I have to put a together a game of tic tact toe via processing for a class, but I'm pretty stumped.
The trick is that we have to use a search and storage algorithm which was given to us;
int pX = map(mouseX, 0, width, 0, gridwidth);
int pY = map(mouseY, 0, height, 0, gridheight);

Any help regarding this would be greatly appreciated, the code I have so far is below.

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//Declarations
char alphabet[]= {
  'X', 'O', 'X', 'O', 'X', 'O', 'X', 'O', 'X'};

PFont dFont;

//grid
int gridWidth = 3;
int gridHeight = 3;

//players
int playCounter = 0;
// x=1, O=0
int currentPlayer = 1;

void setup(){
  size(300,300);
  background(255);
  noCursor();

  //set style
  dFont = loadFont("ActionJackson-48.vlw");
  textFont(dFont);
  textAlign(LEFT,TOP);


}//end setup

void draw (){
  background (255);

  drawGrid();

  readArray();

  checkwin();

  alternatePlayer();


  //map mouse location (in pixels) to a grid location
  int gridRow = (int) map(mouseX, 0, width, 0, gridWidth);
  int gridCol = (int) map(mouseY, 0, height, 0, gridHeight);

  //convert grid location to array location to
  //read which character to display
  int index= (gridRow * gridWidth) + gridCol;

  //text style parameters
  /*fill(0);
   textAlign(CENTER, CENTER);
   text(alphabet[index], mouseX, mouseY);
   */


}//end draw


//the grid
void drawGrid(){

  stroke(0);
  strokeWeight(5);
  line (100,0,100,300);
  line (200,0,200,300);
  line (0,100,300,100);
  line (0,200,300,200);

}

void readArray(){
  //searching 2d space and convertin it to 2D spaace
  for (int gridRow = 0; gridRow < gridHeight; gridRow ++){
    for (int gridCol = 0; gridCol < gridWidth; gridCol++){

      //convert grid location to array location
      int index = (gridRow * gridWidth) + gridCol;
      //output to text window
      // print(alphabet[index]);


      //translate grid location to pixel location
      int pixelX= (int) map(gridRow, 0, gridWidth, 0, width);
      int pixelY= (int) map(gridCol, 0, gridHeight, 0, height);

      //text

        //fill(0);
      //text (alphabet[index], pixelX, pixelY);

    }//end gridCol
  }//end gridRow
}

void checkwin(){
  // check to see if any COLUMNS are filled with same charcter
  // columns are indexed -- > 0,1,2 .. gridWidth
  for (int i = 0; i < gridWidth; i++){
    if (      (alphabet[i] != ' ')
      &&  (alphabet[i] == alphabet[ i + 1 * gridWidth])
      &&  (alphabet[i] == alphabet[ i + 2 * gridWidth]) )
    {
      print("column " + i + " is all " + alphabet[i]);
    }  // end if
  } // end for


  // check to see if any ROWS are filled with same charcter
  // you must look in both cells of the row
  // the first cell in each rows is INDEXED as a multiple of gridWidth
  // therefore the iterator <i> is multiplied by gridWidth -- this ensures that
  // your search begins at the start of each row and carries on across

  for (int i = 0; i < gridHeight; i++){
    if (      (alphabet[i*gridWidth] != ' ')
      &&  (alphabet[i*gridWidth] == alphabet[i*gridWidth+1]) )
    {
      println("\trow " + i + " is all " + alphabet[i*gridWidth]);
    }  // end if
  }
}

void alternatePlayer(){
  if (currentPlayer == 1) {
    println("currentplayer is X");
    fill(0);
    text("X", mouseX, mouseY);
  }

  if (currentPlayer == 0) {
    println("currentplayer is O");
    text("O", mouseX, mouseY);
  }

}

void mousePressed(){
  currentPlayer = 1 - currentPlayer;

  if (currentPlayer == 1) {
    fill(0);
    text("X", mouseX, mouseY);
  }

  if (currentPlayer == 0) {
    text("O", mouseX, mouseY);
  }

}