MATLAB Answers


How to read strings from file with fscanf or sscanf (NOT textscan)?

Asked by LifeSux SuperHard on 17 May 2013

So, of course, I'm having a little trouble right now. I'm trying to read a text file that goes something like this in a columnar order. What I would like to do is store the number, character and string columns seperately in arrays.

[Numbers] [Characters] [Strings] 

Now, while I have figured out how to read the number and character columns into their own arrays, I cannot seem to do so with the string column. At least, not with fscanf or sscanf, which are the commands I want to use.

How can you read a file organized as such using fscanf or sscanf? (I know about textscan, I want to know if this is possible with fscanf or sscanf).

The first thing I tried was the following:

fid = fopen('Data.txt', 'w+'); 
B = fscanf(fid, '%d %c %s', [3,inf]); 

Now while this worked fine for just the numbers and chars (i.e. B = fscanf(fid, '%d %c', [2,inf])), it fails for the above in the sense that it reads everything out of order (e.g. instead of B = [1,2,3...; a,b,c...; ABC, DEF, GHI...] I get B = [1,65,65; 66, 67, 2;...], just junk basically).

So I researched a bunch and tried out this:

fid = fopen('Data.txt', 'w+'); 
i = 1;
while ~feof(fid)
     line = fgets(fid);
     M(i) = sscanf(line, '%d, %c, %s', [3,inf];
     i = i+1; 

This runs, but M ends up coming out only as a row vector consisting of the first column of numbers in the data file. It just completely ignores the existence of chars and strings.

Now, to get a better understanding of the sscanf function I tried the following

fid = fopen('Data.txt', 'w+'); 
    i = 1;
    while ~feof(fid)
         line = fgets(fid);
         M(i) = sscanf(line, '%d, %d, %d', [3,inf];
         i = i+1; 

For a sample set of data consisting of just columns of numbers. This, incidentally, does exactly the same thing as previously; it just reads the first number column of the data and quits. So, I don't even know how to use sscanf, feof, or fgets properly, basically. So I could also use some help here as well.

And I know trying to read just columns of numbers is trivial with fscanf, but I'm trying to understand sscanf and fgets here.


per isakson
on 17 May 2013

"just junk basically" or is it ascii code?

    >> double('ABC')
    ans =
        65    66    67
Cedric Wannaz
on 17 May 2013

Could you provide us with a few lines of your data file? There are several options that we can discuss.. meanwhile, be aware that 65 is the ASCII code of character 'A', 66 for 'B', etc, so what you thought was junk is not; it is 'ABC' not read/interpreted/displayed as a string.

LifeSux SuperHard on 17 May 2013

It is ASCII code, I imagine.

10 A ABC

I created this expressly for testing these functions, but that is how my real data is organized (well, sort of, my data only has numbers and strings, and the strings are only two letters long, not three).

And yes, when I tried this with a column of numbers and a column of characters with fscanf I get

1    65
2    65
3    65
...  ...

but this is simple to convert back to a character array (c = char(B(:,2)')). However, with the numbers, strings, and characters I get the following with fscanf

1  65 65
66 67 2
65 65 66
... ... ...

Where I would want something like

1 65 ?
2 65 ? 
3 65 ?
... ... ...


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2 Answers

Answer by Cedric Wannaz
on 18 May 2013
Edited by Cedric Wannaz
on 18 May 2013
 Accepted answer

Just a few alternate thoughts (and I'll think about FSCANF over the week end a little more).

=== Using REGEXP (available in almost all languages):

.. and the following content (to illustrate the flexibility):

 1 A ABC
 2 B ABC
 4 D ABC
 6 F ABC
 7 G ABC
 8 H ABC
 9 I ABC
 10 J ABC


 >> buffer = fileread('data.txt') ;    % Could be performed with FOPEN/FREAD 
                                       % to be more generic.
 >> pattern = '(?<Column1>\d+)\s(?<Column2>\w+)\s+(?<Column3>.*?)[\r\n]' ;
 >> n = regexp(buffer, pattern, 'names')
 n = 
 1x10 struct array with fields:
 >> n(2)
 ans = 
    Column1: '2'
    Column2: 'B'
    Column3: 'ABC'
 >> n(3)
 ans = 
    Column1: '3'
    Column2: 'C'
    Column3: 'ABC DEF'
 >> str2double({n(:).Column1})
 ans =
     1     2     3     4     5     6     7     8     9    10

etc .. here I used named tokens and a struct array output, just for the fun of it. I don't think that it is what you are looking for, but I just wanted to illustrated a regexp-based approach for the record.

=== Reading array of chars and converting to cell array based on position of spaces and \n and/or \r:

... to update if asked by OP.

=== Using FSCANF:

.. and the following, more regular content:

 1 A ABC
 2 B ABC
 3 C ABC
 4 D ABC
 5 E ABC
 6 F ABC
 7 G ABC
 8 H ABC
 9 I ABC
 10 J ABC


 fid  = fopen('data_regular.txt', 'r') ;
 data = cell(1e6, 3) ;                    % Prealloc.
 rCnt = 0 ;                               % Row counter.
 while ~feof(fid)
    rCnt = rCnt + 1 ;
    data{rCnt,1} = fscanf(fid, '%d', 1) ;
    data{rCnt,2} = fscanf(fid, '%s', 1) ;
    data{rCnt,3} = fscanf(fid, '%s', 1) ;
 fclose(fid) ;
 data = data(1:rCnt,:) ;                  % Truncate.

Using this, we get:

 >> data
 data = 
    [ 1]    'A'    'ABC'
    [ 2]    'B'    'ABC'
    [ 3]    'C'    'ABC'
    [ 4]    'D'    'ABC'
    [ 5]    'E'    'ABC'
    [ 6]    'F'    'ABC'
    [ 7]    'G'    'ABC'
    [ 8]    'H'    'ABC'
    [ 9]    'I'    'ABC'
    [10]    'J'    'ABC'

Note that EOF should be tested a little better (and not every three FSCANF, which assumes a well formed file). The whole could be in a TRY/CATCH statement otherwise.

=== Using FGETL + SSCANF:

It is more complicated than FSCANF, because the later moves forward an internal file pointer/counter as it reads the content, so the next read operation takes what follows. SSCANF doesn't work like this and you have to indicate what to extract and what to skip in the format. To illustrate:

 >> s = '12 A ABC' ;
 >> sscanf(s, '%d')                 % OK for the number.
 ans =
 >> sscanf(s, '%s')                 % Can we do the same for the 2nd col? KO.
 ans =
 >> sscanf(s, '%*d %s', 1)          % Skip # and read a 1 char string => KO, ASCII.
 ans =
 >> char(sscanf(s, '%*d %s', 1))    % => char, OK.
 ans =
 >> char(sscanf(s, '%*d %s %*s'))   % Or read a string and skip next.
 ans =
 >> char(sscanf(s, '%*d %*s %s'))   % Same for 3rd column, but dim KO.
 ans =
 >> char(sscanf(s, '%*d %*s %s')).' % Transpose, OK.
 ans =


Answer by per isakson
on 17 May 2013
Edited by per isakson
on 17 May 2013

What you see is as documented. Clip from on-line help:

    sscanf finds three word matches for %s and two numeric matches for %d. Because
    the format specifier has a mixed %d and %s format, sscanf converts all 
    nonnumeric characters to numeric:
    [str count] = sscanf('5 strings and 4 spaces', '%d%s%s%d%s');
      Columns 1 through 9
         5   115   116   114   105   110   103   115    97
      Columns 10 through 18
       110   100     4   115   112    97    99   101   115
    count =

sscanf returns a numeric or a character array.

textscan can produce the output you are looking for.


per isakson
on 17 May 2013

Your comment, "just junk basically" , gave me the wrong impression, sorry!

LifeSux SuperHard on 17 May 2013

yeah no problem, it was a little vague.

per isakson
on 18 May 2013

"the sscanf problem" does that refer to "everything out of order"? I assume so.

The source of the "problem" is that Matlab reads data in column order, which is because of early influences from FORTRAN.

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