How to transform a matrix from a text file to numeric matrix to perform calculations with.

Hello.

I have a matrix in a file(which i have attached) and after using fopen i have used fgetl to read the matrix but it reads into a string and i need to perform calculations with every single onde of the elements in the matrix and it has to have the elements in the exact order as they where in the text file. In the end what i basically want is a matrix where afterwards i can use it to acess it's elements and do calculations with them.

I am only allowed to use functions that are in the book "MATLAB A Pratical Introduction to Programming and Problem Solving" so i am having some troubles with this.

while feof(f) == 0
      matrix = fgetl(f);
   end

Thanks for the help.

[Stephen23]

The function dlmread imports it perfectly into a matrix:

 mat = dlmread('grafo.txt')

[Sofia Batista]

Thank you for answering!

Indeed it works but i don´t think I´m allowed to use that function. What i was looking for was a way for fgetl to read into lines and not on a single string line probably using while and some type of instruction and after that use str2num to converted to a numerical matrix.

[Guillaume]

I have no idea what is in the book you refer to, but if a book titled "MATLAB A Pratical Introduction to Programming and Problem Solving" does not talk about dlmread or its related csvread then its title is very misleading and the book not worth buying.

Answer 1

[Guillaume]

As per Stephen's comment, I'd recommend using dlmread (or similar csvread) or at a push textscan (which makes the code more complicated that it needs to be). If, for some reason, that is not allowed and you really want to do it the complicated and slow way, then you can indeed use fgetl.

The first problem with your code

while feof(f) == 0
    matrix = fgetl(f);
end

is that each time you call fgetl you overwrite the matrix variable (whose name is really misleading, it's just one line of text) with the content of the line.

You can either store each line as row of a char matrix (fragile, will error if a line has more characters than the others):

lines = '';
while feof(f) == 0
    lines(end+1, :) = fgetl(f);
end

Or as a cell array of char vectors:

lines = {};
while feof(f) == 0
    lines{end+1} = fgetl(f);
end

Or convert as you read the line, and then store in a matrix or cell array of numbers:

matrix = {};   %or matrix = [];
while feof(f) == 0
    line = fgetl(f);
    matrix{end+1, 1} = str2double(strsplit(line));
    %or matrix(end+1, :) = str2double(strsplit(line)); %but will error if not all lines have the same number of elements
end
%only for cell arrays:
try
   matrix = cell2mat(matrix);
catch
   error('some rows have more numbers than others');
end

Note that I use str2double instead of str2num. str2num isn't safe. It will happily format your hard drive if you pass it 'rmdir('c:\', 's'). Since you have no idea what is in the text file you're parsing, you should assume it can be hostile.

[Sofia Batista]

Hi!Thank you so much for your time.

Sadly it´s not working. It shows

    Subscripted assignment dimension mismatch.
Error in Receber1 (line 10)
       matriz(end+1, :) = str2double(strsplit(line));

I have used

function [n_nos, matriz] = Receber1( fich )
f = fopen(fich, 'r');
if f == -1
    disp('Insucesso ao abrir o ficheiro.');
else
    n_nos = 0;
    matriz = [];
    while feof(f) == 0
        line = fgetl(f);
        matriz(end+1, :) = str2double(strsplit(line));
        n_nos = n_nos+1;

end  
 fechar = fclose(f);
    if fechar == 0
        disp('Ficheiro carregado com sucesso.');

      else
          disp('Insucesso ao fechar o ficheiro.');

      end
  end
  end

This is my full code. There's also iteration because i need the number of rows in the matrix. Maybe fscanf is easier to use because it reads directly into a matrix(of doubles i think) but i dont know how to reshape(since it reads into the matrix columnwise) it and if using something like

[nl nc]= size(matriz)

will do de trick after performing fscanf code and reshaping.

[Guillaume]

As per the comment I wrote at the end of the line that is giving you error:

%but will error if not all lines have the same number of elements

This is exactly what you're running into. Because the last line of your text file has an extra space at the end, strsplit creates 7 strings instead of 6 (the last one being empty). For this case, the fix is trivial, trim the line to remove surrounding spaces.

matrix(end+1, :) = str2double(strsplit(strtrim(line)));

Regarding the other comments, you don't need the n_nos iteration, its value is always going to be size(matrix, 1)

You could indeed use fscanf but you'd be better off upgrading to textscan or even better, as said in the comment, dlmread or csvread and avoid all the hassle.

[Sofia Batista]

Thank you so much!! It's finally working!

[Sofia Batista]

Sorry but i have a question again! I went and showed my code to my teacher and he said he didn´t know the strsplit function so i could not use it. Is there any way to go around that?

Answer 2

[David Barry]

Here's an example of how you can import the data as doubles. I'll leave you to reshape the matrix if you need to.

fid = fopen('grafo.txt');
matrix = textscan(fid, '%f');
matrix = matrix{1};
fclose(fid);