That version is incomplete -- there's a full PDF at the Computer History museum's Software Preservation Group page (http://www.softwarepreservation.org/projects/apl/book/APROGR...).

I always wondered whether APL is a language worth learning. I would love to hear some opinions about APL or its children (J, K, ...).

APL was the first language I learned - I had a very interesting and forward looking Engineering Professor who taught us how to solve complex systems of equations using APL. I loved it, and am sorry that I don't use it anymore.

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I'm interested... what level of mathematics do you need to know to fully understand this text?