I am trying to execute a bit of code that scans for a user input value. This action is contained in a custom method I wrote, named getTriangleDim(); the method reads in the users int value, makes sure it is within a certain range, and then returns the int value that was entered. The method works great and I have no issues with it.
The problem arises when I enter a non-int value for my getTriangleDim() method. It gives me an InputMismatchException error. I have written a try-catch statement into a do-while loop to attempt to fix this issue. But this is the first time I have ever used a try-catch statement, and I am apparently missing something.
Here is the code for the try-catch statement nested within the do-while loop:
//loop to scan for triangle dimension
boolean bError = true;
int triangle;
do{
try {
triangle = getTriangleDim();
bError=false;
}
catch (Exception e){
System.out.println("You did not enter an integer, please enter an integer value");
triangle = getTriangleDim();
}
}while (bError);
if i test it by entering a char value in place of the int, it actually catches the error once, and then prints my "you did not....." statement. But if I re-enter another non-int number, I get a runtime error again that says.......you guessed it........ InputMismatchException error.
The code for my method is here:
//method for scanning triangle dimensions from keyboard
public static int getTriangleDim(){
int triangle = 0;
Scanner keyboard = new Scanner(System.in);
do{
System.out.print("Enter a non-zero integer length (+/-1 - +/-16): ");
triangle = keyboard.nextInt();
if((!(triangle <= 16 && triangle >= 1))&&(!(triangle >= -16 && triangle <= -1)))
System.out.println("Inpute value outside of range");
}while((!(triangle <= 16 && triangle >= 1))&&(!(triangle >= -16 && triangle <= -1)));
return triangle;
}
I need the Do-While loop to continue, but I keep getting these errors.
There's no need to ask for input in the catch block. You're already in a loop, so you can catch the exception, tell the user to give you valid input, and then you don't have to do anything else--you'll loop back around to the beginning.
do{
try {
triangle = getTriangleDim();
bError=false;
} catch (Exception e){
System.out.println("You did not enter an integer, please enter an integer value");
// Don't do anything else in here: we will loop back to the beginning again and get new input!
}
}while (bError);
As a side note (and as you've noticed), your code won't currently compile if you try to use triangle outside of that try block, because "triangle might not have been initialized". This is due to the fact that the compiler cannot determine at compile-time what your program will do at run-time: that is, the compiler cannot see that triangle will always be initialized within that loop. So your variable declaration ought to also set triangle to some default value. 0 is the normal default for int, but use whatever makes sense in your program (it looks like 0 is fine, based on your getTriangleDim code).
int triangle = 0;
do { // etc.
This way you can "promise" the compiler that triangle will have a value by the time you leave the loop, and you'll be able to use it elsewhere.
Related
trying to make program rotation of array, it was giving an Exception by user misinput so I use try block but now, under try block it is not initializing values....
Can some one tell the reason or solution for this....
public static void main (String[] args) {
Scanner sc = new Scanner(System.in);
ArrayRotation ar = new ArrayRotation();
System.out.println("Enter T : ");
int t = sc.nextInt();
sc.nextLine();
while(t!=0){
System.out.println("\nEnter N D : ");
String s = sc.nextLine();
s.trim();
String st[] = s.split(" ");
int n,d;
try{
n = Integer.parseInt(st[0]);
d = Integer.parseInt(st[1]);
}catch(Exception e){ System.out.println("Exception"+e.getMessage()); }
System.out.println("Enter Element : ");
s=sc.nextLine();
st = s.split(" ");
ar.rotateArray(st,n,d);
t--;
}
}
If you need valid input and you did not get valid input, the thing to do is to try again to get valid input, after telling the user the input was invalid. Don't just proceed with the invalid data. You therefore need an inner loop:
while (t!=0) {
boolean validInput = false;
while (!validInput) {
System.out.println("\nEnter N D : ");
String s = sc.nextLine().trim();
String[] st = s.split(" ");
int n,d;
try {
n = Integer.parseInt(st[0]);
d = Integer.parseInt(st[1]);
validInput = true;
}
catch (Exception e) {
System.out.println("Invalid input");
}
}
… process n and d as before …
}
For my taste the loop to get the valid input would be better off being a subroutine in its own right - for clarity.
Variable in local scope should be initialized , that is what error , so do initialize the variables n and d to some integer value say as below
int n = 0 ,d = 0;
try{
n = Integer.parseInt(st[0]);
d = Integer.parseInt(st[1]);
}
When a method throws an exception, that method never returns.
This means that if n = Integer.parseInt(st[0]); throws an exception, it does not return a value, which means n will not be assigned a value (since there is no return value to assign to it).
You are ignoring the exception and trying to continue as if nothing went wrong. But something did go wrong—n was never assigned a value. So the compiler tells you that it is not safe to use n in any subsequent code.
To solve this, you first must decide what to do if the user provides invalid input. You can’t just ignore the exception. If the input doesn’t represent two integers, you don’t have any values to work with. You can’t continue in any meaningful way.
The best course of action is to remove your try and catch. This will cause the program to terminate if Integer.parseInt fails, which is almost certainly what you want (unless your assignment requires you to do something different). Remember that it is not possible to continue in any meaningful way without values assigned to n and d.
In other words, change this:
int n,d;
try{
n = Integer.parseInt(st[0]);
d = Integer.parseInt(st[1]);
}catch(Exception e){ System.out.println("Exception"+e.getMessage()); }
to this:
int n = Integer.parseInt(st[0]);
int d = Integer.parseInt(st[1]);
As a side note, this line does nothing:
s.trim();
…because Strings cannot be changed. s.trim() returns a new String which you must capture in a variable. You probably want to do this:
s = s.trim();
Your image is quite misleading and really doesn't point to the actual problem, it only points to what you perceive to be the problem. Your mistake was placing the initialization of variables n and d into a try block which takes your call to the rotateArray() method out of scope for that initialization of those variables.
The bigger problem is... What in the world are you rotating? Where is the Array to rotate? Is it actually a String Array or is it suppose to be an Integer Array? Please don't tell me it's the st[] String Array (which is what you're trying to do) because according to your code that array is used to establish the array size (n) portion to work with and the number of elements (d) the User wants to rotate by. No rocket science to rotate an Array with only two elements. Give the rotateArray() method an array to actually rotate.
Let's provide an Integer Array and a way to do this without a try/catch mechaism:
// The Array to carry out rotations on.
int[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
String ls = System.lineSeparator();
Scanner sc = new Scanner(System.in);
ArrayRotation ar = new ArrayRotation();
int t = 0;
String value = "";
while (value.equals("")) {
System.out.print("Enter Number of times to Rotate Array: --> ");
value = sc.nextLine().trim();
/* Make sure a String representation of a Integer value is supplied.
The regular Expression "\\d+" in the String#matches() method
ensures the a numerical integer string is supplied. */
if (!value.matches("\\d+" || value.length() > 9) {
/* Will handle situations where nothing is supplied, alpha
characters might be supplied, or the supplied numerical
value is outrageously large. */
System.out.println("Invalid integer numerical value supplied!" + ls);
value = "";
}
}
t = Integer.parseInt(value);
while (t != 0) {
System.out.println();
System.out.print("Enter the Size (portion) of the Array to consider and the" + ls
+ "number of elements to rotate (separated with a space): --> ");
String s = sc.nextLine().trim();
// Quit if anything starting with the letter "Q"
// (like "q" or "quit") is supplied.
if (s.substring(0, 1).equalsIgnoreCase("q")) {
System.out.println("\"Quit\" Supplied!");
System.exit(0);
}
String st[] = s.split(" ");
/* Make sure two values were supplied and that
they are both numerical integer strings. */
if (st.length != 2 || !st[0].matches("\\d+") || !st[1].matches("\\d+")) {
System.out.println("Invalid Input! Try again and make sure 'both' "
+ "values are numerical Integers.");
continue;
}
// Declare and initialize the n and d variables.
int n = Integer.parseInt(st[0]);
int d = Integer.parseInt(st[1]);
/* If the number of elements to rotate is greater
than the portion of array to rotate in. */
if (d >= n) {
System.out.println("Invalid Input! The size of the Array portion to rotate" + ls
+ "must be greater than the number of elements to rotate.");
}
/* If the supplied Array size to deal with is
out of bounds of the Array itself. */
else if (n < 1 || n > array.length) {
System.out.println("You have supplied an invalid Array Size! (" + n
+ ") Size must be between 1 and " + array.length + "!"
+ ls);
}
/* If the supplied number of elements to rotate
is less than 1 or greater than the total number
of elements - 1. */
else if (d < 1 || d > (array.length - 1)) {
System.out.println("You have supplied an invalid number of elements to rotate! (" + d
+ ") Value must be between 1 and " + (array.length - 1) + "!" + ls);
}
// All is good - Do the rotation.
else {
ar.rotateArray(array, n, d); // Rotate the Array
// Display the current rotation...
System.out.println("Current Rotation: --> " + Arrays.toString(array));
t--;
}
}
// Done
The error you outline in your image is a general compilation error and is relatively generic for all data types. This error occurs when you are trying to use a local variable without first initializing it. You won't get this error if you use a uninitialized class or instance variable because they are initialized with their default value (for example: Reference types are initialized with null and integer types are initialized with zero), but if you try to use an uninitialized local variable in Java, you will get this error. This is because Java has the rule to initialize the local variable before accessing or using them and this is checked at compile time. If the compiler believes that a local variable might not have been initialized before the next statement which is going to use it, you will receive this error. You of course will not get this error if you just declare the local variable but don't use it but then, why declare it in the first place.
Everyone is stating to initialize the local variables n and d because in reality, in order to successfully compile your code that is exactly what needs to be done in order for the rotateArray() method (which uses these uninitialized variables) to function. Again in reality, you do initialize them however your code does it within a try{} block which alters scope and the compiler is smart enough to know that if the initialization fails within the try{} block then the catch{} block could let that failure be ignored. In fact, if you were to place the call to the rotateArray() method within that try{} block then you would not get this compile time error since the call is within the scope of of where the variables n and d are actually initialized. You know, a decent IDE (line Eclipse, NetBeans, InteliJ, etc) should catch this error for you long before you try to compile.
According to your code, the actual intent of the try/catch blocks would be to handle the case of invalid input whereas a non-numerical integer value was supplied by the User. In this case it would be up to your catch{} block to handle that particular situation which should be to inform the User of the invalid input and then continue to re-prompt for proper input. At compile time the compiler really doesn't care about this mechanism since this would be a Runtime Error unless of course it is syntax related.
Nothing wrong with try/catch, I just like to avoid them if I can.
I apologize if this question is uber-simplistic, but I'm still in the early stages of learning Java. I have an example program that calls other methods within the class, and I'm not totally following a few of the elements - hoping someone can clarify. It's a simply random number guessing game and it works fine, but I want to better understand some components. Specifically:
There is a boolean variable (validInput) that is declared but never appears to be used anywhere in the methods
There are 2 methods (askForAnotherRound and getGuess) with a 'while' loop that just has 'true' as the variable(?) - i.e. "while (true)."
This code is directly from the example in the book and, again, it works. I just want to better understand those 2 elements above. (I think the validInput variable is not useful as when I 'comment out' that line the code still executes). I'm curious, though, about the "while (true)" element. There is an option to set, in the askForAnotherRound, to set the return value to false (ending the program). Are boolean methods defaulted to 'true' when they are first executed/called?
Again...understand this is probably a super-simple question for most folks on here, but as a newb I just want to understand this as best I can...
Thanks!!!
// Replicates the number guessing game using 4 separate methods.
import java.util.Scanner;
public class GuessingGameMethod2
{
static Scanner sc = new Scanner(System.in);
public static void main(String[] args)
{
System.out.println("Let's play a guessing game!");
do
{
playARound();
}while (askForAnotherRound());
System.out.println("Thanks for playing!");
}
public static void playARound()
{
boolean validInput;
int number, guess;
String answer;
//Pick a Random Number
number = getRandomNumber();
//Get a guess
System.out.println("\nI'm thinking of a number between 1 and 10.");
System.out.print("What do you think it is? ");
guess = getGuess();
//Check the guess
if (guess == number)
System.out.println("You're right!");
else
System.out.println("You're wrong! The number was " + number + ".");
}
public static int getRandomNumber()
{
return (int)(Math.random() * 10) + 1;
}
public static int getGuess()
{
while (true)
{
int guess = sc.nextInt();
if (guess < 1 || guess > 10)
{
System.out.print("I said, between 1 and 10. Try again");
}
else
return guess;
}
}
public static boolean askForAnotherRound()
{
while (true)
{
String answer;
System.out.print("\nPlay again? Y or N: ");
answer = sc.next();
if (answer.equalsIgnoreCase("Y"))
return true;
else if (answer.equalsIgnoreCase("N"))
return false;
}
}
}
I don't see boolean validInput being used either. But if it were to be used somewhere it would probably be to check that you guess satisfies 1 <= guess <= 10.
When it comes to askForAnotherRound and getGuess here's what you should know:
while(true) is always executed. One way you can get out of the while loop is if you use the break statement or if the loop is in a function you can return something. The method askForAnotherRound() will always return either true or false. Depending on the returned value of askForAnotherRound() you will either play another round or not. Note that when you have
`do{
...
someActions()
...
}while(booleanValue)`
someActions() will be executed at least once before it checks for the value of booleanValue which, if it turns out false you'll exit out of the do/while loop. Boolean methods do not default to anything, you have to give them a value.
Hope this helps! I'm also in the process of learning Java right now, so good luck!
As I see you're absolutely true about validInput - it isn't used. May be it will be used in the following chapters.
As for askForAnotherRound() - no, boolean methods don't evalute to true, by default. Even more, Java compiler throw an error if it find a method which does not return value and finish it execution in some cases.
while(true) - it's infinite loop, so it will be executed untill some instruction which interrupts loop, in general it's return statement.
askForAnotherRound() do the following:
- asks user if he/she want to play again
- returns true if user input "Y"
- returns false if user input "Y"
- asks again in all other cases(so it doesn't finish execution) and etc.
Hope it'll help
The validInput is indeed worthless.
The infinite loops are required to read from the console to get a valid input. e.g
while (true)
//start infinite loop
{
int guess = sc.nextInt();
if (guess < 1 || guess > 10)
{
//continue the loop the input is not between 1-10
System.out.print("I said, between 1 and 10. Try again");
}
else
//break out of infinite loop, valid int
return guess;
}
If we take this method without the infinite loop, and i recommend trying this, it will simply return the value read even if it was not valid.
For example.
return sc.nextInt();
will allow any int returned, if we returned anything outside of the bounds in the current impl it would loop again until you enter a value between 1-10
The same is also true for ask for next round, its looping until a valid input is given.
I would bet the next exercises will use the validInput var as both these methods loop until a valid input is given.
You are right about validInput. It is not used. And probably missed after some code change. Should be removed.
while(true) - true is not variable but a boolean constant. It will basically make program run for ever in this case unless somebody kills program. Another alternative would have been to use break to exit out of loop on some condition.
I am wondering why one loop works while the other doesn't even though they are very similar.
The dmCyl loop I would like to loop until a positive int is entered. This is the loop that does not work. However, the hgtCyl loop, which loops until the user enters an int, does work. It seems like the (dmCyl < 0) loop is just completely ignored. Why is that?
Also, the loop that works does so whether I put hgtCyl = scnr.nextInt(); in the while body (after scnr.next()) or outside of it. Does it's location change anything?
Thanks!
EDIT:
Setting the first while to a do while loop solves the problem.
Thanks again!
Solution:
dmCyl = scnr.nextInt();
do{
System.out.print("please insert positive int");
dmCyl = scnr.nextInt();
}while (dmCyl < 0);
Original Code:
int dmCyl = 0;
int hgtCyl = 0;
Scanner scnr = new Scanner(System.in);
while (dmCyl < 0) {
System.out.print("Please enter an integer");
scnr.next();
}
dmCyl = scnr.nextInt();
And the loop that works:
while (!scnr.hasNextInt()){
System.out.print("Please enter an integer");
scnr.next();
}
hgtCyl = scnr.nextInt();
Use do/while
do {
System.out.print("Please enter an integer");
dmCyl = scnr.nextInt();
} while (dmCyl < 0);
You are setting dmCyl to zero initially then you are telling the loop to only go while it is less than zero. Since at the point it starts the loop it has already satisfied the condition and never enters the loop. Set dmCyl equal to -1 instead.
You initialize dmCyl = 0, so your statement dmCyl < 0 is false.. you should try to add something like this: while (dmcyl <= 0)
Here part of the official description of while from the Java Language Specification 14.12, relevant text marked (bold) by me:
A while statement is executed by first evaluating the Expression. If the result is of type Boolean, it is subject to unboxing conversion (§5.1.8).
If evaluation of the Expression or the subsequent unboxing conversion (if any) completes abruptly for some reason, the while statement completes abruptly for the same reason.
Otherwise, execution continues by making a choice based on the resulting value:
If the value is true, then the contained Statement is executed. Then there is a choice:
If execution of the Statement completes normally, then the entire while statement is executed again, beginning by re-evaluating the Expression.
If execution of the Statement completes abruptly, see §14.12.1.
If the (possibly unboxed) value of the Expression is false, no further action is taken and the while statement completes normally.
That means, the expression is checked before executing the loop content for the first time. If it results in false the content is not executed and the loop terminated.
I am having a heck of a time trying to figure out why I can't leave a loop. What I need to do is leave the loop if my Boolean, forward, is set to true. (The Boolean has been set to false above the while loop.
When I run the code snippet below I and enter a positive number I can only enter an unlimited amount a numbers. When I enter a negative number I get one prompt telling me that's not allowed and to try again. After than I am stuck in the similar situation above. It doesn't matter what I enter next it will just keep letting input over and over again.
{
while (forward == false)
if (n2 == 0)
{
System.out.println("Sorry, the 0 is not a valid entry for the second number, try again!");
n2 = in.nextInt();
}
else
{
forward = true;
}
}
You can get away with using no extra variables
n2 = in.nextInt();
while (input == 0){
System.out.println("Sorry, 0 is not valid input");
n2 = in.nextInt();
}
as the forward is a boolean, you can use it directly instead of compare
forward == false.
If you wanna use this variable (I will not go thru the path of the best way achieve your aim), you can do the follow:
while (!forward) {
if (n2 == 0) {
System.out.println("Sorry, the 0 is not a valid entry for the second number, try again!");
n2 = in.nextInt();
} else {
forward = true;
}
}
Do you have an outer while loop that makes you go back into your while loop? An efficient way to solve loop issues is to debug the code a line at a time.
If you are in Eclipse, set a breakpoint (a place where your program will pause) by clicking on the line number just before entering the while loop. Then run the program, and you will see the current line highlighted. Then move line by line by pressing F6 repeatedly. In the bottom pane you can also find the current values of variables.
Now if you inspect your code line by line you will have a better idea of what's going on.
I've nested an IF ELSE statement inside a WHILE statement, but am confused as to why the WHILE is interpreted before the ELSE (when the IF fails). A user is asked to enter a number from 1-10 (inclusive). If the number is inside that range, the program ends. If it's outside of that range, I want to display an error and then prompt them to again enter a number.
It works well if I put the "prompt" before the WHILE, but then I have to put it again inside the ELSE statement for it to show up again. I found this question, but it didn't appear to answer the issue I'm running into. I'm admittedly a Java novice, so I apologize if I'm missing some fundamental aspect of Java.
import java.util.Scanner;
public class Rangecheck {
private static int userNumber; //Number input by user
private static boolean numberOK = false; //Final check if number is valid
//String that will be reused in the DO statement
private static String enterNumber = "Please enter a number from 1 to 10: ";
public static void main(String[] args) {
//Print string
while(!numberOK) // Repeat until the number is OK
{ System.out.println(enterNumber);
Scanner input_UserNumber = new Scanner(System.in); //input integer
userNumber = input_UserNumber.nextInt();
if (10>= userNumber && userNumber >= 1) //Check if 10>=input>=1
{
/*
** If number was valid, congratulate the user and mark numberOK true
*/
System.out.println("Good job! The number you entered is "+userNumber+".");
numberOK = true; // Congratulate user / exit loop if successful
}
else ; //if (10 < userNumber && userNumber < 1)
{
System.err.println("The number entered was not between 1 and 10!");
System.err.print(enterNumber); // Error; user retries until successful
}
}
}
}
I'd expect the System.err.println() to be evaluated in the else statement and then the whileto be evaluated, so that this gets returned:
The number entered was not between 1 and 10!
Please enter a number between 1 and 10:
I've sort of worked around this by putting enterNumber just before while, then putting a second
println in the else statement immediately following the error. It returns what I expect, but I believe I'm fundamentally misunderstanding something.
Let's suppose you have the following code:
while (whileCondition) {
//Inside while, before if
if (ifCondition) {
//Inside if
} else {
//Inside else
}
}
This cycle will execute repeatedly, until the whileCondition becomes false. Whenever the ifCondition is true, the operations inside if will be executed, otherwise the operations inside else will be executed.
Back to your problem:
Your line of
System.out.println(enterNumber);
is at the start of the while. So before the code even gets to your if, the content of enterNumber is displayed on the console. Later, your if is evaluated and if you enter, let's say 22, the condition given to the if will be false and the content of the else block will be evaluated.
The else statement is repeated before the while statement. There can however sometimes be a problem with Streams. A Stream does not necessarily print the data immediately. Especially in the case one uses two different streams, the order of printing the output data can be interleaved.
You can use:
System.err.flush();
to ensure the data is written to the console first.
Add this statement in else body:
numberOk=false;