I've been searching overflow questions and googling it for about half an hour and can't find an answer for this.
At first I thought it might be that I'm not closing my Scanner object. I added
inp.close();
after all my code,but still nothing.
I'm using Eclipse to create a simple Binary Search algorithm.
My problem is that it is not keeping my input. And what's even weirder is that it only accepts "5".
After pressing enter it only creates more spaces. It doesn't move on to the rest of the program.
I've also tried entering more values under the "skipped" ones without any success.
Here's some screenshots
nextInt() reads scans the next token of the input as an int.
If the input is not an int, then an InputMismatchException is thrown.
You can use next() to read the input, whatever its type is. And you can hasNextInt() to make sure the next input is an int:
Scanner inp = new Scanner(System.in);
if(inp.hasNext()) {
if(inp.hasNextInt()) {
int n = inp.nextInt();
// do something with n
} else {
String s = inp.next();
// do something with s
}
}
Actually, I have a theory - your Scanner code is working just fine, it's your binary search code that's broken. Whatever it's doing works on an input of 5 but falls into an infinite loop for other inputs.
Consider breaking up your input parsing code from your binary searching code (e.g. do input parsing in main() and define a binarySearch() function that main() calls) so that you can test them separately.
Related
So, before I start I just wanted to say that I'm very new to Java as a language and I've been reading a text book that was recommended to me.
One of the examples provided within the text book on for loops had the following code, which is meant to generate an infinite for loop until the user presses the character 'S' on their keyboard.
Here is the code:
class ForTest {
public static void main(String args[])
throws java.io.IOException {
int i;
System.out.println("Press S to stop.");
for (i = 0; (char) System.in.read() != 'S'; i++)
System.out.println("Pass #" + i);
}
}
I copied the code exactly as it was written within the book but when I run the program, to my surprise, it doesn't start printing out numbers onto the console. Additionally, whenever I press any keys on the keyboard it generates three numbers within the sequence. Example shown below:
I have also included a screenshot of the code from the book below:
I was wondering whether anyone knows why this is the case!
Any help would be greatly appreciated thanks.
The reason it's printing multiple times is because multiple characters are detected.
In your case, it's printing twice because you entered a value (Pass 1) and a new line (Pass 2)
The problem you have is not with System.in.read(), but because the console is usually using a buffered approach. Meaning that data is only transferred to the System.in.read() once you press enter.
So to get the example working, you would have to switch the console to an unbuffered mode, but there is no portable way to do this, because there are so much different types of consoles. Maybe have a look at what editor/console the book is using
This block of code looks like it was written by someone who was deliberately trying to make it obtuse and difficult to comprehend for a beginner.
The middle expression of a for statement is the criterion for taking the next step of the loop. It is evaluated before each step of the loop to determine whether the for loop is complete yet. In this case, it calls in.read() and checks if the input is S before each step of the loop.
in.read() waits for the next line of input it gets. When you enter a value and press Enter, that line gets read, so the loop takes a step. And a new line is also entered, so the loop takes a second step.
It will not print lines to the console unless you enter lines, because in.read() causes the program to block (wait) for the next input.
I want to output a question to the console and then get the next line of input after the question was output.
For example, my program could be sleeping or doing some time-consuming computation, and while the user is waiting they might decide to type some notes into the console (perhaps without hitting enter, or perhaps over several lines). Once the sleep is completed, the program then asks the user a question, "What is your name?" and then it should wait for the next line of input containing the user's name, and ignore any random notes the user made while the sleep was going on.
Here's some code that tries to do that:
public static void main(String[] args) throws InterruptedException {
Scanner scanner = new Scanner(System.in);
Thread.sleep(10_000);
System.out.println("What is your name?");
// while (scanner.hasNext()) {
// scanner.nextLine();
// }
String name = scanner.nextLine();
System.out.println("Hi, " + name);
}
This behaves as follows when I type a couple of lines during the sleep:
gpeo
hpotWhat is your name?
Hi, gpeo
The problem is that scanner will read the next input continuing from the last input it read, not from the last System.out.println() (which makes sense). The commented out code tries to rectify that problem by reading past all earlier input first, then waiting on one more line to assign to name. However, scanner.hasNext() does not work as I was hoping, since when there is no next token it does not simply return false but waits for another token (so I don't know why it bothers to return a boolean at all).
Another thing that baffles me is that during the sleep if you type stuff on a single line, that single does in fact get ignored:
brbr irgjojWhat is your name?
A
Hi, A
I thought it was going to output Hi, brbr irgjojA, so that makes me think I might be misunderstanding how console input and Scanner work.
Edit: The last example was from a run within IntelliJ. When I run from my Bash commandline instead I get Hi, brbr irgjojA. The output of the first example does not change though.
Also, I was asked if this question is the same as this, and apparently I have to explain why it's not here or it will appear on the question. The issue in that post (and others like it) is that he/she is mixing scanner.nextLine() with scanner.nextInt() and similar methods that do not read the whole line or the line ending. I am only using nextLine() to read input, and my issue is quite different.
Further edit
I managed to discard the first line of random notes based on this answer to another question. Here is the new code:
public static void main(String[] args) throws InterruptedException, IOException {
Scanner scanner = new Scanner(System.in);
Thread.sleep(10_000);
System.out.println("What is your name?");
while (System.in.available() > 0) {
scanner.nextLine();
}
String name = scanner.nextLine();
System.out.println("Hi, " + name);
}
Here are some test runs in IntelliJ:
grgWhat is your name?
A
Hi, A
ghr
rhWhat is your name?
A
Hi, A
rghr
hrh
htWhat is your name?
Hi, hrh
uirh
iw
hjrt
sfWhat is your name?
Hi, iw
And here are similar tests in Bash:
htrWhat is your name?
A
Hi, htrA
rgj
hrWhat is your name?
A
Hi, hrA
rjkh
ry
jWhat is your name?
Hi, ry
ryi
rj
rd
jrWhat is your name?
Hi, rj
As you can see, the line inside the while loop never appears to get executed more than once for some reason. I tried adding a sleep inside the loop or using other InputStream methods like skip() and readAllBytes(), but these didn't seem to help at all.
I think there might not be anything one can do about the incomplete line that is a problem for Bash, but I'm sure there must be a way to throw out all the completed lines (rather than just the first one). The solution doesn't have to use Scanner, it should just behave as intended.
The Scanner uses a buffer. It’s default size is 1024 characters. So by the first nextLine() call, it reads up to 1024 of the available characters into the buffer. This is necessary, as the Scanner doesn’t even know how many characters belong to the next line, before filling the buffer and searching for a line break in the buffer.
Therefore, if there are less pending characters than the buffer size, the loop will iterate only once. But even when there are more characters, and more loop iterations, the resulting state likely is to have some pending lines in the buffer.
As long as the Scanner’s buffer is in its initial empty state, you can flush the source stream directly, instead of using the scanner:
Scanner scanner = new Scanner(System.in);
Thread.sleep(10_000);
while(System.in.available() > 0) {
System.in.read(new byte[System.in.available()]);
}
System.out.println("What is your name?");
String name = scanner.nextLine();
System.out.println("Hi, " + name);
Note that it would be natural to use System.in.skip(System.in.available()); instead of read, but while trying it, I encountered a bug that the underlying stream did not update the available() count after a skip when reading from a console.
Note that if the Scanner is not in its initial state but has some buffered content already, there is no way to flush the buffer, as its API is intended to make no distinction between buffered and not yet buffered, so any attempt to match all content would result in reading from the source (and blocking) again. The simplest solution to get rid of the buffered content would be
scanner = new Scanner(System.in);
*EDIT - SOLVED: After instantiating the Scanner Object, I used a delimiter as follows:
scanner.useDelimiter("");
Prior to this, I did try a delimiter that looked something like this (the exact code is available on Stack Overflow):
scanner.useDelimiter("\\p{javaWhitespace}");
...but it didn't work very well.
Thank you, everyone. If you're having this very same issue, try the first delimiter. If it doesn't work, upgrade your JDK to 13 then try it again.
Ok, my goal is to have a user input a credit card number which I would then like to store in an ArrayList of Integers and subsequently pass this list to my functions which will perform the Luhn algorithm in order to validate the provided number. Once the user presses Enter, the processing begins. This is a console application, nothing fancy.
Everything works beautifully...except the user-input part. None of the user-input is being stored into the declared ArrayList. I've inserted a print message to give me the size of the list just after the pertinent while-loop and....yep, 0. I also pass this list into a custom lengthChecker(ArrayList<Integer> list){} function subsequent to the relevant while-loop and it's printing my custom error-message.
I have declared local int variables within the scope of the while-loop and that wasn't helping much. I have tried getting the user's input as Strings and storing them in an ArrayList<String> list; then parsing the input but that didn't work very well (especially as I need the Enter key to behave as a delimiter such that the next steps can take place)
Anyways, here is the code to the function in question. Am I missing something obvious or should I just quit programming?
public void userInput() {
Scanner scanner = new Scanner(System.in);
List<Integer> list = new ArrayList<Integer>();
System.out.println("Please input the card-number to be checked then press Enter: ");
while(scanner.hasNextInt()) {
list.add(scanner.nextInt());
}
System.out.println("Length of list: " + list.size());
listLengthChecker(list);
scanner.close();
}
Thank you in advance.
I don't have the full context on all the code you've written to be able to solve your problem, but I can guess at what's going on. If you want to run any user I/O (such as the scanner), it must occur within the main method. I can only assume that you run your userInput() function within the main method in your class. However, because your userInput() function doesn't have the static keyword in its definition, it can't be accessed without initialising an object of the class - but as far as I can tell from your code, there is no object that the method could refer to. Add the static keyword (i.e. initialise the method as public static void userInput()) to be able to run the function as you intend.
As for the while loop - there's a small chance that this is a difference in Java versions (I use Java 11), but while(scanner.hasNextInt()) won't stop being true at the end of your line or when you press enter - only when you insert something (such as a character) that cannot be interpreted as an integer.
This while loop untill you enter any non integer value.
You finished entering all the integer values and then your program will print your list elements.
I'm trying to make a tic-tac-toe game and I'm encountering a lot of copy-paste work for inputs. I'm trying to figure out what design pattern and implementation works for prompting the user, collecting their input, comparing it and then acting by assigning a value. Right now my code looks like this.
public void promptPlayerCount(BufferedReader in) throws IOException {
String input;
// initial prompt
System.out.println("How many players?");
input = "try again";
while (input.equals("try again")) {
input = in.readLine();
// extract data and check it
switch (Integer.parseInt(input)) {
case 1:
// assignment
playerCount = 1;
break;
case 2:
playerCount = 2;
break;
default:
input = "try again";
// clarified instructions
System.out.println("please enter 1 or 2");
}
}
}
There's a part of me that thinks I could make a function (maybe a factory?) that allows me to generate a function by passing the constructing function the details of the initial prompt, the extraction method, the assignment action and the clarification message.
Would this be best done with lambda functions?
Text input is hard, especially if you can't trust your user (like in a game). Your parseInt will throw a nasty exception right off if your value isn't an integer.
Also standard in is not friendly. I assume this is for an assignment so I won't fault you for using it, but in anything where you don't HAVE to use stdin, don't. The problem is that it's amazingly difficult to get Java to respond to anything less than an entire line with an enter at the end.
When dealing with user input I almost always trim it (Just because they love to insert random white spaces at the beginnings and end) and check to see if it's empty. This could probably be put into a function that also either shows an error or exits the program on "Empty" and otherwise returns a string.
If you often want int values, write a second function that calls the first. Have the second function return an int, but have it catch the exception if the text is invalid and prompt the user again. You could even have this function take a "Range" of integers as a parameter and provide a prompt. So what you have above could look like this:
playerCount = getUserInput("Please enter the number of users", 1, 2);
The rest is wrapped in simple non-redundant functions.
Won't write the code for you because A) it's probably a homework assignment and the fun part is actually coding it and B) someone else probably will provide a full solution with code before I'm done typing this :(
Good luck.
We need to make something of a lexical analyzer, and I have had some trouble with a particular function of mine, useLoad, or more accurately, what happens in Main after useLoad is used.
I figured out that that was because...for some reason, buffer=keyboard.nextLine() is throwing the error, because it's not getting more input from the keyboard for some reason. I thought that .nextLine() should force it to get more input from the user. And I don't know why it's throwing that exception specifically after this one particular method. It can do other methods just fine and not lose its ability to read. Is it because I have a variable called keyboard in another object and closed it? That seems doubtful. Just tried changing the name. Didn't make a difference.
Variables used but not declared in the below code: Keywords[0] is the String "load ". initial = the scanner string that's passed in to the function. offset = a counter variable, to see how far in to the line we've read.
The useLoad function (which is what I think is messing up somehow), is at the bottom, but I included everything it runs through (with each method separated by a horizontal rule), in chronological order, just in case I'm just not seeing what's going on.
public static void main(String[] args) {
Scanner keyboard = new Scanner(System.in); //the scanner for keyboard
int i = 0;
String buffer ="";
boolean loopControl == true;
SymbolTable mySym = new SymbolTable();
System.out.println("READY FOR INPUT\n");
while (loopControl == true){
//read in the line
buffer = "";
buffer = keyboard.nextLine();
if(!mySym.checkStop(buffer)){ //if we didn't stop
mySym.primary(buffer);
}
else{//if we did stop
closeLoop();
}
if (i >= 55){
loopControl = false;
System.out.println(("You have gone over the limit ("+i+" lines) per execution. Please continue by running this program again.").toUpperCase());
//just a safety precaution...you know... in case closeLoop doesn't work
}
i++;
}
keyboard.close();
}
if(initial.substring(0, Keywords[0].length()).equals(Keywords[0])){ //Load
//if this is working as expected, then we simply need to do what the keyword says to do.
offset += Keywords[0].length(); //we have moved this much deeper in to the line
useLoad(offset, initial);
offset = 0; //just make sure, once we are done with the line, we start back at the start of the next line.
return; //we found what we were looking for, get out.
}
private void useLoad(int offsetIn, String readIn) {
double doubIn = 0;
//now get the value of the
Scanner keyboard = new Scanner(System.in); //the scanner for keyboard
System.out.println("\nENTER VALUE FOR " + readIn.toUpperCase());
doubIn = keyboard.nextDouble();
keyboard.close();
variables.create(readIn.substring(offsetIn), doubIn);
}
I think I've figured out your problem.
Java docs for both Java 7 and 8 include this line in Scanner's close method documentation:
If this scanner has not yet been closed then if its underlying readable also implements the Closeable interface then the readable's close method will be invoked.
Looking into the docs for System, I've found that System.in is of type InputStream which, you guessed it, implements Closeable. The close method documentation for InputStream says that it does nothing; however, InputStream is abstract and close is not marked as final, which means it can be overridden. System.in returns an InputStream which could potentially - and clearly does - do something.
So the problem is, you are creating multiple Scanners with System.in, and each time you close any one of them, you close System.in, rendering it useless!
This problem has actually been discussed in another question here, with a solution given. That said, for your program I would suggest one of two approaches:
The first approach is mentioned there: Either use a pre-made wrapper class or make your own, which accepts an InputStream in its constructor. Have this class' InputStream implementation call all the methods of its wrapped object, except for the close method which is a no-sell, and then pass Wrapper(System.in) to Scanner instead of System.in directly. However, I would be wary of taking this approach except in very specific circumstances, because any time you use one of these wrappers you would need to remember to close its wrapped object at the end of its use, unless it's something like System.in.
Second approach: Use a runner class in your program, and initialize the scanner there. Pass the scanner into the required objects during construction for them to hold a reference to, and then allow them to complete their required tasks without closing Scanner within the class. Once the exit condition is specified, return to the runner class and close Scanner from there.
You close the keyboard each time round the loop. Therefore the second time around you read from a closed keyboard object.
A quick look at the documentation for Scanner.nextLine contains the news that it might throw:
NoSuchElementException - if no line was found