Algorithms of adjacency

Today at work I wanted to test the relationships between the contents of a vector in a sequential manner: cell i+1 should have this relationship to cell i kind of thing.

Question:

Does the algorithm header support that?

Answer:

Yes, two ways.

Question:

Can you tell just by reading a list of the calls?

Answer:

Maybe. How much attention are you paying to the signatures?

The key word here is "adjacent". Two of the algorithm header routines have that word in their name: adjacent_find and adjacent_differernce, and both of them have an overload where you supply a binary operator.¹

Of course, the issue of difficult naming rears its ugly head here: adjacent_difference can perform any computation it wants to create the output values written (i.e. it is a version of the pairwise overloads of transform specialized for adjacent cells). Kate Gregory's story about partial_sort_copy is relevant here.

So that's cool: what I wanted was right there.

Only I started thinking² about the other things I might want to do this way, and I noticed that the library doesn't give me a built in way to count pairs that meet some predicate. Of course I can build it out of adjacent_find fairly easily,³ but it's not baked in.

My problem at work? Well, that was test code, and the predicate was a lot clearer if you knew where you were in the container you were, so in the end I wrote for (size_t i=1; i<container.size(); ++i) { const auto thing1 = container[i-1]; const auto thing2 = container[i]; // ... } The library routines are great, but knowing when to not use them is great, too.

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¹ You can also acheive same two operations by using mismatch and transform, though I feel the "adjacent" named routines make the intent clearer. I takes a moment to suss out the meaning of something like const auto [i2, i1] = std::mismatch(++container.begin(), container.end(), container.begin(), my_comparator); aferall, while adjacent_find is pretty plain (though you'll have to parse the behavior of the supplied comparator in either case).

² I know. I know. That was my mistake.

³ If I had more spare time I'd poke ino some of he standard library implementations to see if people build count_if from find.