I am fairly new to Java and I am trying to work on my data validation. The code runs fine when I use valid data, but when I put in a string instead of an integer the code just loops forever. It just loops the, "Bad input. Please enter a number." Thanks in advance!
//Get input from user
System.out.print("What is your name (Last, First)? ");
String name = scan.nextLine();
System.out.print("enter a date:");
String datein = scan.nextLine();
boolean valid = false;
while (valid != true)
{
System.out.print("Electricity used (KW):");
if (scan.hasNextDouble())
{
electricityUsed = scan.nextDouble();
valid = true;
}
else
System.out.println("Bad input. Please enter a number.");
}
because hasNextDouble always returns false.
here is the doc. You answer your own question :
but when I put in a string instead of an integer
To fix it add a scan.nextLine() to your else branch.
A simpler approach is to use the following.
System.out.print("Electricity used (KW):");
while(!scan.hasNextDouble()) {
scan.nextLine();
System.out.println("Bad input. Please enter a number.");
System.out.print("Electricity used (KW):");
}
double electricityUsed = scan.nextDouble();
Related
I just started learning Java... Sorry if this is just a way too dumb question.
I was trying to compare the user input. If the input is not either "Yes" or "No" then force the user to input either one of them... but my code don't work...
Compiling has no issue, but even if the input is "Yes" or "No" the while loop just keep looping.
Tried printing out the value of "userInput" within the loop but it shows "Yes" or "No" correctly when inputted, yet the loop just goes on.
protected static boolean askUser() {
String userInput = "x";
boolean userChoice;
System.out.println("Do you have a question you want to know the answer too? (Yes/No): ");
userInput = input.nextLine();
while (!userInput.equalsIgnoreCase("Yes") || !userInput.equalsIgnoreCase("No")) {
System.out.println("Please input only \"Yes\" or \"No\": ");
userInput = input.nextLine();
}
if (userInput.equalsIgnoreCase("Yes")) {
userChoice = true;
} else {
userChoice = false;
}
return userChoice;
}
Any idea on how to fix this code?
!userInput.equalsIgnoreCase("Yes") || !userInput.equalsIgnoreCase("No") is always true because Yes is not No and No is not Yes.
You will want to loop while the input is not Yes and not No, so the condition should be !userInput.equalsIgnoreCase("Yes") && !userInput.equalsIgnoreCase("No").
Can someone please advise why the inner loop of the code below will not exit?
I've added an inner loop to check if input from the user of of a particular value and if not prompts for the correct input. When debugging the code and passing in a value which should force the loop to end it doesn't although I can see the correct value in the variable:
while (finished.equalsIgnoreCase("n")) {
System.out.println("Enter a persons name");
names = in.nextLine();
writer.println(names);
System.out.println("Finished? (Y/N)");
finished = in.nextLine();
while( !finished.equalsIgnoreCase("y") || !finished.equalsIgnoreCase("n")) {
System.out.println("Invalid choice; (Y/N)");
finished = in.nextLine();
}
}
Every string is either not not equal to y or not equal to n. You probably meant to use the && operator:
while(!finished.equalsIgnoreCase("y") &&
!finished.equalsIgnoreCase("n")) {
// Code...
I'm trying to prevent the user from inputting spaces or no values.
but nothing works. Even with no entered values program goes further without printing my error. What am I doing wrong?
my code example
Scanner nameScan = new Scanner(System.in);
System.out.print("Input your name: ");
String newcomer = nameScan.nextLine();
player.setName(newcomer);
String userName = player.getName();
userName = userName.trim();
if (userName.length()==0) {
System.out.println(" ");
System.out.println("You have to set up a player name first... ");
System.out.println(" ");
}
else {...
As #11thdimension said, you have to validate the input.
You can do something like:
if (newcomer.isEmpty()) {
System.out.println("Please write something");
}
Or you can do a while loop and keep asking for a correct input.
Your code
if(username.length() == 0)
will not check whether the username contains space because space is counted towards the length of the String.
To check for empty String input(which may contain space(s)), you can do:
if("".equals(username.replaceAll("\\s+",""))) //or
if("".equals(username.trim()) //or
if(username.isEmpty())
Further more, you would want to use a do-while loop for validation instead of using an if-statement.
I'm trying to only accept numbers from a user. This code works for giving them an error message if they enter a letter. But it doesn't work for if they hit Enter or just white space. I've tried initializing a String called test as null and then setting scnr.nextLine() = test, and then checking if test is empty, but I didn't understand how to keep the rest of the program operating correctly when I did that. Scanner is very tricky to me. Please help!
double mainNumber = 0;
System.out.print("Enter a number: ");
if (scnr.hasNextDouble() ){
mainNumber = scnr.nextDouble();
System.out.println(mainNumber);
scnr.nextLine();
}
else {
System.out.println("Sorry, please enter a number.\n");
scnr.nextLine();
}
You have to use while-cycle and loop input as long as needed before user put a valid number.
This code
public static void main(String[] args) {
Scanner scnr = new Scanner(System.in);
double mainNumber = 0;
boolean isValidNumber = false;
System.out.print("Enter a number: ");
while (isValidNumber == false) {
String line = scnr.nextLine();
try {
mainNumber = Double.valueOf(line);
isValidNumber = true;
} catch (NumberFormatException e){
System.out.print("Sorry, please enter a number.\n");
}
}
System.out.println("Main number is: " + mainNumber);
}
Having this sample output :
Enter a number: sdfgsgxb
Sorry, please enter a number.
xcvbxcvb
Sorry, please enter a number.
gsfdfgsdf
Sorry, please enter a number.
aearg
Sorry, please enter a number.
15.77
Main number is: 15.77
Well I guess your code is in a while loop or something ? So that it keep asking until the user enter the right value.
Then you should (for convenience) use String str = scnr.nextString() instead of nextDouble() and analyze the string it returned.
You can use str.trim() to remove whitespaces (and then check if string is empty with str.isEmpty() ), and to check if it's a number you can use regexp ( How to check that a string is parseable to a double? and any regex tutorial you can find ) or just use this regex: str.matches("\\d+") (returns true if str is a number, but no comma here).
Of course, don't forget to cast your String as double after: Double.parseDouble( str.replace(",",".") );. I hope the "replace" part is obvious ;)
You might use the following snippet to read one double value:
Scanner scanner = new Scanner(System.in);
try {
double number = Double.parseDouble(scanner.nextLine());
} catch (NumberFormatException e) {
e.printStackTrace();
}
Everything of my guessing game is alright, but when it gets to the part of asking the user if he/she wants to play again, it repeats the question twice. However I found out that if I change the input method from nextLine() to next(), it doesn't repeat the question. Why is that?
Here is the input and output:
I'm guessing a number between 1-10
What is your guess? 5
You were wrong. It was 3
Do you want to play again? (Y/N) Do you want to play again? (Y/N) n
Here is the code:(It is in Java)
The last do while loop block is the part where it asks the user if he/she wants to play again.
import java.util.Scanner;
public class GuessingGame
{
public static void main(String[] args)
{
Scanner input = new Scanner(System.in);
boolean keepPlaying = true;
System.out.println("Welcome to the Guessing Game!");
while (keepPlaying) {
boolean validInput = true;
int guess, number;
String answer;
number = (int) (Math.random() * 10) + 1;
System.out.println("I'm guessing a number between 1-10");
System.out.print("What is your guess? ");
do {
validInput = true;
guess = input.nextInt();
if (guess < 1 || guess > 10) {
validInput = false;
System.out.print("That is not a valid input, " +
"guess again: ");
}
} while(!validInput);
if (guess == number)
System.out.println("You guessed correct!");
if (guess != number)
System.out.println("You were wrong. It was " + number);
do {
validInput = true;
System.out.print("Do you want to play again? (Y/N) ");
answer = input.nextLine();
if (answer.equalsIgnoreCase("y"))
keepPlaying = true;
else if (answer.equalsIgnoreCase("n"))
keepPlaying = false;
else
validInput = false;
} while (!validInput);
}
}
}
In your do while loop, you don't want the nextLine(), you just want next().
So change this:
answer = input.nextLine();
to this:
answer = input.next();
Note, as others have suggested, you could convert this to a while loop. The reason for this is that do while loops are used when you need to execute a loop at least once, but you don't know how often you need to execute it. Whilst it's certainly doable in this case, something like this would suffice:
System.out.println("Do you want to play again? (Y/N) ");
answer = input.next();
while (!answer.equalsIgnoreCase("y") && !answer.equalsIgnoreCase("n")) {
System.out.println("That is not valid input. Please enter again");
answer = input.next();
}
if (answer.equalsIgnoreCase("n"))
keepPlaying = false;
The while loop keeps looping as long as "y" or "n" (ignoring case) isn't entered. As soon as it is, the loop ends. The if conditional changes the keepPlaying value if necessary, otherwise nothing happens and your outer while loop executes again (thus restarting the program).
Edit: This explains WHY your original code didn't work
I should add, the reason your original statement didn't work was because of your first do while loop. In it, you use:
guess = input.nextInt();
This reads the number off the line, but not the return of the line, meaning when you use:
answer = input.nextLine();
It immediately detects the leftover carriage from the nextInt() statement. If you don't want to use my solution of reading just next() you could swallow that leftover by doing this:
guess = input.nextInt();
input.nextLine();
rest of code as normal...
The problem really lies in a completely different segment of code. When in the previous loop guess = input.nextInt(); is executed, it leaves a newline in the input. Then, when answer = input.nextLine(); is executed in the second loop, there already is a newline waiting to be read and it returns an empty String, which activates the final else and validInput = false; is executed, to repeat the loop (and the question).
One solution is to add an input.nextLine(); before the second loop. Another is to read guess with nextLine() and then parse it into an int. But this complicates things as the input could not be a correct int. On a second thought, the code already presents this issue. Try entering a non-numeric response. So, define a function
public static int safeParseInt(String str) {
int result;
try {
result= Integer.parseInt(str) ;
} catch(NumberFormatException ex) {
result= -1 ;
}
return result ;
}
And then replace your first loop with:
do {
validInput= true ;
int guess= safeParseInt( input.nextLine() ) ;
if( guess < 1 || guess > 10 ) {
validInput= false ;
System.out.print("That is not a valid input, guess again: ");
}
} while( !validInput );
PS: I don't see any problem with do-while loops. They are part of the language, and the syntax clearly indicates that the condition is evaluated after the body is executed at least one time. We don't need to remove useful parts of the language (at least from practice) just because others could not know them. On the contrary: if we do use them, they will get better known!
validInput = false;
do {
System.out.print("Do you want to play again? (Y/N) ");
answer = input.next();
if(answer.equalsIgnoreCase("y")){
keepPlaying = true;
validInput = true;
} else if(answer.equalsIgnoreCase("n")) {
keepPlaying = false;
validInput = true;
}
} while(!validInput);
I changed the coding style as I find this way more readable.
Your problem is that nextInt will stop as soon as the int ends, but leaves the newline in the input buffer. To make your code correctly read the answer, you'd have to enter it on the same line as your guess, like 5SpaceYReturn.
To make it behave more than one would expect, ignore the first nextLine result if it contains only whitespace, and just call nextLine again in that case without printing a message.
I believe the output of input.nextLine() will include the newline character at the end of the line, whereas input.next() will not (but the Scanner will stay on the same line). This means the output is never equal to "y" or "n". Try trimming the result:
answer = input.nextLine().trim();