Java: 2d array columns and rows

Question

Thank you for taking the time to read,

I have a 2d matrix of booleans and im trying to change each of the border values to false. I have this working for the top and left-most borders but cant seem to figure out a working code for the bottom and right-most, any help would be appreciated, these are the codes for the upper and left-most:

// Changes border values to false

    for (int row = 0; row <matrix.length; row++) {
        int col = 0;
        matrix[row][col] = false;
    }

    for (int col = 0; col <matrix.length; col++) {
        int row = 0;
        matrix[row][col] = false;
    }

Square matrix? Homework?

trashgod
– trashgod

2011-09-06 02:13:24 +00:00
Commented Sep 6, 2011 at 2:13 — trashgod
– trashgod, Commented Sep 6, 2011 at 2:13

Yasin Okumuş · Accepted Answer · 2011-09-06 13:25:12Z

5

Make the row and col values the last values instead of zero;

for the last column:

for (int row = 0; row <matrix.length; row++) {
    int col = matrix[0].length-1;
    matrix[row][col] = false;
}

and last row:

for (int col = 0; col <matrix[0].length; col++) { 
    int row = matrix.length-1;
    matrix[row][col] = false;
}

Or better aproach to avoid creating new instances of cols and rows inside the loops;

for (int row = 0; row <matrix.length; row++) {
    matrix[row][matrix[0].length-1] = false;
}
for (int col = 0; col <matrix[0].length; col++) { 
    matrix[row][matrix.length-1] = false;
}

For col, the last value must be the length of a row, so I write matrix[0].length it might be any other suitable number too instead of 0, but out of bounds.

edited Sep 6, 2011 at 13:25

answered Sep 6, 2011 at 2:17

Yasin Okumuş

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2 Comments

aymeric Over a year ago

Avoid useless col and row instanciations inside the loops. Do it once outside.

Yasin Okumuş Over a year ago

You are right, it was just to make him understand with his own code.

LoveAndCoding · Accepted Answer · 2011-09-06 02:20:06Z

0

Well, the easiest way I can think of is to just set row to the max value of rows through the loop (looks to be matrix.length - 1 each time) as well. So, for example:

for (int row = 0; row < matrix.length; row++) {
    matrix[row][0] = false;
    matrix[row][matrix.length - 1] = false;
}

and do the same for your columns.

answered Sep 6, 2011 at 2:20

LoveAndCoding

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4 Comments

Justin Over a year ago

Thank you for the help, I very much appreciate such a quick reply

LoveAndCoding Over a year ago

No problem, though @Yasin beat me to it. I think mine is a more efficient way of doing it (since you are doing both at the same time, and not create extra variables) but both are correct.

Yasin Okumuş Over a year ago

@Justin, you should choose an answer at least :)

Justin Over a year ago

oop sorry, first time ever using this site

HRgiger · Accepted Answer · 2011-09-06 02:36:51Z

0

Or something like that;

 for(int c=0;c<matrix.length;c++){
              System.out.println("");
              for(int r=0;r<matrix[0].length;r++){
                if(c==0||r==0||c==matrix.length-1||r==matrix[0].length-1){
                    matrix[c][r] = false;
                }else{
                    matrix[c][r] = true;
                }
                System.out.print(" " + matrix[c][r]);
              }
          }

answered Sep 6, 2011 at 2:36

HRgiger

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