Writing Efficient Code in Dyalog

Writing Efficient Code in Dyalog

Roger Hui

Abstract

Part I. Performance Tips

A review of performance tips and rules of thumb useful for writing efficient code.

Part II. Special Code

Many primitives contain special code for certain arguments to effect time and/or space savings not available to general arguments; moreover, some phrases are “recognized” and are supported by special code. (These are in addition to idioms.) We explore special codes with emphasis on the newer constructs.

Part I. Performance Tips

•	`cmpx` and `wsreq`
	• `cmpx ⍵` compares expression timings
	• `1 1 cmpx ⍵` gets actual times
	• `wsreq ⍵` workspace required

•	work on big chunks of data
	• amortize fixed costs over more data

•	keep data together
	• arrays are stored in row-major (ravelled) order
	• cache misses are expensive

•	order of complexity
	• know the order of complexity of primitives
	• avoid O(`n*2`) operations

•	sorting and searching
	• sorting is faster than grading
	• exploit small range integers
	• `x⍳x` is useful (and optimized)
	• use `⎕ct←0` if possible
	• beware of O(`n*2`) cases

•	utilities
	• utilities create centers of gravity
	• dfns are faster than tradfns
	• imitate primitives from other languages

Part II. Special Codes

•	within a primitive
	• datatype
	• boolean data
	• special but common cases

•	Idioms

•	operators
	• `⍤`
	• `⌸`

•	trains
	• `f¨ ⊂`
	• `1 ⍳⍨ comp`
	• `≢∪`

References

[0]	Hui, Roger, Performance Tips, Dyalog ’12, Elsinore, Denmark, 2012-10-14.
[1]	Hui, Roger, Exploring Index-Of, Dyalog ’14, Eastbourne, UK, 2014-09-21.
[2]	Hui, Roger, A Speed-Up Story, Dyalog Blog post, 2014-11-05.
[3]	Hui, Roger, Exploring Key, Dyalog Blog post, 2015-04-13.
[4]	Hui, Roger, A Dialog on APL, Dyalog Blog post 2015-05-18.

Presented at Dyalog ’15, 2015-09-10.

created:	2015-05-24 22:25
updated:	2015-08-04 19:05