How to set a marker in a matrix?

Dear all

#1,#2,#3 are just numbers.

I have created a 3D cell matrix(300,300,300) and what my script does is that it will read a number of tab delimited text files. fscanf was use to read each line and each line have information of #1,#2,#3 and position x,y,z for the matrix.

It will then go to the position in the 3D cell matrix (x,y,z) and add #2 and #3 to the existing # in that position(x,y,z) in column 1 and 2. If the cell is empty, it will append to it.

After reading the whole file, some positions in the matrix will have a variable while some are still empty depending on the information from the textfile. The next step of the script would be to perform some calculation using #2 and #3 to obtain #4 and input to column 3 at the positions where information was added to.

A simple way would be to do 3 for loops that loop through position x,y,z for all cell in the matrix.

for i = 1:300
  for j = 1:300
    for k = 1:300
      % Calculate #4 if information is added in this position
    end 
  end 
end

But this would be very time consuming, is there a way simpler way or how can a marker be set in which information has been introduced to the position?

Thank you in advance!