I am new in java. I have written a program in which the user chooses how many numbers he wants to add. If the user enters a string it will throw an exception and the programs tell the user to enter all details again.
My problem is i want the program to ask the user to re enter details from that number which he entered wrong.
Eg: the user chooses to add 4 numbers but he enters the third number as string, the program should ask the user to re-enter from the third number and not the entire details again.
My code is as follow:
import java.io.*;
import java.util.*;
class Add
{
public static void main(String args[]) throws Exception
{
boolean loop=true;
while(loop)
try
{
String yn;
do
{
Scanner s=new Scanner(System.in);
System.out.println("Enter how many numbers to add: ");
int num=Integer.parseInt(s.next());
int a,sum=0;
for(int i=1;i<=num;i++)
{
System.out.println("Enter number["+i+"]: ");
a=Integer.parseInt(s.next());
sum=sum+a;
}
System.out.println("The Sum is:"+sum);
System.out.println("Do you want to continue?(Y/N):");
yn=s.next();
} while(yn.equals("y")||yn.equals("Y"));
}
catch(Exception e)
{
System.out.println("Try Again\n");
}
}
}
You can resolve this, by placing the "retry" one level deeper in the code:
import java.io.*;
import java.util.*;
class Add {
public static void main(String args[]) throws Exception {
boolean loop=true;
Scanner s=new Scanner(System.in);
while(loop) {
try {
String yn;
do {
loop = true;
System.out.println("Enter how many numbers to add: ");
int num=Integer.parseInt(s.next());
int a,sum=0;
for(int i=1;i<=num;) {
try {
System.out.println("Enter number["+i+"]: ");
a=Integer.parseInt(s.next());
sum=sum+a;
i++;
}
catch(Exception e) {
System.out.println("Invalid input. Try Again.\n");
}
}
System.out.println("The Sum is:"+sum);
System.out.println("Do you want to continue?(Y/N):");
yn=s.next();
loop = yn.equals("y")||yn.equals("Y");
} while(loop);
}
catch(Exception e) {
System.out.println("Number of elements invalid. Try Again.\n");
}
}
}
}
As #Zhuinden indicates, one better uses Scanner.nextInt, since it is more generic...
Below code should help
int i =1;
while(i <=num){
System.out.println("Enter number["+i+"]: ");
try{
a=Integer.parseInt(s.next());
sum=sum+a;
i++;
}catch(NumberFormatException ex){
System.out.println("Please enter a valid interger");
}
}
Use additional method that reads single entry until it is valid number:
private static int readNumber(Scanner s) {
Integer value = null;
while (value == null) {
try {
value = Integer.parseInt(s.next());
} catch (NumberFormatException e) {
System.out.println("bad format, try again...");
}
}
return value;
}
Then you can use this method whenever you can read valid number:
System.out.println("Enter how many numbers to add: ");
int num = readNumber(s);
...
System.out.println("Enter number[" + i + "]: ");
a = readNumber(s);
....
Method getNumber(s) guarantee to return only when user give correct integer.
works according to what you want, but not ethical
import java.io.*;
import java.util.*;
class Add
{
public static void main(String args[]) throws Exception
{
boolean loop=true;int pos=0;int num=0,sum=0;
while(loop)
try
{
String yn;
do
{sum=0;num=0;
Scanner s=new Scanner(System.in);
System.out.println("Enter how many numbers to add: ");
num=Integer.parseInt(s.next());
int a;
for(int i=1;i<=num;i++)
{
System.out.println("Enter number["+i+"]: ");
a=Integer.parseInt(s.next());
sum=sum+a;pos=i;
}
System.out.println("The Sum is:"+sum);
System.out.println("Do you want to continue?(Y/N):");
yn=s.next();
} while(yn.equals("y")||yn.equals("Y"));
}
catch(Exception e)
{
Scanner s=new Scanner(System.in);
int a=0;
for(int i=pos+1;i<=num;i++)
{ System.out.println("Enter number["+i+"]: ");
a=Integer.parseInt(s.next());
sum=sum+a;}
System.out.println("The Sum is:"+sum);
}
}
}
Related
So basically I've been trying to get this small simple code to work but I'm running into the problem of making a loop. What I want to happen is basically this: User enters an Integer, if its not an integer it will display an error and ask for an Integer until and Integer is given. I'm having a difficult time setting up a loop cause I don't quite know what to do. Im pretty new and dumb so this is probably really easy but I'm kind of an idiot and suck at this but I'm learning.
Here's what I have.
import java.util.Scanner;
public class Loop{
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter an Integer: ");
if (scan.hasNextInt()) {
int Index = scan.nextInt();
scan.nextLine();
System.out.println("Index = " + Index);
}
else if (scan.hasNextDouble()) {
System.out.println("Error: Index is Double not Integer.");
}
else {
System.out.println("Error: Index is not Integer.");
}
}
}
You can use while loop for that.
while (true) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter an Integer: ");
if (scan.hasNextInt()) {
int Index = scan.nextInt();
scan.nextLine();
System.out.println("Index = " + Index);
break;
} else if (scan.hasNextDouble()) {
System.out.println("Error: Index is Double not Integer.");
} else {
System.out.println("Error: Index is not Integer.");
}
}
You need to use a loop (for or while) to your code. You can do it like this.
import java.util.Scanner;
public class Loop {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
while (true) {
System.out.println("Enter an Integer: ");
if (scan.hasNextInt()) {
int Index = scan.nextInt();
scan.nextLine();
System.out.println("Index = " + Index);
} else if (scan.hasNextDouble()) {
System.out.println("Error: Index is Double not Integer.");
} else {
System.out.println("Error: Index is not Integer.");
}
// add same condition to break the loop
}
// close the scanner
scan.close()
}
}
I have written a JAVA program that takes input from the user and check if the user has entered the correct thing or not. I have taken the input from the Scanner class. If the user enters an invalid character like a String, I want to display 'Invalid Input'.
public class Main {
public static void main(String[] args) {
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
int enteredNumber = takeInteger.nextInt();
}
}
Just ask the Scanner whether the next input is a valid int value, e.g.
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
while(!takeInteger.hasNextInt()) {
System.out.println("Invalid Input: " + takeInteger.next());
}
int enteredNumber = takeInteger.nextInt();
This will retry the operation until the user entered a number. If you just want a single attempt, use something like
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
if(!takeInteger.hasNextInt()) {
System.out.println("Invalid Input: " + takeInteger.next());
}
else {
int enteredNumber = takeInteger.nextInt();
// ... proceed with the input
}
You will get an Exception that is InputMismatchException when an invalid input is passed.(i.e except integer value),you can use a try-catch block to hold the exception and inform the user about the invalid input. Try block , Catch block
import java.util.*;
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
try{
int enteredNumber = takeInteger.nextInt();
}
catch(InputMismatchException e) {
System.out.println("Enter a valid input");
}
You can use Exception handling for the same.
public class Main {
public static void main(String[] args) {
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
try {
int enteredNumber = takeInteger.nextInt();
}
catch (Exception e) {
System.out.println("Invalid Input");
}
}
}
You can add a try-catch block in your program to check if the user's input is a number or not.
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
String input = takeInteger.next();
int enteredNumber;
try
{
enteredNumber = Integer.parseInt(input); // Input is a number
}
catch(NumberFormatException ex)
{
System.out.println("Wrong Input!"); // Invalid input
}
You need to call the .nextLine method of the Scanner class and then parse to the desired type.
Example:
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Enter a number");
String line = sc.nextLine();
try {
int enteredNumber = Integer.parseInt(line);
System.out.println("You have entered: " + enteredNumber);
} catch (Exception e) {
System.out.println("Invalid Input");
}
}
}
Result with a number:
Enter a number
12
You have entered: 12
Result a text:
Enter a number
abcd
Invalid Input
How can I use any alternative to 'goto' in java?
I tried using break label. But since I am not breaking out of any loop, it is giving undefined label error.
import java.io.*;
class $08_02_Total_Avg_Marks
{
public static void main(String args[]) throws IOException
{
//declare and initialize variables
int subNo = 0, totalMarks = 0;
float avg = 0.0F;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
label1:
System.out.println("Enter no. of subjects");
//check if input is integer
try
{
subNo = Integer.parseInt(br.readLine().trim());
}
catch(NumberFormatException e)
{
System.out.println("Please enter a whole number.");
//goto label1
}
int[] marksArray = new int[subNo];
for(int i=0; i<marksArray.length; i++)
{label2:
System.out.println("Enter marks for subject " + (i+1));
try
{
marksArray[i] = Integer.parseInt(br.readLine().trim());
}
catch(NumberFormatException e)
{
System.out.println("Please enter a whole number.");
//goto label2
}
}
}
}
I was terminating the program on invalid input. But I need to execute the same lines on invalid input.
Rather than wanting to go to a specific point explicitly, wrap the bit you might want to repeat in a loop. If you don't want to execute the loop again, break.
For the first one:
while (true) {
System.out.println("Enter no. of subjects");
//check if input is integer
try
{
subNo = Integer.parseInt(br.readLine().trim());
break;
}
catch(NumberFormatException e)
{
System.out.println("Please enter a whole number.");
// Nothing required to continue loop.
}
}
For the second one, wrap the loop body in loop:
for(int i=0; i<marksArray.length; i++)
{
while (true) {
System.out.println("Enter marks for subject " + (i+1));
try
{
marksArray[i] = Integer.parseInt(br.readLine().trim());
break;
}
catch(NumberFormatException e)
{
System.out.println("Please enter a whole number.");
}
}
}
Or, probably better, write a method wrapping this loop:
int getInt(BufferedReader br) throws IOException {
while (true) {
try
{
return Integer.parseInt(br.readLine().trim());
} catch(NumberFormatException e) {
System.out.println("Please enter a whole number.");
}
}
}
and then call this method:
System.out.println("Enter no. of subjects");
int subNo = getInt(br);
for(int i=0; i<marksArray.length; i++) {
System.out.println("Enter marks for subject " + (i+1));
marksArray[i] = getInt(br);
}
This code snippet will loop until a correct number is inserted, in this example (it solves your first goto problem)
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
boolean noNumberEntered; //Default on false
System.out.println("Enter no. of subjects");
//TODO: check if input is integer
while(!noNumberEntered){
try
{
subNo = Integer.parseInt(br.readLine().trim());
noNumberEntered = true;
}
catch(NumberFormatException e)
{
System.out.println("Please enter a whole number.");
}
}
I have reformatted your code a little bit. My basic idea was: All of the goto statements can be written in equivalent loops. The first one has now been made with a while loop, which terminates ones there comes NO exception. As for the second label, that has been done with the same mechanism (so a while-loop), however, with a label that can be exited/terminated with a "break + nameOfYourLable" - statement.
import java.io.*;
class $08_02_Total_Avg_Marks
{
public static void main(String args[]) throws IOException
{
//declare and initialize variables
int subNo = 0, totalMarks = 0;
float avg = 0.0F;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
boolean goToLabel1 = true;
while (goToLabel1) {
System.out.println("Enter no. of subjects");
//check if input is integer
try
{
subNo = Integer.parseInt(br.readLine().trim());
goToLabel1 = false; //parsing succeeded, no need to jump to label1
}
catch(NumberFormatException e)
{
System.out.println("Please enter a whole number.");
//goto label1
}
}
int[] marksArray = new int[subNo];
for(int i=0; i<marksArray.length; i++)
{
label2: while (true) {
System.out.println("Enter marks for subject " + (i+1));
try
{
marksArray[i] = Integer.parseInt(br.readLine().trim());
break label2;
}
catch(NumberFormatException e)
{
System.out.println("Please enter a whole number.");
}
}
}
}
}
You can use a do while loop and a boolean instead, like that :
class $08_02_Total_Avg_Marks
{
public static void main(String args[]) throws IOException
{
//declare and initialize variables
int subNo = 0, totalMarks = 0;
float avg = 0.0F;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
boolean goodEntry = true;
do {
goodEntry = true;
System.out.println("Enter no. of subjects");
//check if input is integer
try
{
subNo = Integer.parseInt(br.readLine().trim());
}
catch(NumberFormatException e)
{
System.out.println("Please enter a whole number.");
goodEntry = false;
}
} while(!goodEntry);
}
You can do the same with your second goto.
There are many ways to do that (with while loop and a boolean, with breaks...), but loops are better then goto.
I'm trying to create simple Menu for my program. I use InputMismatchException to catch exception when user enter char instead of integer but the program is looping when I enter char.
I left callMenu method empty but I'll fill it after solving this problem.
I am not sure is the mistake in the main menu or in catch part. When I do not write kb.nextInt() in catch section I'm getting infinitive loop. In another case it displayes mistake
public static void main(String[] args) {
do {
Menu.showMenu();
Menu.callMenu(Menu.getChoice());
} while (true);
}
}
import java.util.Scanner;
import java.util.Scanner.*;
import java.util.InputMismatchException;
public class Menu {
public static void showMenu() {
System.out.println("----------Menu----------");
System.out.println("1 - ");
System.out.println("2 - ");
System.out.println("0 - Exit");
}
public static int getChoice() {
Scanner kb = new Scanner(System.in);
System.out.print("\n Enter menu number:");
int choice = kb.nextInt();
while (!isValidChoice(choice)) {
try {
System.out.println("\n INVALID INPUT. Try again.");
System.out.print("Enter menu number: ");
choice = kb.nextInt();
} catch (InputMismatchException e) {
System.out.println("Error");
kb.next();
}
}
return choice;
}
public static boolean isValidChoice(int ch) {
return ch >= 0 && ch <= 9;
}
public static void callMenu(int menuNum) {
switch (menuNum) {
case 1:
break;
case 2:
break;
case 0:
System.exit(0);
}
}
}
I am not going to modify your code a lot. Here is what i can see wrong and what i think you can change
The point that your are getting the first input will throw an exception if you do not enter a integer. You do not have a catch block around it. So you should take that inside the try catch block to capture any invalid inputs
The main method will keep running infinitely when you check the value as "true" in while condition. Change that to false.
What i am providing you below assumes that your program will exit if an character/Non-Integer value in entered by displaying an error message.If the inputs are integers, your code will loop until one of your correct menu items are reached
I am only putting in the changes and not the whole code below. Rest remains same.
public static int getChoice() {
Scanner kb = new Scanner(System.in);
System.out.print("\n Enter menu number:");
int choice=-1;
try{
choice = kb.nextInt();
while (!isValidChoice(choice)) {
System.out.println("\n INVALID INPUT. Try again.");
System.out.print("Enter menu number: ");
choice = kb.nextInt();
}
}catch (InputMismatchException e) {
System.out.println("Error");
}
return choice;
}
In your main method
public static void main(String[] args) {
do {
Menu.showMenu();
Menu.callMenu(Menu.getChoice());
} while (false);
}
Hope this works out. Let me know
package exercises;
import java.util.*;
public class Try_and_catch {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int x=1;
do
{
System.out.println("Enter first number");
int n1 = input.nextInt();
System.out.println("Enter second number");
int n2 = input.nextInt();
int sum= n1/n2;
System.out.println(sum);
} while(x==1);
}
}
The code above requires input only integers, my question is how to handle the error whenever the user input a character?
Use a try block:
boolean again = true;
int n1;
while (again) {
try {
System.out.println("Enter first number");
input.nextInt();
again=false;
}
catch(InputMismatchException ime)
{
// do nothing!
}
}
What happens here is pretty simple: if we get an exception, then "again" is not set to true and we go back around in the loop. If we get out of the try block without an exception, then again is toggled and we go merrily on our way.