The Twin Algorithms
summary
Argues that several STL numerics (iota, accumulate, partial_sum, inner_product) descend from APL and that twin algorithms — scans and reductions — are visually and semantically paired, a fact revealed quickly on tryapl.org. Demonstrates an APL not-equal scan to strip HTML tags, then translates it into C++20/23 ranges (views, zip_transform, partial_sum later in C++26), showing the C++ version works but needs temporaries and verbose plumbing. Shows outer product to count LRUD moves, cites kx sale (~$100M for a 50KB k binary) and the three-reverse rotate implementation (Doug McIlroy).
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chapters (7)
- 0:00Introduction and motivation
- 16:51Connections between C++ and APL
- 43:37Reduce and scan algorithms
- 54:26Filtering HTML tags with scan
- 1:09:53Outer product in C++ and APL
- 1:23:29Rotate and reverse symmetry
- 1:31:48Summary and call to learn languages
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