It's worth noting that on modern OSes, the OS will forcibly zero out memory when it's allocated from the OS, so if you're allocating a really large sparse set (large enough to not come from existing free heap RAM), and don't intend to clear it a lot, sparse sets won't save any time, and calloc (which can skip the zeroing if it's getting new pages from the OS) will probably be a better idea.
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Also for people new to C programming, don't forget about calloc!
It's worth noting that on modern OSes, the OS will forcibly zero out memory when it's allocated from the OS, so if you're allocating a really large sparse set (large enough to not come from existing free heap RAM), and don't intend to clear it a lot, sparse sets won't save any time, and calloc (which can skip the zeroing if it's getting new pages from the OS) will probably be a better idea.