sscanf to extract numbers from string

Hi everyone,

I have a data file like this:

 1     .00  80.00 160.00 240.00 320.00 400.00 480.00 560.00 640.00 720.00
 1 -250.00-250.00-250.00-255.00-255.00-260.00-265.00-270.00-265.60-260.00
 1  800.00 880.00 960.001040.001120.001200.001280.001360.001440.001520.00
 1 -255.00-255.00-263.30-286.70-310.60-320.00-313.90-290.00-267.80-260.00
....

The format doens't change.

I try to read each line using sscanf, but when the numbers doenst have space, I can't read them.

In otherwords, when I use:

ff = fgetl(fid)
aff = sscanf(ff,'%f')

This works fine for the first line, because the numbers has space between them.

But doens't work for the rest of lines.

I also tried the command:

ff = fgetl(fid)
aff = sscanf(ff,'%2f %7.2f%7.2f%7.2f%7.2f%7.2f%7.2f%7.2f%7.2f%7.2f%7.2f',[1 11])

But without success.

Someone can help me?

Best regards.

Answer 1

[Walter Roberson]

textscan() and fscanf() and sscanf() all have the same problem: Their counts for formats such as %7f start only after leading space has been skipped. You can see this in particular in the space 960.00 no-space 1040.11 entries on the third line: the count starts after the space, so the 960.001 is what gets parsed.

To handle fixed-width inputs, you have a small number of choices:

1. There is a File Exchange contribution to handle fixed-width input
2. if you bash your head against the problem for long enough you can convince textscan() to work fixed width; this is not easy
3. you can use array indexing to break the text into fields that you convert to numeric such as with str2double
4. you can use regexp to break the text into fields that you convert to numeric such as with str2double
5. with R2017a or later, you can use readtable() in a fixed-width mode. See https://www.mathworks.com/help/matlab/ref/matlab.io.text.fixedwidthimportoptions.html

regexp() can be pretty useful for a purpose such as this.

[Bruno Goncalves]

Hi Walter,

I overcome this problem using your third option.

First I get the data:

filename='data.txt';
fid=fopen(filename,'rt');
line1 = fgets(fid);

Second calculate the length of each line:

sline = length(line1);

Third, I edit to work properly with my file.

nsline=(sline-4)/7;

The key is the result of nsline.

In my data type, nsline will one number between 1 and 10.

With this size, I use str2double to convert the string to number, using different IF-END Loops, depending the value of nsline.

That is, each size of nsline will give me different size of reading.

For nsline=1.

   if nsline==1
       tmp1 = [str2double(line1(1:2)) str2double(line1(3:10))  ];
   end

For nsline=2

 if nsline==2
     tmp1 = [str2double(line1(1:2))  str2double(line1(3:10))  str2double(line1(11:17))];
 end

.

And so on....

.

For nsline=10

if nsline==10
      tmp1 = [str2double(line1(1:2))   str2double(line1(3:10))  str2double(line1(11:17))...
                str2double(line1(18:24)) str2double(line1(25:31)) str2double(line1(32:38))...
                str2double(line1(39:45)) str2double(line1(46:52)) str2double(line1(53:59))...
                str2double(line1(60:66)) str2double(line1(67:73))];
end

As you can see, I has read specific positions inside string vector. That is possible because the format of my file doesnt change.

Because of that, it is possible to convert String to Number using str2double.

Thanks again Walter Robertson.

Best regards.

[Walter Roberson]

str2double( cellstr( reshape(line(3:end).', 7, []).' ) )

[Bruno Goncalves]

Error using reshape
Product of known dimensions, 7, not divisible into total number of elements,
72.

[Walter Roberson]

Use fgetl() rather than fgets(), and use 4:end instead of 3:end .

Your code used one more character for the second entry on the line (first floating point entry), but there is no reason to expect that the extra character is used; it is more likely that there is just an extra blank in the format at that point.

Answer 2

[Guillaume]

sscanf format is different from sprintf. In particular there's no .2 notation for %f, so your .2 is interpreted as a literal .2 and of course does not match anything.

There's no need to fgetl and then sscanf. You can read the whole file in one go with fscanf instead, so:

fid = fopen(somefile, 'rt');
assert(fid > 0, 'Failed to open file');
aff = fscanf(fid, '%2f %7f%7f%7f%7f%7f%7f%7f%7f%7f%7f', [11, Inf])'
fclose(fid);

[Walter Roberson]

This turns out to fail on the space 960.00 no-space 1040.11 pair on the third line.

[Bruno Goncalves]

Hi Guilaume,

Thank you, but I have to apologize.

The format changes.

I attached a file containing the correct format.

I have to read line by line because, depending on the value you are reading, it will or not will change another variable.

In a first situation, I used the code bellow to read the values.

This works fine when I have space between numbers.

But, doenst work when they are attached each others, as that case.

filename='v2.txt';
fid=fopen(filename,'rt');

ppcntr=300; 
ppvel=300; 
nrzmax=ppcntr/10; 
nrvmax=ppvel/10;
icnt=1;
player=12;  % numero maximo de camadas
ncont=1;    % numero de camadas atual

ntotal = fscanf(fopen(filename,'rt'),'%f',Inf);
ntotal = ntotal(end-3)-1;
zmaximo = fscanf(fopen(filename,'rt'),'%f',Inf);
zmaximo = zmaximo(end);

% velocity model
for icont=1:ntotal
    nrz=1;
    while ( icnt == 1 )
        if ( nrz > nrzmax ); break; end
        line1 = fgets(fid);
        line2 = fgets(fid);
        line3 = fgets(fid);
        tmp1 = sscanf(line1,'%f');
        tmp2 = sscanf(line2,'%f');
        tmp3 = sscanf(line3,'%f');
        ntmp1 = length(tmp1); 
        ntmp2 = length(tmp2); 
        ntmp3 = length(tmp3);
        if ( nrz == 1 )
            j1=1; j2=ntmp1-1;
        elseif ( nrz > 1 )
            j1=j2+1; j2=j2+(ntmp1-1);
        end
        ilyr = fix(tmp1(1));
        xm(icont,j1:j2) = tmp1(2:ntmp1);
        icnt = fix(tmp2(1));
        zm(icont,j1:j2) = tmp2(2:ntmp2);
        ivarz(icont,j1:j2) = tmp3(1:ntmp3);
        nrz = nrz + 1;
        xm_pts(icont) = j2;
        zm_pts(icont) = j2;
        ivarz_pts(icont) = j2;
        if ( icnt == 0 ); break ; end
        %disp("parei..."); pause
    end
    %  sprintf('Interface %d primeiro grupo lido',icont)
    %  pause
    nrv = 1;
    icnt = 1;
    while ( icnt == 1 )
        if ( nrv > nrvmax ); break ; end
        line1 = fgets(fid);
        line2 = fgets(fid);
        line3 = fgets(fid);
        tmp1 = sscanf(line1,'%f');
        tmp2 = sscanf(line2,'%f');
        tmp3 = sscanf(line3,'%f');
        ntmp1 = length(tmp1); ntmp2 = length(tmp2); ntmp3 = length(tmp3);
        if ( nrv == 1 )
            j1=1; j2=ntmp1-1;
        elseif ( nrv > 1 )
            j1=j2+1; j2=j2+(ntmp1-1);
        end
        ilyr = fix(tmp1(1));
        xvel(icont,1,j1:j2) = tmp1(2:ntmp1);
        icnt = fix(tmp2(1));
        vf(icont,1,j1:j2) = tmp2(2:ntmp2);
        ivarv(icont,1,j1:j2) = tmp3(1:ntmp3);
        nrv = nrv + 1;
        xvel_pts(icont,1) = j2;
        vf_pts(icont,1) = j2;
        ivarv_pts(icont,1) = j2;
        if ( icnt == 0 ); break ; end
    end
    %  sprintf('Interface %d segundo grupo lido',icont)
    %  pause
    nrv = 1;
    icnt = 1;
    while ( icnt == 1 )
        if ( nrv > nrvmax ); break ; end
        line1 = fgets(fid);
        line2 = fgets(fid);
        line3 = fgets(fid);
        tmp1 = sscanf(line1,'%f');
        tmp2 = sscanf(line2,'%f');
        tmp3 = sscanf(line3,'%f');
        ntmp1 = length(tmp1); ntmp2 = length(tmp2); ntmp3 = length(tmp3);
        if ( nrv == 1 )
            j1=1; j2=ntmp1-1;
        elseif ( nrv > 1 )
            j1=j2+1; j2=j2+(ntmp1-1);
        end
        ilyr = fix(tmp1(1));
        xvel(icont,2,j1:j2) = tmp1(2:ntmp1);
        icnt = fix(tmp2(1));
        vf(icont,2,j1:j2) = tmp2(2:ntmp2);
        ivarv(icont,2,j1:j2) = tmp3(1:ntmp3);
        nrv = nrv + 1;
        xvel_pts(icont,2) = j2;
        vf_pts(icont,2) = j2;
        ivarv_pts(icont,2) = j2;
        if ( icnt == 0 ); break ; end
    end
    %  sprintf('Interface %d terceiro grupo lido',icont)
    %  pause
    ncont=ncont+1;
    icnt=1;
    % disp('Tecle enter ler a interface ' + icont)
    %  pause
end
sprintf('Total interfaces lidas %d ',ncont-1)
disp('Fim da leitura...')

fclose(fid)