I am expecting input with the scanner until there is nothing (i.e. when user enters a blank line). How do I achieve this?
I tried:
while (scanner.hasNext()) {
// process input
}
But that will get me stuck in the loop
Here's a way:
Scanner keyboard = new Scanner(System.in);
String line = null;
while(!(line = keyboard.nextLine()).isEmpty()) {
String[] values = line.split("\\s+");
System.out.print("entered: " + Arrays.toString(values) + "\n");
}
System.out.print("Bye!");
From http://www.java-made-easy.com/java-scanner-help.html:
Q: What happens if I scan a blank line with Java's Scanner?
A: It depends. If you're using nextLine(), a blank line will be read
in as an empty String. This means that if you were to store the blank
line in a String variable, the variable would hold "". It will NOT
store " " or however many spaces were placed. If you're using next(),
then it will not read blank lines at all. They are completely skipped.
My guess is that nextLine() will still trigger on a blank line, since technically the Scanner will have the empty String "". So, you could check if s.nextLine().equals("")
The problem with the suggestions to use scanner.nextLine() is that it actually returns the next line as a String. That means that any text that is there gets consumed. If you are interested in scanning the contents of that line… well, too bad! You would have to parse the contents of the returned String yourself.
A better way would be to use
while (scanner.findInLine("(?=\\S)") != null) {
// Process the line here…
…
// After processing this line, advance to the next line (unless at EOF)
if (scanner.hasNextLine()) {
scanner.nextLine();
} else {
break;
}
}
Since (?=\S) is a zero-width lookahead assertion, it will never consume any input. If it finds any non-whitespace text in the current line, it will execute the loop body.
You could omit the else break; if you are certain that the loop body will have consumed all non-whitespace text in that line already.
Scanner key = new Scanner(new File("data.txt"));
String data = "";
while(key.hasNextLine()){
String nextLine = key.nextLine();
data += nextLine.equals("") ? "\n" :nextLine;
}
System.out.println(data);
AlexFZ is right, scanner.hasNext() will always be true and loop doesn't end, because there is always string input even though it is empty "".
I had a same problem and i solved it like this:
do{
// process input
}while(line.length()!=0);
I think do-while will fit here better becasue you have to evaluate input after user has entered it.
Related
I need to read a user input that begins with "begin", "BEGIN" or "Begin" and the input can be one or more rows until user writes "end", "END" or "End". End is separated from previous String(s) with non-letter character (new line, space or "}" and so on).
I have tried something like this, but I know that's wrong.
String everything = sc.next();
while (true) {
String part = sc.next();
part.toUpperCase();
if (part.equals("END")) {
everything = everything.concat(part);
break;
} else {
everything = everything.concat(part);
}
}
I think your trying to do too many things at once. Your approach could work, but it's making things more complicated. First, get the user input and store it in a list. Once he presses END, then write some code that concatenates it, or whatever you want to do with it.
Scanner sc=new Scanner(System.in);
ArrayList<String> all=new ArrayList<>();
while (true) {
String part = sc.next();
if (part.toUpperCase().equals("END")) {
break;
}
all.add(part);
}
//then do whatever you want with that list.
I'm trying to read from a file line by line that is tab delimited. Here's an example of a file:
state 0 start
state 5 accept
transition 0 0 1 x R
transition 1 0 0 x R
I create a Scanner object for the file and set the delimiter to \t. I loop through the file as long as there is a next line. I want to check whether a line begins with state or transition and then get the following information. In the case of lines that begin with state, I use nextInt() and then next() to obtain 0 and start respectively. The issue then arises of going to the next line and repeating the process. I use nextLine() and that's where things get ugly.
I'm aware that nextLine() does not consume the newline character, so I thought to use it twice and it creates more of an issue it seems.
Scanner sc = new Scanner(file);
sc.useDelimiter("\t");
while(sc.hasNextLine())
{
if(sc.next().equals("state") == true)
{
int stateNumber = sc.nextInt();
String state = sc.next();
sc.nextLine();
}
sc.nextLine();
}
That seems to be the relveant code that is creating issues.
Am I misunderstanding how next() works or am i missing something else entirely?
One option here would be to simply read in each entire line in one go, and then split by a delimeter (tab) to get the individual components:
which (sc.hasNextLine()) {
String line = sc.nextLine();
String[] parts = line.split("\\t");
if (parts[0].equals("state")) {
// construct custom object here
}
}
If you want to stick with your original approach then use this:
while(sc.hasNextLine()) {
if (sc.next().equals("state")) {
int stateNumber = sc.nextInt();
String state = sc.next();
}
// consume the entire remaining line, line break included
sc.nextLine();
}
For those lines containing "state" you were making two calls to nextLine(), which would result in an entire line being lost.
I'm having trouble understanding the source below:
myChar1 = myScanner.findWithinHorizon(".",0).charAt(0);
System.out.println(myChar1);
myChar2 = myScanner.findWithinHorizon(".",0).charAt(0);
System.out.print(myChar2);
I understand what it does, but I'm just having a bit of a trouble understanding how it works.
The actual prompting of the user for input is done at the first line right? but the real meaning of the first line is: "put the first char of input in myChar1". Then what happens? It seems the input still stays inside myScanner because when I use it in myChar2 I get the second char, but why? why not the first char? Does findWithinHorizon(".",0).charAt(0) deletes the char that is assigned to the variable?
And last question: if in the first line the program prompts the user for input why doesn't it do it again in the second line?
Also, a quick recap of the (".",0) would be helpful as well.
Perhaps the piece you are missing is that findWithinHorizon actually takes a regular expression as the String argument. In a regular expression, . matches any character (except a new line).
A call to findWithinHorizon(".", 0) simply finds the next character in the input and advances the Scanner past whatever was found.
So for example,
Scanner in = new Scanner("abc123");
for(;;) {
String found = in.findWithinHorizon(".", 0);
if(found == null) break;
System.out.println(found);
}
the output is:
a
b
c
1
2
3
The reason it does not prompt for input at the second line is that is the way Scanner and System.in work together. They will only block and prompt for input if there is no more existing input to consume. As a short example, try this out:
Scanner in = new Scanner(System.in);
while(true) {
System.out.println(in.findWithinHorizon(".", 0));
}
That will loop infinitely, repeating back whatever you input, character by character. It will only prompt for more when it's done with the prior input. On the first iteration, Scanner will call read on System.in which will block and wait for input. When the input runs out, read will block again.
I will admit, this is a school assignment... But I simply cannot figure out what I am doing wrong.
I have a hash table with an insert function. The following code is supposed to take a line of data from System.in in the format "Long String" (i.e. "32452 John"). The first token must be a Long for the ID number, and it must be followed by a String token for the name. When I run the program and I get to the portion where this must be executed (It is in a switch statement), I entered 'a' and hit enter. The command line immediately reads "Invalid value." (note: not VALUES, as that would mean it hit the nested if statement. It won't let me type in any data. Thank you in advance!
System.out.println("Enter ID and Name.");
//temp to take in the next line entered by the user
//inScan is the Scanner for System.in
temp = inScan.nextLine();
//Create Scanner for the line
Scanner tempScan = new Scanner(temp);
if(tempScan.hasNextLong()){
thisID = tempScan.nextLong();
if((tempScan.hasNext()) && (thisID>0)){
thisName = tempScan.next();
//The data will only be inserted if both segments of data are entered
myTable.insert(new Student(thisID, thisName));
}else{
System.out.println("Invalid values.");
}
}else{
System.out.println("Invalid value.");
}
Why do you need the second Scanner?
Example
String input = scanner.nextLine();
String[] tokens = input.split(" ");
Long id = Long.parseLong(tokens[0]);
String name = tokens[1];
And if you wanted to add your validation:
String input = scanner.nextLine();
if(input.contains(" ")) {
// You know there's a space in it.
String[] tokens = input.split(" ");
if(tokens.length == 2) {
// You know it's a value, followed by a space, followed by a value.
if(tokens[0].matches("[0-9]+")) {
// You know it only contains numbers.
Long id = Long.parseLong(tokens[0]);
}
}
}
I've not run it, but i guess your problem is that when you enter the text 'a' and hit enter, this line is false:
if(tempScan.hasNextLong()){
as you haven't entered a number. hence why it drops to the next block. If you enter something numerical first, i suspect your code with work. you probably need to add a 'while' loop around it, to run until it gets a number.
You already have a Scanner which reads from System.in, there's no need for another one. The second one you've made is a scanner for a String, which will never have a nextLong as it has nothing to scan after your String.
I won't write any code for you as this is homework, but stick to your original scanner when checking for user input instead.
Ok, so I can't seem to get this to work, though many people have told me the syntax and logic is correct. Can anyone reveal for me what I could possibly be doing wrong?
public Scanner in = new Scanner(System.in);
public void movePlayer() {
System.out.print("move: ");
String str = in.nextLine();
in.nextLine();
char c = str.charAt(0);
if (c == 'l' || c == 'L') {
player.moveLeft();
}
}
The program gets caught at char c = str.charAt(0);
And I am being returned this error:
java.lang.StringIndexOutOfBoundsException:
String index out of range: 0 (in java.lang.String)
you did not input anything though the console, so str is empty. this is the reason why chatAt(0) throw an exception
You don't want to use nextLine(). You want to use next().
String str = in.next();
This is the Javadoc for nextLine()
Advances this scanner past the current line and returns the input that was skipped. This method returns the rest of the current line, excluding any line separator at the end. The position is set to the beginning of the next line.
You want next() instead:
Finds and returns the next complete token from this scanner. A complete token is preceded and followed by input that matches the delimiter pattern. This method may block while waiting for input to scan, even if a previous invocation of hasNext() returned true.
This will stop you from consuming the empty line and raising an exception.
This means that str is empty. You should check if it is not null and not empty.
if (str != null && !str.isEmpty()) {
...
}
Add a check for Empty String and Null as well . You will avoid a lot of headaches.
If you press Enter key in console, Scanner will be considered a complete line, regardless of whether or not there is text entered.
Press Enter at the beginning of a line, returns a String "" to the method Scanner.nextLine().
Add a check with str.lenght () > 0 before str.charAt(0).
Use in.next() instead. For whatever reason, nextLine() doesn't work with CharAt() sometimes.