Variable Width Columns in .txt Files - matlab

Question

I have a function that takes data and imports that data into a text file. The issue that I am having is with formatting. I want to be able to set the width of the columns based on the widest array of characters in that column. So, in the code below I have labels and then data. My idea would be to take the length of each individually and find the largest value. Say the second column labels has 15 chars and that is longer than any data array, then I want to set the width of that column to 15 + 3 (white spaces) making it 18. If column 3 had a max of 8 chars for a member of data, then I would like to set the width to 11. I have found plenty of literature on fixed width, and I found that I could do '-*s', *width, colLabels; but I am having difficulty figuring out how to implement that.
Below is my code and it doesn't fail but it takes forever and then won't open because there is not enough memory. I have really tried to work through this to no avail.
Thanks in advance and if there is any other information I can provide, then let me know.
for col = 1:length(this.colLabels) % iterate through columns
colLen = length(this.colLabels{col}); % find the longest string in labels
v = max(this.data(:,col)); % find the longest double in data
n = num2str(v, '%.4f'); % precision of 4 after decimal place
dataLen = length(n);
% find max width for column and add white space
if colLen > dataLen
colWidth = colLen + 3;
else
colWidth = dataLen + 3;
end
% print it
fprintf(fid, '%-*s', this.colWidth, this.colLabels{col}); % write col position i
fprintf(fid, '\n');
fprintf(fid, '%-*s', this.colWidth, this.colUnits{col});% write unit position i
fprintf(fid, '\n');
fprintf(fid, '%-*s', this.colWidth, this.data(:,col)); % write all rows of data in column i
end

Answer 1

There are few places where you are making mistakes:
The size of number is not necessarily related to its size when printed. Consider 1.1234 and 1000, one of these is a larger string and the other is a larger number. This may or may not matter for your data ...
Two, it is best to use the correct format strings when printing to string. %s is for strings, not numbers.
Perhaps most importantly, text appears on multiple lines because of the newline character which ends one line and starts another. This means you essentially have to write one row at a time, not one column at a time.
I tend to prefer creating the text in memory then writing to a file. The following isn't the cleanest implementation but it works.
this.colLabels = {'test' 'cheese' 'variable' 'really long string'};
this.colUnits = {'ml' 'cm' 'C' 'kg'};
n_columns = length(this.colLabels);
%Fake data
this.data = reshape(1:n_columns*5,5,n_columns);
this.data(1) = 1.2345678;
this.data(5) = 1000; %larger number but smaller string
%Format as desired ...
string_data = arrayfun(#(x) sprintf('%g',x),this.data,'un',0);
string_data = [this.colLabels; this.colUnits; string_data];
%Add on newlines ...
%In newer versions you can use newline instead of char(10)
string_data(:,end+1) = {char(10)};
string_lengths = cellfun('length',string_data);
max_col_widths = max(string_lengths,[],1);
%In newer versions you can use singleton expansion, but beware
n_spaces_add = bsxfun(#minus,max_col_widths,string_lengths);
%left justify filling with spaces
final_strings = cellfun(#(x,y) [x blanks(y)],string_data,num2cell(n_spaces_add),'un',0);
%Optional delimiter between columns
%Don't add delimiter for last column or for newline column
final_strings(:,1:end-2) = cellfun(#(x) [x ', '],final_strings(:,1:end-2),'un',0);
%Let's skip last newline
final_strings{end,end} = '';
%transpose for next line so that (:) goes by row first, not column
%Normally (:) linearizes by column first
final_strings = final_strings';
%concatenate all cells together
entire_string = [final_strings{:}];
%Write this to disk fprintf(fid,'%s',entire_string);

Answer 2

The data in the text file is stored one line after the other, so you cannot write column by column. You need first to determine the width of the columns and write the label/unit header, then write all the data. All we need to have is a proper format string for fprintf: fixed width format and fprintf is extremely useful for exporting column delimited data.
The first part of the code is ok in order to determine the width of the columns (assuming the data only has positive samples). You only need to store it in an array.
nCol=length(this.colLabels);
colWidth = zeros(1,nCol);
for col = 1:nCol
colLen = length(this.colLabels{col}); % find the longest string in labels
v = max(this.data(:,col)); % find the longest double in data
n = num2str(v, '%.4f'); % precision of 4 after decimal place
dataLen = length(n);
% find max width for column and add white space
colWidth(col)=max(colLen,dataLen);
end
Now, we need to build format string for the labels and data, to use with sprintf. The format string will look like '%6s %8s %10s\n' for the header and '%6.4f %8.4f %10.4f\n' for the data.
fmtHeader=sprintf('%%%ds ',colWidth);
fmtData=sprintf('%%%d.4f ',colWidth);
%Trim the triple space at the end and add the newline
fmtHeader=[fmtHeader(1:end-3) '\n'];
fmtData =[fmtData(1:end-3) '\n'];
We use the fact that, when sprintf is given an array as input, it will iterate through all the values to produce a long string. We can use the same trick to write the data, but singe we write line by line and Matlab stores data in column major order, a transpose is necessary.
fid=fopen('myFile.txt');
fprintf(fid,fmtHeader,this.colLabels{:});
fprintf(fid,fmtHeader,this.colUnits{:});
fprintf(fid,fmtData,transpose(this.data));
fclose(fid);
For the headers, the cell can be converted to a comma separated list with {:}. This is the same as writing fprintf(fid,fmtHeader,this.colLabels{1},this.colLabels{2},...)
Using the same test data from #Jimbo 's answer and fid=1; to output the fprintf to the screen the code gives:
test cheese variable really long string
ml cm C kg
1.2346 6.0000 11.0000 16.0000
2.0000 7.0000 12.0000 17.0000
3.0000 8.0000 13.0000 18.0000
4.0000 9.0000 14.0000 19.0000
1000.0000 10.0000 15.0000 20.0000
Finally, the most compact version of the code is:
fid=1; %print to screen for test purpose
colWidth =max( cellfun(#length,this.colLabels(:)') , max(1+floor(log10(max(this.data,[],1))) , 1) + 5); %log10 to count digits, +5 for the dot and decimal digits ; works for data >=0 only
fprintf(fid,[sprintf('%%%ds ',colWidth(1:end-1)) sprintf('%%%ds\n',colWidth(end))],this.colLabels{:},this.colUnits{:}); %print header
fprintf(fid,[sprintf('%%%d.4f ',colWidth(1:end-1)) sprintf('%%%d.4f\n',colWidth(end))],this.data'); %print data