Fix write() extreme slowdown for filenames containing underscores

[Copilot]

Writing NIfTI files via write() was up to 1000x slower for filenames like TYPEB_FITHRF.nii vs TYPEBFITHRF.nii — after 3 minutes, only 39.7 MB of a 13.9 GB file had been written.

Root cause: str2num in make_img_filename uses eval internally. For non-numeric identifiers, eval('[TYPEB_FITHRF]') triggers a full MATLAB path search. On clusters with large, NFS-mounted MATLAB paths, this lookup is expensive and happens once per volume written.

Changes

• str2num → str2double in make_img_filename (both iimg_reconstruct_vols.m and iimg_reconstruct_3dvol.m): str2double never invokes eval — non-numeric strings return NaN instantly with no path search. Added isnan fallback to preserve existing behavior.

  % Before: triggers eval('[<identifier>]') → full MATLAB path search
  n = str2num(n1);
  
  % After: direct numeric parse, NaN for non-numeric, no path lookup
  n = str2double(n1);
  if isnan(n)
      n = imagenum;
  end

• Fix warning('off') → warning('off', 'all') with save/restore in both files: the bare warning('off') is deprecated since MATLAB R2020a and may silently fail to suppress warnings, causing per-volume warning output during large writes.

  % Before
  warning('off');
  % ...
  warning('on');
  
  % After
  warnstate = warning('off', 'all');
  % ...
  warning(warnstate);

• Fix O(N²) char concatenation in slice write path (iimg_reconstruct_vols.m): replaced the growing char array built row-by-row in a loop with a pre-allocated cell array converted via char() at the end.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Write function very slow for certain filenames containing "_" but very fast without it.</issue_title>
<issue_description>I encountered a very weird issue while using the toolbox on Ubuntu (but presumably this is also the reason why it takes so long to write stuff on our cluster).

When I am trying write to a .nii file using the write() function I get very different writing speeds depending on the name of the file everything else being the same.

Base code

data = nifti_data_obj;
data.dat = modelmd;
data.fullpath = file_name;
write(data,'overwrite');

If the file name is test.nii, test_test.nii or even TYPEBFITHRF.nii , it is very fast (01 minutes, 01.632 seconds). However if I use the name TYPEB_FITHRF.nii it takes so long to write the 13.9 GB file that in the testing process I never finished it. To give you an idea how slow: After 3 minutes, TYPEB_FITHRF.nii is still only 39.7 MB in file size.

Unfortunately I am not very versed in MATLAB so I have no idea what's going on. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Write function very slow for certain filenames containing "_" but very fast without it. #67

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