input.length() < 0)
return false;
input = input.trim();
if ("".equals(input))
return false;
if (input.startsWith("-")) {
return input.substring(1).matches("[0-9]*");
} else {
return input.matches("[0-9]*");
}
}
EDIT:
allowing user to re-enter untill valid number is entered
boolean validInput = false;
do
{
System.out.println("Enter the number ");
// get user input
String input sc.nextLine();
if(isDigit(input))
validInput = true;
else
System.out.println("Enter valid Number");
}
while (!validInput );

I am trying to make a program which accepts two numbers and finds the prime, even and odd numbers in between

I am trying to make a program which will accept a starting number and ending number (int) and find the prime, even and odd numbers in between. I am not able to set my if and else conditions properly. It would be great if someone could even tell me a simpler method.
package proj2;
import java.util.Scanner;
public class Proj2
{
public void number(int sn,int en)
{
if (sn <= en)
{
int number = 2;
if (sn > number && sn % number != 0)
{
System.out.println(sn + " : " + "Prime");
number++;
sn++;
number(sn++,en);
}
else if (sn % 2 == 0)
{
System.out.println(sn + " : " + "Even");
sn++;
number(sn++,en);
}
else
{
System.out.println(sn + " : " + "Odd");
sn++;
number(sn,en);
}
}
}
public static void main(String [] abc)
{
Scanner sc = new Scanner(System.in);
System.out.println("Enter the starting number:");
int x = sc.nextInt();
System.out.println("Enter the ending number:");
int y = sc.nextInt();
Proj2 p = new Proj2();
p.number(x,y);
}
}

Being prime and odd are not mutually exclusive, your primality test is incorrect, and you are recursively calling number(). Instead use:
private boolean isPrime(int num){
// assuming positive ints
if( num <= 2 || num % 2 == 0)
return false;
for(int divisor = 3; divisor <= (int)Math.pow(num, 0.5); divisor+=2)
if( num % divisor == 0)
return false;
return true
}
public void number(int start, int end){
if( start <= end ){
for( int curr = start; curr <= end; curr++ ){
if( curr % 2 == 1 ) {
System.out.println(curr + " : Odd");
if(isPrime(curr))
System.out.println(curr + " : Prime");
} else System.out.println(curr + " : Even");
}
}

Validating against duplicate user entries when using sets

I am currently studying Java and have being using normal arrays so far. Next semester we will be using data structures like ArrayList etc, but i decided to read ahead. I have read that for storing data that can not be a duplicate, Sets were the data structure of choice, but in the code below the user can still enter a duplicate entry? Can any body explain the process to me and perhaps a solution to my problem?
public class Lotto {
private static final int INPUT_SIZE = 6;
private static final int MIN_NUMBER_POSSIBLE = 0;
private static final int MAX_NUMBER_POSSIBLE = 25;
private Set<Integer> userNumbers = new HashSet<Integer>();
private Set<Integer> randomNumbers = new HashSet<Integer>();
public static void main(String[] args) {
Lotto c = new Lotto();
c.generateRandomNumbers();
System.out.println("Pick " + INPUT_SIZE + " numbers from "
+ MIN_NUMBER_POSSIBLE + " to " + MAX_NUMBER_POSSIBLE + ".");
c.readUserNumbers();
if (c.doUserNumbersMatchRandomNumbers()) {
System.out.println("You win :) !");
} else {
System.out.println("Sorry you failed :( !");
c.showRandomNumbersToUser();
}
}
private void generateRandomNumbers() {
Random random = new Random();
for (int i = 0; i < INPUT_SIZE; i++) {
randomNumbers.add(random.nextInt(MAX_NUMBER_POSSIBLE));
}
}
private void showRandomNumbersToUser() {
System.out.println("\nRandom numbers where : ");
for (Integer randomNumber : randomNumbers) {
System.out.println(randomNumber + "\t");
}
}
private void readUserNumbers() {
Scanner input = new Scanner(System.in);
int inputSize = 1;
while (input.hasNextInt() && inputSize < INPUT_SIZE) {
int numberChoosen = input.nextInt();
if (numberChoosen < MIN_NUMBER_POSSIBLE
|| numberChoosen > MAX_NUMBER_POSSIBLE) {
System.out.println("Your number must be in "
+ MIN_NUMBER_POSSIBLE + " - " + MAX_NUMBER_POSSIBLE
+ " range.");
} else {
userNumbers.add(numberChoosen);
inputSize++;
}
}
}
private boolean doUserNumbersMatchRandomNumbers() {
for (Integer userNumber : userNumbers) {
if (!randomNumbers.contains(userNumber)) {
return false;
}
printMatchingNumber(userNumber);
}
return true;
}
private void printMatchingNumber(int num) {
System.out.println("Your number, " + num + ", has been called.");
}
}

Set#add(Object) returns true when the object was added successfully and false otherwise. It will not throw an exception, so you need to add a conditional to check whether the operation was successful:
if (userNumbers.add(numberChoosen)) {
System.out.println("Number added successfully");
} else {
System.out.println("Duplicate number detected");
}

Sets do not prohibit entering a unique value more than once. It is on you to check preconditions, see e.g Set.add.
So instead of just calling:
userNumbers.add(numberChoosen);
try
if (!userNumbers.contains(numberChoosen)) {
userNumbers.add(numberChoosen);
} else {
// do stuff...
}

GUI to check for prime numbers

I have updated the code to reflect the BigInteger num. But I am now receiving errors with my display message. Syntax error is under IsPrime. How can I change this string output to get the correct display message to the user? Here is my code:
public static void main (String [] args){
//prompt user to input a number
String input = JOptionPane.showInputDialog("Enter number ");
// change string to int
int number = Integer.parseInt(input);
//display message to user of their results
BigInteger num = new BigInteger(input);
String output = number + " is" + (BigInteger(input) ? " " : " not ") + "a prime number: " + BigInteger(input);
JOptionPane.showMessageDialog (null, output);
}
public static Boolean IsPrime(BigInteger num) {
// check if number is a multiple of 2
if (num.mod(new BigInteger("2")).compareTo(BigInteger.ZERO) == 0) {
return false;
}// if not, then just check the odds
for (BigInteger i = new BigInteger("3"); i.multiply(i).compareTo(num) <= 0; i =
i.add(new BigInteger("2"))) {
if (num.mod(i).compareTo(BigInteger.ZERO) == 0) {
return false;
}
}
return true;
}
}

Your IsPrime method takes a big int - but does all the checking with standard ints - that's going to cause problems if your user wants to be difficult. This is working code for checking prime numbers for general BigIntegers.
public static Boolean IsPrime(BigInteger num) {
// check if number is a multiple of 2
if (num.mod(new BigInteger("2")).compareTo(BigInteger.ZERO) == 0) {
return false;
}// if not, then just check the odds
for (BigInteger i = new BigInteger("3"); i.multiply(i).compareTo(num) <= 0; i =
i.add(new BigInteger("2"))) {
if (num.mod(i).compareTo(BigInteger.ZERO) == 0) {
return false;
}
}
return true;
}

Try this one to check for prime number
boolean isPrime(int n) {
for(int i=2;2*i<n;i++) {
if(n%i==0)
return false;
}
return true;
}
A more faster way to check for prime number
boolean isPrime(int n) {
//check if n is a multiple of 2
if (n%2==0) return false;
//if not, then just check the odds
for(int i=3;i*i<=n;i+=2) {
if(n%i==0)
return false;
}
return true;
}
--EDIT--
As per your updated question and comments
Try this one
String output = number + " is" + (isPrime(number) ? " " : " not ") + "a prime number.";

Credit card type and validation

I would like to run a program that can determine the validation and type of credit card number based of number entered. Compiler shows notification that there is an error in my coding but I cannot detect where is it. The program is also cannot be run. Below is the coding,
import java.util.*;
public class CreditCard {
public static void main(String args[]) {
String CType;(String number) {
if (number.startsWith("4"))
return "Visa";
else if (number.startsWith("5"))
return "MasterCard";
else if (number.startsWith("6"))
return "Discover";
else if (number.startsWith("37"))
return "American Express";
else
return "Unknown type";
};
Scanner input = new Scanner(System.in);
System.out.println("Enter a credit card number: ");
long number = input.nextLong();
long total = sumOfEvenPlaces(number) + (sumOfOddPlaces(number)*2);
if (isValid(total)) {
System.out.println("The "+CType+" card number is valid");
} else {
System.out.println("The "+CType+" card number is invalid.");
}
}
public static boolean isValid(long total) {
if (total % 10 != 0) {
} else {
return true;
}
return false;
}
public static int sumOfEvenPlaces(long number) {
int sum = 0;
int remainder;
while (number % 10 != 0 || number / 10 != 0) {
remainder = (int) (number % 10);
sum = sum + getDigit(remainder * 2);
number /= 100;
}
return sum;
}
public static int getDigit(int number) {
if (number > 9) {
return (number % 10 + number / 10);
}
return number;
}
public static int sumOfOddPlaces(long number) {
int sum = 0;
int remainder;
number /= 10;
while (number % 10 != 0 || number / 10 != 0) {
remainder = (int) (number % 10);
sum = sum + getDigit(remainder * 2);
number /= 100;
}
return sum;
}
}

I do card type detection with an enum:
package com.gabrielbauman.gist;
import java.util.regex.Pattern;
public enum CardType {
UNKNOWN,
VISA("^4[0-9]{12}(?:[0-9]{3}){0,2}$"),
MASTERCARD("^(?:5[1-5]|2(?!2([01]|20)|7(2[1-9]|3))[2-7])\\d{14}$"),
AMERICAN_EXPRESS("^3[47][0-9]{13}$"),
DINERS_CLUB("^3(?:0[0-5]\\d|095|6\\d{0,2}|[89]\\d{2})\\d{12,15}$"),
DISCOVER("^6(?:011|[45][0-9]{2})[0-9]{12}$"),
JCB("^(?:2131|1800|35\\d{3})\\d{11}$"),
CHINA_UNION_PAY("^62[0-9]{14,17}$");
private Pattern pattern;
CardType() {
this.pattern = null;
}
CardType(String pattern) {
this.pattern = Pattern.compile(pattern);
}
public static CardType detect(String cardNumber) {
for (CardType cardType : CardType.values()) {
if (null == cardType.pattern) continue;
if (cardType.pattern.matcher(cardNumber).matches()) return cardType;
}
return UNKNOWN;
}
}
You can then do CardType.detect("cardnumbergoeshere") and you'll get back CardType.VISA, etc.
There's a unit test over at the gist.
For validation, I have:
public boolean isValid(String cardNumber) {
int sum = 0;
boolean alternate = false;
for (int i = cardNumber.length() - 1; i >= 0; i--) {
int n = Integer.parseInt(cardNumber.substring(i, i + 1));
if (alternate) {
n *= 2;
if (n > 9) {
n = (n % 10) + 1;
}
}
sum += n;
alternate = !alternate;
}
return (sum % 10 == 0);
}
That should do it.
Edit: fixed escape characters in DINERS_CLUB

This may be more along the lines of what you're trying to do:
public static void main(final String args[])
{
String cType = null;
System.out.println("Enter a credit card number: ");
final Scanner input = new Scanner(System.in);
final String cardNumber = input.next();
if (cardNumber.startsWith("4"))
{
cType = "Visa";
}
else if (cardNumber.startsWith("5"))
{
cType = "MasterCard";
}
else if (cardNumber.startsWith("6"))
{
cType = "Discover";
}
else if (cardNumber.startsWith("37"))
{
cType = "American Express";
}
else
{
cType = "Unknown type";
}
final long total = sumOfEvenPlaces(Long.valueOf(cardNumber)) + (sumOfOddPlaces(Long.valueOf(cardNumber)) * 2);
if (isValid(total))
{
System.out.println("The " + cType + " card number is valid");
}
else
{
System.out.println("The " + cType + " card number is invalid.");
}
}
On a stylistic note, CType should start with a lower case letter (e.g. cType). You'll have to experiment with the use of Scanner as well as I'm not sure my implementation will do what you're looking for.