Trouble finding logical error in my for loop

Question

I am using Java and GridBagLayout to recreate the form seen in the image. In order to create the grid lines under the days of the week I am inserting empty JLabels with borders into the empty cells. This worked perfect for me and then I decided to make each empty cell that is shaded on the form to also be shaded in java, and this is where I am struggling.

My thinking is that I can create a "shade pointer" like I did with my x and y coordinates, this shadePtr will begin with the value of 8 since that is the first row to be shadded, when the loop shades a row, then it increments the shadePtr by 3 since row 11 is the next to be shaded, then 14 and so on.

So far the only bit of success I get is if I comment out the last line of code you see (shadePtr = shadePtr + 3) but then only one row is shaded in. I can't seem to figure out what I am doing wrong here and would appreciate your time and effort.

    int yPointer = 7;
    int xPointer = 3;
    int shadePtr = 8;
    for (int j = 0; j <= 299; j++)
    {

        gbc.gridx = xPointer;
        gbc.gridy = yPointer;
        gbc.gridheight = 1;
        gbc.gridwidth = 1;
        if (yPointer == 36) calendarGridLines[j].setBorder(BorderFactory.createMatteBorder(1, 0, 1, 0, Color.BLACK)); //if bottom row
        else calendarGridLines[j].setBorder(BorderFactory.createMatteBorder(1, 0, 0, 0, Color.BLACK));
        if (yPointer == shadePtr){ //if row number = shadePtr then color the cell
            calendarGridLines[j].setOpaque(true);
            calendarGridLines[j].setBackground(Color.GRAY);

        }
        gridbag.setConstraints(calendarGridLines[j], gbc);
        rp.add(calendarGridLines[j]);
        xPointer++; //go to next cell in row
        j++; //use the next jlabel
        gbc.gridx = xPointer;
        gbc.gridy = yPointer;
        gbc.gridheight = 1;
        gbc.gridwidth = 1;
        if (yPointer == 36) calendarGridLines[j].setBorder(BorderFactory.createMatteBorder(1, 1, 1, 1, Color.BLACK)); //if bottom row
        else calendarGridLines[j].setBorder(BorderFactory.createMatteBorder(1, 1, 0, 1, Color.BLACK));
        if (yPointer == shadePtr){ //if row number = shadePtr then color the cell
            calendarGridLines[j].setOpaque(true);
            calendarGridLines[j].setBackground(Color.GRAY);

        }
        gridbag.setConstraints(calendarGridLines[j], gbc);
        rp.add(calendarGridLines[j]);

        xPointer++; //go to next cell in row

        if(xPointer == 13) //if end of column then go to next row and reset column pointer to 3 and increment shade pointer by 3
        {
            yPointer++; //go down a row
            xPointer = 3;
            shadePtr = shadePtr + 3; //when this line is commented out, one row will be colored; when active none are colored
        }
    }

Answer 1

So far the only bit of success I get is if I comment out the last line of code you see (shadePtr = shadePtr + 3) but then only one row is shaded in. I can't seem to figure out what I am doing wrong here and would appreciate your time and effort.

If I understood your code right the thing is:

• yPointer is the "row" number in your grid.
• shadePtr is the index to the next row to be shaded.

The problem is you increase yPointer in 1 unit in each iteration (which is fine) but shadePtr is increased in 3 units also in each iteration. As yPointer starts at 7 and shadePtr starts at 8, then those variables never be equals because shadePtr will always be greater than yPointer.

If you comment that last line in the second iteration yPointer == shadePtr == 8 which is the first shaded row. But later yPointer will be increased and shadePtr still 8 so no row will be shaded anymore.

The number 3 is important to solve your problem but making this little change at the end:

int yPointer = 7;
int xPointer = 3;
int shadePtr = 8;

for (int j = 0; j <= 299; j++) {
    ...
    if(xPointer == 13) { //if end of column then go to next row and reset column pointer to 3 and increment shade pointer by 3
        yPointer++; //go down a row
        xPointer = 3;
        if((j % 3) == 0) {
            shadePtr = yPointer;
        }
    }
}

This means, if j is a multiple of 3 then the next row should be shaded so make this assignment: shadePtr = yPointer after increase yPointer. This will shade these row numbers: 8, 11, 14, 17, 20, 23, 26, 29, 32 and 35.

In this way your problem should be solved by making a little change but note shadePtr is actually unnecessary. You can have a simple boolean to know if the row should be shaded:

int yPointer = 7;
int xPointer = 3;
boolean shade = false;

for (int j = 0; j <= 299; j++) {
    ...
    if(xPointer == 13) { // end of column
        yPointer++; //go down a row
        xPointer = 3;
        shade = (j % 3) == 0;
    }
}