sprintfc – undocumented helper function

Every now and then I discover a useful built-in Matlab function, often by bumping into them within the m-code of standard functions. Such was the case, for example, of the ismembc function, that I described here back in 2009, and the dtstr2dtnummx function that I described in 2011. Today I describe another such function, sprintfc, based on a tip I received from Dirk Engel.

The requirement

The use-case of generating a cell-array of formatted strings is often encountered. Unfortunately, the standard sprintf function generates a single string, rather than a cell-array of strings. So, for example,

>> data = pi * (0:.5:2)
data =
         0    1.5708    3.1416    4.7124    6.2832
 
>> sprintf('%.3f  ',data)
ans =
0.000  1.571  3.142  4.712  6.283

This is nice, but not exactly what we need, which is a cell array that looks like this:

>> c  % c is a 1x5 cell array
c = 
    '0.000'    '1.571'    '3.142'    '4.712'    '6.283'

The standard solutions

There are several standard solution for getting the required cell array. A simple for-loop is one of the fastest of these, due to JIT optimizations in recent Matlab releases. Some of the other alternatives may seem very strange, but they perform faster than the simpler variants. Note that the following code should be run within a testing function, to remove extraneous influences on the timing (JIT compilation etc.):

% 1) Simple for-loop with sprintf
tic
for idx = 1 : 1000
    c = {};  %initialize
    % note: reallocations are not really a factor here, so don't bother preallocating
    for dataElement = data
        c{end+1} = sprintf('%.3f',dataElement);
    end
end
toc
% Elapsed time is 0.076002 seconds.
 
% 2) Using arrayfun + num2str
tic
for idx = 1 : 1000
    c = arrayfun(@(c) num2str(c,'%.3f'), data, 'uniform',false);
end
toc
% Elapsed time is 0.557993 seconds.
 
% 3) Using cellfun + num2str + num2cell
tic
for idx = 1 : 1000
    c = cellfun(@(c) num2str(c,'%.3f'), num2cell(data), 'uniform',false);
end
toc
% Elapsed time is 0.566924 seconds.
 
% 4) Using num2cell + num2str
tic
for idx = 1 : 1000
    c = num2cell(num2str(data','%.3f'),2)';
end
toc
% Elapsed time is 0.173026 seconds.
 
% 5) Using cellstr + num2str
tic
for idx = 1 : 1000
    c = reshape(cellstr(num2str(data(:),'%.3f')),size(data));
end
toc
% Elapsed time is 0.175769 seconds.
 
% 6) Using num2cell + num2str (after Urs Schwartz & Jiro Doke)
tic
for idx = 1 : 1000
    c = reshape(num2cell(num2str(data(:),'%.3f'),2),size(data));
end
toc
% Elapsed time is 0.170247 seconds.
 
% 7) Using strread + sprintf
tic
for idx = 1 : 1000
    c = reshape(strread(sprintf(['%.3f','$'], data),'%s', 'delimiter','$'),size(data));
end
toc
% Elapsed time is 0.059457 seconds.

There are also some File Exchange contributions that do a similar thing, including num2cellstr or mtx2charcell, which basically implement the last two variants.

sprintfc

It turns out that the standard num2str function uses an undocumented built-in function sprintfc that already does this for us in one fell swoop – this is both the simplest and the fastest solution:

% 8) fastest and simplest - sprintfc
tic
for idx = 1 : 1000
    c = sprintfc('%.3f',data);
end
toc
% Elapsed time is 0.015714 seconds.

This is a 35x speedup compared to arrayfun/cellfun, 5x speedup compared to the JIT-optimized for-loop, and 4x speedup compared to the fastest other variant. Useful indeed.

The syntax of the sprintfc function can be seen in the num2str.m function:

[cells, errorMsg, isLeft] = sprintfc(format, data, isImaginary);

where isImaginary is an optional flag (default=false) indicating that data should be formatted as imaginary values with the specified magnitude:

>> data = pi * (-1:.5:1)
data =
   -3.1416   -1.5708         0    1.5708    3.1416
 
>> sprintfc('%.3f', data, false)
ans = 
    '-3.142'    '-1.571'    '0.000'    '1.571'    '3.142'
 
>> sprintfc('%.3f', data, true)
ans = 
    '-3.142i'    '-1.571i'    '+0.000i'    '+1.571i'    '+3.142i'

The second returned argument is an errorMsg string that is returned when sprintfc fails, typically due to an invalid format:

>> [c,errMsg] = sprintfc('%r', data)
c = 
    {''}
errMsg =
Invalid format.

The third returned value, isLeft, is apparently used to indicate whether the output cell-string is left-justified; namely, if the ‘-‘ flag is used within the format (thanks Yaroslav!). For example,

>> [cells, ~, isLeft] = sprintfc('%-8.5g', sqrt(2), false)
cells = 
    '1.4142  '
isLeft =
    1
 
>> [cells, ~, isLeft] = sprintfc('%8.5g', sqrt(2), false)
cells = 
    '  1.4142'
isLeft =
    0

sprintfc has apparently remained in its current stable state since it was first introduced in the last decade (sometime between Matlab release 7.1 [in 2005] and R2008a). I do not know why MathWorks chose to keep sprintfc as an internal undocumented and unsupported function, despite its obvious usefulness.

If anyone finds any other similar useful built-ins, please do let me know.

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Happy Hanukkah/Thanksgiving everyone :-)

Categories: Medium risk of breaking in future versions, Stock Matlab function, Undocumented function

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5 Responses to sprintfc – undocumented helper function

  1. Yaroslav Don says:

    Yair hi,
    After a short inspection I’ve come to the following conclusion: the third variable, isLeft, is used to indicate whether the output cell-string is left-justified; namely, if the '-' flag is used within the format. For example,

    >> [cells, ~, isLeft] = sprintfc('%-8.5g', sqrt(2), false)
    cells = 
        '1.4142  '
    isLeft =
        1

    however,

    >> [cells, ~, isLeft] = sprintfc('%8.5g', sqrt(2), false)
    cells = 
        '  1.4142'
    isLeft =
        0

    Sincerely, Yaroslav

  2. Matteo says:

    Hi Yair

    I tried to figure out how to do this, and with higher dimension cell arrays, when I was still a Matlab newbie. I was obviously ahead of myself, but with a ton of reading, testing, and asking around, I managed. But if I needed to do that again it is good to know about this function. Great tip, nice post. Thanks

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