Develop Reference

How can I avoid my loop to take the previous answer of the user?

I am making a program that will take a user's input on how many numbers he wants and determine the highest number between the given. After that the user will be prompt with a Yes or no question. If the user decides to say yes, the program will loop again and if not, the program will end. Now my question is why does it take the highest number from the previous run?
import java.util.Scanner;
public class IT_VILLAFLOR_Lab1_Prog2
{
public static void main(String[] Args){
int num=1,num2,Largest=0,max;
char YN;
Scanner sc = new Scanner(System.in);
System.out.print("Enter the Max Number = ");
max = sc.nextInt();
for(num=1; num<=max; num++)
{
System.out.print("Enter Number " + num + ": ");
num2 = sc.nextInt();
if(Largest<num2)
{
Largest=num2;
}
else if(num==max)
{
System.out.println("The Biggest number is " + Largest );
System.out.print( "Do you want to try again? Y/N ");
YN = sc.next().charAt(0);
if(YN =='Y'|| YN =='y')
{
num=0;
System.out.print('\f');
System.out.print("Enter the Max Number " );
max = sc.nextInt();
}
else
{
System.exit(0);
}
}
}
}
}

If the user wants to continue, you are resetting num to 0. Along with this, Largest also needs to be reset to 0.
num=0;
Largest=0; //new code
By the way, you need to change the line else if(num==max) to if(num==max) . Try the test case with max of 2 and values as 12 ,23.

Exception Thread in Do-While Loop

I'm working on a project that calculates the value of a bank account based on starting balance(b), interest rate(IR), and quarters to display. My entire code works perfectly, but the very last portion is to make sure the variables like interest rate are within the confines of the boundaries my professor gave me. I do need to display an error message if the user enters a value outside the boundaries and ask for the value again.
For example, the number of quarters to display needs to be greater than zero, and less or equal to 10.
As you can see, pretty much all of my program is in a do-while loop. I know I can have nested loops, but what would I be able to put in my do-while loop that would work in this situation? An if-else statement? Try and catch block? Another while loop?
If I used a try-catch, then could anyone give me an example of how I could do that? Thank you very much for your time, and all help is appreciated! The below is my code for reference.
import java.util.Scanner;
public class InterestCalculator
{
public static void main(String[] args)
{
Scanner scannerObject = new Scanner(System.in);
Scanner input = new Scanner(System.in);
int quartersDisplayed;
double b, IR;
do
{
Scanner keyboard=new Scanner(System.in);
System.out.println("Enter the numbers of quarters you wish to display that is greater than zero and less or equal to 10: ");
quartersDisplayed = keyboard.nextInt();
System.out.println("Next enter the starting balance. ");
System.out.println("This input must be greater than zero: ");
b = keyboard.nextDouble();
System.out.println("Finally, enter the interest rate ");
System.out.println("which must be greater than zero and less than or equal to twenty percent: ");
IR = keyboard.nextDouble();
System.out.println("You have entered the following amount of quarters: " + quartersDisplayed);
System.out.println("You also entered the starting balance of: " + b);
System.out.println("Finally, you entered the following of interest rate: " + IR);
System.out.println("If this information is not correct, please exit the program and enter the correct information.");
double quarterlyEndingBalance = b + (b * IR/100 * .25);
System.out.println("Your ending balance for your quarters is " + quarterlyEndingBalance);
System.out.println("Do you want to continue?");
String yes=keyboard.next("yes");
if (yes.equals(yes))
continue;
else
break;
}
while(true);
}
}

So here's some code to answer your questions and help get you started. However, there are problems with your logic that do not pertain to your question which I will address afterward.
Note: I have added comments to your code. Most of them start with "EDIT:" so that you can tell what I changed. I didn't use this prefix in all cases because some of it is new code and it's obviously my comment
import java.util.Scanner;
public class InterestCalculator {
public static void main(String[] args) {
// EDIT: you already have a scanner defined below with a more meaningful name so I removed this one
// Scanner scannerObject = new Scanner(System.in);
Scanner input = new Scanner(System.in);
//EDIT: defining userResponse outside the loop so we can use it everywhere inside
String userResponse = null;
do {
//EDIT: moved the variables inside the loop so that they are reset each time we start over.
//EDIT: initialize your variable to a value that is invalid so that you can tell if it has been set or not.
int quartersDisplayed = -1;
//EDIT: gave your variables more meaningful names that conform to java standards
double startingBalance = -1, interestRate = -1;
//EDIT: you don't need a second Scanner, just use the one you already have.
// Scanner keyboard = new Scanner(System.in);
do{
System.out.println("Enter the numbers of quarters you wish to display that is greater than zero and less or equal to 10: ");
userResponse = input.next();
try{
quartersDisplayed = Integer.parseInt(userResponse);
}catch(NumberFormatException e){
//nothing to do here, error message handled below.
}
if(quartersDisplayed <= 0 || quartersDisplayed > 10){
System.out.println("Sorry, that value is not valid.");
}else{
break;
}
}while(true);
do{
System.out.println("Enter the starting balance (must be greater than zero): ");
userResponse = input.next();
try{
startingBalance = Double.parseDouble(userResponse);
}catch(NumberFormatException e){
//nothing to do here, error message handled below.
}
if(startingBalance <= 0){
System.out.println("Sorry, that value is not valid.");
}else{
break;
}
}while(true);
do{
System.out.println("Enter the interest rate (greater than zero less than twenty percent): ");
userResponse = input.next();
try{
interestRate = Double.parseDouble(userResponse);
}catch(NumberFormatException e){
//nothing to do here, error message handled below.
}
//Note: I assume twenty percent is represented as 20.0 here
if(interestRate <= 0 || interestRate > 20){
System.out.println("Sorry, that value is not valid.");
}else{
break;
}
}while(true);
System.out.println("You have entered the following amount of quarters: "
+ quartersDisplayed);
System.out.println("You also entered the starting balance of: " + startingBalance);
System.out.println("Finally, you entered the following of interest rate: "
+ interestRate);
System.out.println("If this information is not correct, please exit the program and enter the correct information.");
double quarterlyEndingBalance = startingBalance + (startingBalance * interestRate / 100 * .25);
System.out.println("Your ending balance for your quarters is "
+ quarterlyEndingBalance);
System.out.println("Do you want to continue?");
//EDIT: modified your variable name to be more meaningful since the user's response doesn't have to "yes" necessarily
userResponse = input.next();
// EDIT: modified the logic here to compare with "yes" or "y" case insensitively.
// if (userResponse.equals(userResponse))
if("y".equalsIgnoreCase(userResponse) || "yes".equalsIgnoreCase(userResponse))
continue;
else
break;
} while (true);
Now to address other issues - your interest calculation doesn't seem correct to me. Your formula does not make use of the quartersDisplayed variable at all. I assume you're compounding the interest quarterly so you will definitely need to make use of this when calculating your results.
This may be beyond the scope of your project, but you should not use double or float data types to represent money. There is a stackoverflow question about this topic that has good information.
Possible improvements - since you're asking the user for two values of type double you could create a method to ask for a double value and call it twice instead of repeating the code. This is a better approach because it helps reduce the chance of mistakes and makes testing and maintenance easier.

You can do something like this in your do/while loop:
do
{
Scanner keyboard = new Scanner(System.in);
do
{
System.out.println("Enter the numbers of quarters you wish to display that is greater than zero and less or equal to 10: ");
quartersDisplayed = keyboard.nextInt();
}
while (quartersDisplayed < 1 || quartersDisplayed > 10);
System.out.println("Next enter the starting balance. ");
do
{
System.out.println("This input must be greater than zero: ");
b = keyboard.nextDouble();
}
while (b < 1);
// rest of code ...
}

With the Scanner#hasNextInt (and the equivalent for double), you can avoid having exceptions thrown, and thus don't need try-catch clauses. I think in general if you can avoid try-catch, it's good, because they are clumsy - but I might be wrong.
However, my approach is like this. Inside your outer do-while, have three other do-while-loops to get the three values. The reason is that you want to keep looping until you get a correct value. The explanation of why keyboard.nextLine() is important is covered here.
I didn't include all of your code, only the part in question. Here's my take on it:
import java.util.Scanner;
public class InterestCalculator {
public static void main(String[] args) {
Scanner keyboard = new Scanner(System.in);
int quartersDisplayed = -1;
double b = -1.0;
double IR = -1.0;
do {
do {
System.out.println("Enter the number of quarters.");
if(keyboard.hasNextInt()) {
quartersDisplayed = keyboard.nextInt();
keyboard.nextLine(); //important
} else {
System.out.println("You need to enter an integer.");
continue;
}
} while(quartersDisplayed < 1 || quartersDisplayed > 10);
do {
System.out.println("Enter the starting balance.");
if(keyboard.hasNextDouble()) {
b = keyboard.nextDouble();
keyboard.nextLine();
} else {
System.out.println("You must enter a number.");
continue;
}
} while(b <= 0);
do {
System.out.println("Enter the interest rate.");
if(keyboard.hasNextDouble()) {
IR = keyboard.nextDouble();
keyboard.nextLine();
} else {
System.out.println("You must enter a number.");
continue;
}
} while(IR <= 0 || IR > 20.0);
//... rest of code
} while(true);
}
}

Why is the confirmation statement printing to the console twice? [duplicate]

This question already has answers here:
Scanner is skipping nextLine() after using next() or nextFoo()?
(24 answers)
Closed 7 years ago.
Whenever I run the program, everything works fine but for some reason the Confirmation prints are happening twice and I can't figure out why. Any help would be appreciated.
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
String Z = "Z";
int number;
String Znumber = "";
String Confirmation = "";
int ExamType;
// Ask user for Z number
do {
System.out.println("Please enter your Z number. Example: 12345678");
number = input.nextInt();
Znumber = Z + number;
} while ((Znumber.length() != 9));
//Confirm Z number entry
do {
System.out.printf(
"Your Z number is: %s. Is this correct? Enter Y/N: \n",
Znumber);
Confirmation = input.nextLine();
} while (!Confirmation.equalsIgnoreCase("y"));
// Ask user which exam they would like to take
ExamType = 0;
Confirmation = "";
System.out.println("Which exam would you like to take? Select 1-3: ");
System.out.println("1: English");
System.out.println("2: Spanish");
System.out.println("3: Math");
ExamType = input.nextInt();
do {
System.out.printf("You selected %s. Are you sure? Enter Y/N: \n",
ExamType);
Confirmation = input.nextLine();
} while (!Confirmation.equalsIgnoreCase("y"));
// Begin exam
if (ExamType == 1) {
System.out.println("Welcome to the English exam!");
// Start code from JavaExam.java
} else if (ExamType == 2) {
System.out.println("Welcome to the Spanish exam!");
// Start code from MathExam.java
} else if (ExamType == 3) {
System.out.println("Welcome to the Math exam!");
// Start code from EnglishExam.java

Instead of Confirmation = input.nextLine();, use Confirmation = input.next(); and you should be good. Tested and confirmed.
You really don't need nextLine here in your do-while loop.

Your first loop (// Ask user for Z number)
number = input.nextInt();
does not read the last '\n'.
I think you need to add a nextLine() in that loop.

In your do-while when you run for the first time print Your Z number is: %s. Is this correct? Enter Y/N:. after that the condition !Confirmation.equalsIgnoreCase("y") is evaluated in this case gives true for this cause try to run the loop do-while in second time.
do {
System.out.printf("Your Z number is: %s. Is this correct? Enter Y/N: \n",
....
} while (!Confirmation.equalsIgnoreCase("y"));

Scanner loop doesn't cycle when hitting enter twice

boolean loop = false;
double numberOfStudents;
System.out.print("Enter a number: ");
if ((scnr.nextLine().trim().isEmpty()) ) {
loop = true;
}
while (loop) {
System.out.println("Enter a number");
if (scnr.hasNextDouble() ){
System.out.println("Loop has stopped");
numberOfStudents = scnr.nextDouble();
loop = false;
}
}
System.out.println("You're outside the loop!");
I'm trying to get the program to say "Enter a number" until the user has entered an actual number (no white spaces or letters or signs). When the user has entered a number, it sets numberOfStudents equal to that number and breaks out of the loop.
But if you hit enter twice, it doesn't iterate. It only displays "Enter a number" once.
What is wrong with the loop logic? Why isn't it looping until valid input is taken?

For the actual answer to your question of "Why doesn't 'Enter a number' display more than once?" see Tom's comment (update: Tom's answer).
I've rewritten your loop in a way which preserves your code, but also makes it a little easier to handle format exceptions (though at the risk of silently swallowing an exception -- should be acceptable for this use case).
Can be up to you to use this design, here is an SO post on why empty catch blocks can be a bad practice.
public static void main(String args[])
{
boolean loop = true;
double numberOfStudents;
Scanner scnr = new Scanner(System.in);
while(loop){
System.out.print("Enter a number: ");
String input = scnr.nextLine();
try{
numberOfStudents = Double.parseDouble(input);
loop = false;
}catch(NumberFormatException e){
}
}
System.out.println("You're outside the loop!");
}
Output:
Enter a number:
Enter a number:
Enter a number:
Enter a number: 50
You're outside the loop!

First of all: Since you're reading from System.in a call to the input stream will block until the user entered a valid token.
So let's check first scan using your scnr variable:
scnr.nextLine()
nextLine() reads everything til the next line delimiter. So if you just press return, then it will successfully read it and will perform the next stuff.
The next call is:
scnr.hasNextDouble()
This call expects a "real" token and ignores white spaces, except as a delimiter between tokens. So if you just press return again it doesn't actually read that input. So it still waits for more (for the first token). That is why it stucks in your loop and you won't get another "Enter a number" output.
You can fix that by either enter a real token, like a number, or by changing the loop like trobbins said.
I hope you now understand your program flow a bit more :).

While trobbins code basically solves your problem, it's bad practice to use exceptions for flow control.
I used a small regexp to check if the value is a number. But this example is not complete, it will still crash it the user enters for example two decimal points. So you would need to create a proper number check or just use integers where the check is much easier.
Someone in the comments pointed out that people may want to enter scientific notation like 5e10, so this would also be another case to check for. If this is just some code you need as a proof of concept or something quick and dirty, you can go with the exception handling method but in production code you should avoid using exceptions this way.
double numberOfStudents;
Scanner scnr = new Scanner(System.in);
while(true) {
System.out.print("Enter a number: ");
String input = scnr.nextLine().trim();
if(input.matches("^[0-9\\.]{1,}$")) {
System.out.println("Loop has stopped");
numberOfStudents = Double.parseDouble(input);
break;
}
}
System.out.println("You're outside the loop!");

The following code should help you:
double numberOfStudents = 0;
Scanner scnr = new Scanner(System.in);
boolean readValue = false; //Check if the valid input is received
boolean shouldAskForNumber = true; //Need to ask for number again? Case for Enter
do {
if (shouldAskForNumber) {
System.out.print("Enter a number:");
shouldAskForNumber = false;
}
if (scnr.hasNextDouble()) {
numberOfStudents = scnr.nextDouble();
readValue = true;
} else {
String token = scnr.next();
if (!"".equals(token.trim())) { //Check for Enter or space
shouldAskForNumber = true;
}
}
} while (!readValue);
System.out.printf("Value read is %.0f\n", numberOfStudents);
System.out.println("You're outside the loop!");
Update
Understood the following statement in question different way:
But if you hit enter twice, it doesn't loop back. It only displays
"Enter a number" once.
The code is set to print "Enter a number" only once if the user hits RETURN/ENTER or enters space character. You may remove the special check and use the code if needed.

import java.util.Scanner;
public class Testing {
public static boolean checkInt(String s)
{
try
{
Integer.parseInt(s);
return true;
} catch (NumberFormatException ex)
{
return false;
}
}
public static void main(String[] args) {
boolean loop = false;
double numberOfStudents;
Scanner scnr = new Scanner(System.in);
String input = "";
while (!(checkInt(input))) {
System.out.println("Enter a number");
input = scnr.nextLine();
}
numberOfStudents = Integer.parseInt(input);
System.out.println("Number of students: " + numberOfStudents );
}
}
//this code is working fine, if you want you check it out.
//In your code your taking another input if the first is an int/double; if the first input is not a number then you have mentioned to take input again..

Use a debugger to see what the code is actually doing. Here's a guide on debugging in Eclipse. After you have finished debugging your code, you will probably know what the problem is.

Below code will help you
boolean loop = true;
double numberOfStudents;
Scanner scnr = new Scanner(System.in);
System.out.print("Enter a number: ");
String input = scnr.nextLine();
while(!scnr.hasNextDouble()){
System.out.print("Enter a number: ");
try{
numberOfStudents = Double.parseDouble(input);
break;
}catch(NumberFormatException e){
}
input = scnr.nextLine();
}
System.out.println("You're outside the loop!");

The following code is working,
boolean loop = true;
double numberOfStudents;
Scanner scnr=new Scanner(System.in);
while(loop) {
System.out.println("Enter a number");
if ((scnr.nextLine().trim().isEmpty()) ) {
loop = true;
}
if (scnr.hasNextDouble() ){
System.out.println("Loop has stopped");
numberOfStudents = scnr.nextDouble();
loop = false;
}
}
System.out.println("You're outside the loop!");
The output is,
run:
Enter a number
hj
po
Enter a number
lhf
Enter a number
o
Enter a number
p
Enter a number
a
Enter a number
34
Loop has stopped
You're outside the loop!

You have to scan the next line if you want to get more values form the scanner again. The code should be like:
while (loop) {
System.out.println("Enter a number");
if(!(scnr.nextLine().trim().isEmpty())){
if (scnr.hasNextDouble() ){
System.out.println("Loop has stopped");
numberOfStudents = scnr.nextDouble();
loop = false;
}
}
}

How to make a program run a second time?

import java.util.Scanner;
public class Dice {
public static void main(String[] args) {
//I used 'print' instead of 'println' just to make it look a little cleaner in the console.
System.out.print("Input your first number: ");
Scanner sc1 = new Scanner(System.in);
double num1 = sc1.nextInt();
//I use doubles for my variables just in case the user wants to divide.
System.out.print("Input your second number: ");
Scanner sc2 = new Scanner(System.in);
double num2 = sc2.nextInt();
/* I used words rather than the actual symbols for my operators just to get practice using scanners for strings.
* Until now I'd solely been using them for int variables. And also due to the small detail that before programming,
* I had no idea what a modulo was and I felt that would be confusing to a random person.
*/
System.out.println("What would you like to do with these numbers?(Add, Subtract, Multiply, Divide, or Check Divisibility): ");
System.out.println("Simply type 'check' to check the divisibility of your two numbers.");
Scanner sc3 = new Scanner(System.in);
String str1 = sc3.nextLine().toUpperCase();
/* toUpperCase to prevent the user from creating an error by typing their in put in a 'unique' way.
*It took me several failures to finally look up toUpperCase.
*/
double num3;
switch(str1) {
case "ADD":
num3 = num1 + num2;
System.out.println("The sum is: " + num3);
break;
case "SUBTRACT":
num3 = num1 + num2;
System.out.println("The difference is: " + num3);
break;
case "MULTIPLY":
num3 = num1 * num2;
System.out.println("The product is: " + num3);
break;
case "DIVIDE":
num3 = num1 / num2;
System.out.println("The quotient is: " + num3);
break;
case "CHECK":
num3 = num1 % num2;
System.out.println("The remainder is: " + num3);
break;
default:
System.out.println("Invalid input. Please ensure that two numbers were entered and that you entered a valid math operation.");
break;
}//switch statement
}//main method
}//class
How would I get my code to run again if I wanted to maybe add another number to my answer? I'm just trying to get some practice in with my Java (I'm extremely green) and I apologize in advance if my question is too broad.

Consider the following small program
boolean quit = false;
while(!quit) {
System.out.print("Enter Something:");
Scanner sc1 = new Scanner(System.in);
String input = sc1.nextLine();
if(input.compareToIgnoreCase("quit") == 0) {
quit = true;
continue;
}
System.out.println("You entered " + input);
}
In this sample we keep asking them to enter something and print it out unless that input is "quit" in that case we use the continue statement to skip the rest of the loop and go back to the top of the while loop and re-evaluate the condition for another iteration. If you entered 'quit' this will evaluate to false and stop the loop and exit the program.
Heres a sample input/output from the program. Notice there is no "You entered quit", this is because the continue statement brought us back to the top of the while loop.
Enter Something:hello
You entered hello
Enter Something:quit
Now how can you adapt this to your program? Heres a small sample of how you can do one of your inputs
double num1 = 0;
String input1 = sc1.nextLine();
if(input1.compareToIgnoreCase("quit") == 0) {
// quit was entered, leave the loop
quit = true;
continue;
}
try {
num1 = Double.parseDouble(input1);
} catch(NumberFormatException e) {
// user entered something that isnt a number, quit the program for now
// you can change this to whatever behavior you like in the future
quit = true;
continue;
}
This will likely leave you with some validation questions like "I want to have my user try again if they input an invalid number" Those are all possible using this method and it leads you in the right direction.

Remember, main() is a callable method. Instead of using a while or for loop, you could just call it again at the end of the main method method.
// Put this at the end of your main method
System.out.print("Do you want to execute again? (yes/no)");
boolean repeat = sc1.nextLine().toUpperCase().equals("YES");
if (repeat) {
main(null); // You're not using any arguments in main()
}
On a separate note, you don't need all three of sc1, sc2, and sc3. They're basically the same. You could probably use sc1 everywhere and remove sc2 and sc3 completely.

// something like this then ask if to do another run if not set flag false
boolean flag = true;
while(flag)
{
System.out.print("Input your first number: ");
Scanner sc1 = new Scanner(System.in);
double num1 = sc1.nextInt();

You should put all your logic around a while loop which will grant to you to repeat your task until a condition is reached.
Maybe you can ask to the user to insert the string "EXIT" when he wants to exit from your program.
In your case I'll do something like this:
boolean exitFlag = false;
do {
// <put your logic here>
String answer = sc3.nextLine().toUpperCase();
if (answer.equals("EXIT")) {
exitFlag = true;
}
} while(!exitFlag);