Fixed issue with loading all-zero sparse matrices, as the data and ir…

… pointers are not stored in the .mat file. (#57)

Co-authored-by: S. Breedveld <[email protected]>

mat73/core.py

@@ -118,8 +118,13 @@ def unpack_mat(self, hdf5, depth=0, MATLAB_class=None, force=False):
         if 'MATLAB_sparse' in hdf5.attrs:
             try:
                 from scipy.sparse import csc_matrix
-                data = hdf5['data']
-                row_ind = hdf5['ir']
+                if 'data' in hdf5:
+                    data = hdf5['data']
+                    row_ind = hdf5['ir']
+                else:
+                    data = []
+                    row_ind = []
+
                 col_ind = hdf5['jc']
                 n_rows = hdf5.attrs['MATLAB_sparse']
                 n_cols = len(col_ind) - 1
@@ -348,4 +353,4 @@ def savemat(filename, verbose=True):
 
 
     # file = '../tests/testfile8.mat'
-    # data = loadmat(file)
+    # data = loadmat(file)