var table = document.getElementById("mytab1");
for (var i = 0, row; row = table.rows[i]; i++) {
//iterate through rows
//rows would be accessed using the "row" variable assigned in the for loop
for (var j = 0, col; col = row.cells[j]; j++) {
//iterate through columns
//columns would be accessed using the "col" variable assigned in the for loop
}
}