General Java Question concerning byte operator and byte rotation

[Winterrage]

Greetins everyone,

I'm trying to understand the logic in some operation with different byte operator.

I'm looking at:
http://www.exampledepot.com/egs/Programs/sudoku_solver_SudokuSolver.html

More specifically, this operation from the set function:

boolean canSet = cells[loc] == 0
&& (colsSet[c] & (1<<num)) == 0
&& (rowsSet[r] & (1<<num)) == 0
&& (subgridSet[blockLoc] & (1<<num)) == 0;
if (!canSet)
{
return false;
}

From this function: public boolean set(int loc, int num)
colSet and rowSet are filled with a lots of 0.

The question is what is colsSet[c] & (1<<num) used for?

Thanks again :)

[JosAH]

Greetins everyone,

I'm trying to understand the logic in some operation with different byte operator.

I'm looking at:
http://www.exampledepot.com/egs/Programs/sudoku_solver_SudokuSolver.html

More specifically, this operation from the set function:

boolean canSet = cells[loc] == 0
&& (colsSet[c] & (1<<num)) == 0
&& (rowsSet[r] & (1<<num)) == 0
&& (subgridSet[blockLoc] & (1<<num)) == 0;
if (!canSet)
{
return false;
}

From this function: public boolean set(int loc, int num)
colSet and rowSet are filled with a lots of 0.

The question is what is colsSet[c] & (1<<num) used for?

Thanks again :)

It's a bit of 'bit flag packing'. The expression '1<<num' shifts the number/bit 1
to the left by 'num' positions; here's a little table:

num= 0: result: 0x01
num= 1: result: 0x02
num= 2: result: 0x04
num= 3: result: 0x08
num= 4: result: 0x10
num= 5: result: 0x20
num= 6: result: 0x40
num= 7: result: 0x80
 

I bet colSet and rowSet are int or byte arrays where each bit of every element
of the array is used as a 'boolean' flag.

The '& operator checks a single bit here: it is either zero or not.

The code fragment must have something to do with a chess board or similar.

kind regards,

Jos

[Winterrage]

Thanks a lot Jos :)

It's a Sudoku table, I understand what is going on with that part.

And now this part is bugging me:

colsSet[c] ^= (1<<num);
rowsSet[r] ^= (1<<num);
subgridSet[blockLoc] ^= (1<<num);

Any other help would be appreciated!

Thanks again

[JosAH]

Thanks a lot Jos :)

It's a Sudoku table, I understand what is going on with that part.

And now this part is bugging me:

colsSet[c] ^= (1<<num);
rowsSet[r] ^= (1<<num);
subgridSet[blockLoc] ^= (1<<num);

Any other help would be appreciated!

Thanks again

Those expressions 'toggle' the value of a particular bit: if it was one (1) it becomes
equal to zero (0) and vice versa. Have a look the the Java artices series. There's
an article in there describing a Sudoku solver; it uses plain booleans instead of
those bitflags.

kind regards,

Jos

[Winterrage]

Those expressions 'toggle' the value of a particular bit: if it was one (1) it becomes
equal to zero (0) and vice versa. Have a look the the Java artices series. There's
an article in there describing a Sudoku solver; it uses plain booleans instead of
those bitflags.

kind regards,

Jos

Thanks a lot, i'll take a look at that :)