sparse_matrix = []
for k,g in groupby(wikipedia_links[:50], key=itemgetter(0)):
    sparse_matrix.append([k] + map(itemgetter(1), g)

"""
Old code (new code doesn't cover _all_ the same functionality, but most of it
sparse_matrix, these_links, last_link_from = [], [1], 1
for link_from, link_to in wikipedia_links:
    if link_from != last_link_from:
        sparse_matrix.append(these_links)
        while link_from != last_link_from+1:
            sparse_matrix.append([])
            last_link_from += 1
        these_links = [int(link_from)]
    if link_to <= wikipedia_links[-1][0]-1:
        these_links.append(int(link_to))
    last_link_from = link_from
"""