How to use a Scanner to get two sets of data from stdin?

[Anil Philip]

I am using a Java IDE (IntelliJ).
I want to read in an array of integers (nums) and then a single integer (target).
I found that using CTRL-D to end the array input will reset stdin, so thought I should read the array as a string (to parse later) and then read the integer (target).
But I am unable to get Scanner to read the second integer.

import java.util.ArrayList;
import java.util.Scanner;

public class TwoSum_Scanner {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        Integer target = -1;
        System.out.println("input the array of integers");
        String s = null;
        while (scanner.hasNextLine()) {
            s = scanner.nextLine();
        }
        System.out.println("s " + s);
        // TODO parse the string into ints and add to array list

        System.out.println("input the target integer");
        while (scanner.hasNextInt()) {
            target = scanner.nextInt();
        }
        System.out.format("target %d", target);
    }
}

OUTPUT

input the array of integers
1 3 5
4^D
s 1 3 5
input the target integer
target -1
Process finished with exit code 0

[Ron McLeod]

You could just trying to read integers from the scanner - if the next token can not be transformed to an int, then Scanner#hasNextInt will return false.

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class TwoSum_Scanner {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("input the array of integers, end with a dot character");
        List<Integer> ints = new ArrayList<>();
        while (scanner.hasNextInt()) {
            ints.add(scanner.nextInt());
        }
        System.out.println("ints: %s".formatted(ints));
    }
}

$ java TwoSum_Scanner.java input the array of integers, end with a dot character 22 45 6 . ints: [22, 45, 6] $ java TwoSum_Scanner.java input the array of integers, end with a dot character 45 68 . ints: [45, 68] $ java TwoSum_Scanner.java input the array of integers, end with a dot character 43 56 555 . ints: [43, 56, 555]

[Anil Philip]

Ron McLeod wrote:You could just trying to read integers from the scanner - if the next token can not be transformed to an int, then Scanner#hasNextInt will return false.

Thank you for your reply.
I want to read the array of ints and then a target int.
I did try hasNextInt() and nextInt() but was unable to read the separate field, the integer 'target'.
Are you able to do that?

        Scanner scanner = new Scanner(System.in);
        ArrayList al = new ArrayList();
        Integer target = -1;
        System.out.println("input the array of integers");
        while (scanner.hasNextInt()) {
            al.add(scanner.nextInt());
        }
        System.out.println("al " + al);
        System.out.println("input the target integer");
        while (scanner.hasNextInt()) {
            target = scanner.nextInt();
        }
        System.out.format("target %d", target);

OUTPUT

input the array of integers
1 3 5 .
al [1, 3, 5]
input the target integer
target -1
Process finished with exit code 0

[Ron McLeod]

Maybe this? :

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class TwoSum_Scanner {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("input the array of integers, end with a dot character");
        List<Integer> ints = new ArrayList<>();
        while (scanner.hasNextInt()) {
            ints.add(scanner.nextInt());
        }
        scanner.next(); // consume the dot character
        System.out.println("ints: %s".formatted(ints));

        System.out.println("%ninput the target integer".formatted());
        int target = -1;
        if (scanner.hasNextInt()) {
            target = scanner.nextInt();
        }
        System.out.println("target: %d".formatted(target));
    }

$ java TwoSum_Scanner.java input the array of integers, end with a dot character . ints: [] input the target integer 53 target: 53

[Campbell Ritchie]

I prefer to use a utility class to wrap a Scanner for keyboard input; you can prevent exceptions if the entries are of the wrong format.

Anil Philip wrote:. . . I found that using CTRL-D to end the array input will reset stdin. . . .

No, don't use ctrl‑D; it can close System.in, leaving you with no way to reopen it. And it's probably better to call it System.in rather than stdin when writing Java® please.
I think I would use a Stream (some of these methods were not available in Java8).

Scanner input = ...;
System.out.print("Please enter ...: ");
int[] numbers =
        input.tokens()
             .takeWhile(s -> new Scanner(s).hasNextInt())
             .mapToInt(Integer::praseInt) // mistake in this line
             .toArray();
int i = input.nextInt();

Line 1 creates the Scanner

Line 2 issues instructions

Line 3 declares an array to be produced by the code following.

Line 4 takes all the tokens entered and creates a Stream<String> to manipulate them.

Line 5 uses a λ expression to test the input by passing it to a new Scanner object. That may seems slow, but if it takes 1 second to enter a number on the keyboard, and maybe 0.05μs to create the Scanner, you haven't got any problems. The hasNextInt() method will return false if you pass anything not an int, and then takeWhile() will finish populating the Stream.

Line 6 uses the mapToInt() method to create a special Stream for ints only. Note I have passed a method references to this method and made a mistake writing the code.

In line 7, this method directly produces an int[].

To get to line 8, enter one token that is not an int, and you can then read the number. You will have to push the enter key to get all the tokens processed.

[Campbell Ritchie]

Anil Philip wrote:. . . I want to read in an array of integers  . . .

Don't confuse arrays and array lists. A list isn't an array, and you are creating a list.

[Anil Philip]

Ron McLeod wrote:Maybe this? :

Thanks!
I was trying to use skip() but I did not know how to specify CTRL-D in the pattern.
Your method of using dot to end input, and using next() to skip input, works.
Thank you!

[Anil Philip]

Campbell Ritchie wrote:
I think I would use a Stream (some of these methods were not available in Java8).

Thank you for the streams version; it is instructive. I was unable to figure out how I would do it using streams!
But it seems inefficient to create a new Scanner for each input integer.

[Anil Philip]

Campbell Ritchie wrote:passing it to a new Scanner object.

Is there a way to reuse the Scanner instead of creating a new one each time?
When I try your version, I get a ConcurrentModificationException

import java.util.ArrayList;
import java.util.Scanner;

public class TwoSum_Scanner {
    public static void main(String[] args) {
        final Scanner scanner = new Scanner(System.in);
        Integer target = -1;
        System.out.println("input the array of integers");

        int[] nums = scanner.tokens().takeWhile(s->scanner.hasNextInt())
                .mapToInt(Integer::parseInt)
                .toArray();
        System.out.println("nums " + nums);
        System.out.println("input the target integer");
        target = scanner.nextInt();
        System.out.format("target %d", target);
    }
}

input the array of integers
1 3 5 . 4
Exception in thread "main" java.util.ConcurrentModificationException
at java.base/java.util.Scanner$TokenSpliterator.tryAdvance(Scanner.java:2851)
at java.base/java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:129)
at java.base/java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:527)
at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:513)
at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:575)
at java.base/java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260)
at java.base/java.util.stream.IntPipeline.toArray(IntPipeline.java:562)
at two_sum.TwoSum_Scanner.main(TwoSum_Scanner.java:14)

Process finished with exit code 1

[Ron McLeod]

There is probably a better way to work with a stream of tokens, but this is my first thought:

import java.util.Arrays;
import java.util.Objects;
import java.util.Scanner;
import java.util.function.Function;

public class TwoSum_Scanner {

    static Function<String, Integer> parseIntegerFromToken = s -> {
        try {
            return Integer.valueOf(s);
        } catch (NumberFormatException e) {
            return null;
        }
    };

    public static void main(String[] args) {
        final Scanner scanner = new Scanner(System.in);
        System.out.println("input the array of integers");

        int[] nums = scanner.tokens()
                .map(parseIntegerFromToken )
                .takeWhile(Objects::nonNull)
                .mapToInt(Integer::intValue)
                .toArray();
        System.out.println("nums " + Arrays.toString(nums));
    }
}

[Anil Philip]

Ron McLeod wrote:There is probably a better way to work with a stream of tokens, but this is my first thought:

Does not compile using intValue.

java: incompatible types: invalid method reference
   method intValue in class java.lang.Integer cannot be applied to given types
     required: no arguments
     found:    java.lang.String
     reason: actual and formal argument lists differ in length

Using parseInt, it will compile but then

Exception in thread "main" java.lang.NumberFormatException: For input string: "."

import java.util.ArrayList;
import java.util.Objects;
import java.util.Scanner;

public class TwoSum_Scanner {
    public static void main(String[] args) {
        final Scanner scanner = new Scanner(System.in);
        Integer target = -1;
        System.out.println("input the array of integers");

        int[] nums = scanner.tokens().takeWhile(Objects::nonNull)
                .mapToInt(Integer::parseInt)
                .toArray();
        System.out.println("nums " + nums);
        scanner.next();
        System.out.println("input the target integer");
        target = scanner.nextInt();
        System.out.format("target %d", target);
    }
}

[Carey Brown]

import java.util.Arrays;
import java.util.Objects;
import java.util.Scanner;
import java.util.function.Function;

public class ScannerInputArray
{
	static final Scanner	scanner = new Scanner( System.in );

	public static void main( String[] args )
	{
		System.out.println( "input the array of integers followed by a dot (.): " );
		int[] nums = scanner.tokens()
			.map( parseIntegerFromToken )
			.takeWhile( Objects::nonNull )
			.mapToInt( Integer::intValue )
			.toArray();
//		scanner.next();
		System.out.println( "nums " + Arrays.toString( nums ) );
		
		System.out.println( "input the target integer: " );
		int			target = scanner.nextInt();
		System.out.printf( "target %d%n", target );
	}

	static Function<String,Integer> parseIntegerFromToken = s ->
	{
		try
		{
			return Integer.valueOf(s);
		}
		catch( NumberFormatException e )
		{
		}
		return null;
	};
}

[Campbell Ritchie]

Anil Philip wrote:. . . Is there a way to reuse the Scanner instead of creating a new one each time?

No. You are looking at the token previously read, not the next token. I have already told you, the overhead of creating multiple objects is tiny compared to the time it takes to enter numbers.

When I try your version, I get a ConcurrentModificationException . . .

That is because you are not using what I wrote. You are trying to do two things simultaneously with the Scanner, which it doesn't like. I have only seen that exception with Iterators . . . . but a Scanner IS‑AN Iterator<String>.

[Campbell Ritchie]

Ron, I am afraid I think the parsing method is using exceptions for flow control. Not something I like.
AP: Since Scanner#tokens() never includes null, that takeWhile() call will never terminate the Stream, and you will suffer an exception.

[Carey Brown]

A variation that (mostly) avoids using exception handling for flow control.

import java.util.Arrays;
import java.util.Scanner;
import java.util.regex.Pattern;

public class ScannerInputArray
{
	static final Scanner	scanner = new Scanner( System.in );
	static final Pattern	INT_ABSOLUTELY = Pattern.compile( "-?\\d{1,9}" );
	static final Pattern	INT_PROBABLY = Pattern.compile( "-?\\d{10}" );

	public static void main( String[] args )
	{
		System.out.println( "input the array of integers followed by a dot (.): " );
		int[] nums = scanner.tokens()
			.takeWhile( s -> isInt(s) )
			.map( Integer::valueOf )
			.mapToInt( Integer::intValue )
			.toArray();
		System.out.println( "nums " + Arrays.toString( nums ) );
		
		System.out.println( "input the target integer: " );
		int			target = scanner.nextInt();
		System.out.printf( "target %d%n", target );
	}

	static boolean isInt( String s )
	{
		if( INT_ABSOLUTELY.matcher( s ).matches() )
			return true;
		if( ! INT_PROBABLY.matcher( s ).matches() )
			return false;
		
		// Usually returns by now but there's an edge case that could slip
		// through that still needs to be caught.
		try
		{
			Integer.parseInt( s );
		}
		catch( NumberFormatException ex )
		{
			return false;
		}
		return true;
	}
}

[Campbell Ritchie]

That isInt() method looks like the sort of thing Scanner does already.