CODEWITHSUNDEEP
java
user10755417
user10755417

Reputation:

Creating Binary World Representation - How to Set a Bit in a Long and then Print the Long

I am using java to model Conway's Game of Life. I am setting a bit in a long and then printing the bit.

My current setCell method is changing the specified bit and all of the bits after that bit index, rather than just the bit at the specified index.

How do I need to change my class?

public class TinyLife {

private final int side_length = 8;
private long world;

public TinyLife(long world){
    this.world = world;
}

public void printWorld(){
    System.out.println("\nThe current world:");
    for (int row = 0; row < side_length; row++){
        for (int col = 0; col < side_length; col++){
            System.out.print(getCell(col, row) ? " # " : " _ ");
        }
        System.out.println();
    }
}

public boolean getCell(int col, int row){
    int index = (row * side_length) + col;
    long a = world >> index; // Shift the bits by position.
    long c = a & 1;
    return (c == 1);
}

public void setCell(int col, int row, boolean value) {
    int index = col + (row * side_length);
    System.out.println(index);
    /* I think that the error is below. Why are all the bits to the left of the bit at the index also being set? */
    if (value) {
        world |= (1 << index);
    }
    else world = world & ~(1 << index);
}

public static void main(String[] args) {
    long first_world = 0x20272000027202L;
    TinyLife world = new TinyLife(first_world);
    world.printWorld();
    System.out.println("Binary rep before setCell = " + Long.toBinaryString(world.world));
    world.setCell(7, 3, true);
    System.out.println("Binary rep after setCell = " + Long.toBinaryString(world.world));
    world.printWorld();
}
}

Expected printout:

The current world:

 _  #  _  _  _  _  _  _ 
 _  #  _  _  #  #  #  _ 
 _  #  _  _  _  _  _  _ 
 _  _  _  _  _  _  _  _ 
 _  _  _  _  _  #  _  _ 
 #  #  #  _  _  #  _  _ 
 _  _  _  _  _  #  _  _ 
 _  _  _  _  _  _  _  _ 

Binary rep before setCell = 100000001001110010000000000000000000100111001000000010
31
Binary rep after setCell = 100000001001110010000010000000000000100111001000000010

The current world:

 _  #  _  _  _  _  _  _ 
 _  #  _  _  #  #  #  _ 
 _  #  _  _  _  _  _  _ 
 _  _  _  _  _  _  _  # 
 _  _  _  _  _  #  _  _ 
 #  #  #  _  _  #  _  _ 
 _  _  _  _  _  #  _  _ 
 _  _  _  _  _  _  _  _

Actual printout:

The current world:

 _  #  _  _  _  _  _  _ 
 _  #  _  _  #  #  #  _ 
 _  #  _  _  _  _  _  _ 
 _  _  _  _  _  _  _  _ 
 _  _  _  _  _  #  _  _ 
 #  #  #  _  _  #  _  _ 
 _  _  _  _  _  #  _  _ 
 _  _  _  _  _  _  _  _ 

Binary rep before setCell = 100000001001110010000000000000000000100111001000000010
31
Binary rep after setCell = 1111111111111111111111111111111110000000000000100111001000000010

The current world:

 _  #  _  _  _  _  _  _ 
 _  #  _  _  #  #  #  _ 
 _  #  _  _  _  _  _  _ 
 _  _  _  _  _  _  _  # 
 #  #  #  #  #  #  #  # 
 #  #  #  #  #  #  #  # 
 #  #  #  #  #  #  #  # 
 #  #  #  #  #  #  #  #

(Note that I added a few line breaks to the text above to try to make my question easier to read)

Thank you very much for any help.

Upvotes: 0

Views: 50

Answers (2)

Oleg Cherednik
Oleg Cherednik

Reputation: 18245

The problem is that 1 << index is an int operation, not a long operation.

That's correct from Andreas. I would like to add to this, that you use index calculation in 3 places, but this problem appears in only one of them. I think you could extract it to the separate method and use it. It could be more clear.

// convert given `cal` and `row` to long with single bit set
private long bit(int col, int row) {
    // optionally you could add check for correct col and row (0 <= col (row) <= side_length - 1)
    return 1L << (row * side_length) + col;
}

then you can simplify your code:

public boolean getCell(int col, int row) {
    return (world & bit(col, row)) != 0;
}

public void setCell(int col, int row, boolean value) {
    if (value)
        world |= bit(col, row);
    else
        world &= ~bit(col, row);
}

Upvotes: 1

Andreas
Andreas

Reputation: 159106

The problem is that 1 << index is an int operation, not a long operation.

With inputs col = 7, row = 3, the calculation index = col + (row * side_length) yields the value 31, which in an int value is the sign bit.

world |= (1 << index) then widens the int value to a long using sign-extension, setting all upper 32 bits to 1's.

Change code to world |= (1L << index), so the bit-shift is done as a long. That way it'll also work for index value above 31.

Upvotes: 1

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