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Here I'm not interested in accuracy (see 13614) but rather in raw speed. You'd think that for a complex machine-precision number z, calculating Abs[z]^2 should be faster than calculating Abs[z] because the latter requires a square root whereas the former does not. Yet it's not so:

s = RandomVariate[NormalDistribution[], {10^7, 2}].{1, I};
Developer`PackedArrayQ[s]
(* True *)
Abs[s]^2; // AbsoluteTiming // First
(* 0.083337 *)
Abs[s]; // AbsoluteTiming // First
(* 0.033179 *)

This indicates that Abs[z]^2 is really calculated by summing the squares of real and imaginary parts, taking a square root (for Abs[z]), and then re-squaring (for Abs[z]^2).

Is there a faster way to compute Abs[z]^2? Is there a hidden equivalent to the GSL's gsl_complex_abs2 function? The source code of this GSL function is simply to return Re[z]^2+Im[z]^2; no fancy tricks.

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    $\begingroup$ Here's an even slower way: (Re[#]^2 + Im[#]^2) & /@ s. And even slower still: Total[ReIm[#]^2] & /@ s $\endgroup$
    – bill s
    May 10, 2019 at 14:24

2 Answers 2

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There's Internal`AbsSquare:

s = RandomVariate[NormalDistribution[], {10^7, 2}].{1, I};
foo = Internal`AbsSquare[s]; // AbsoluteTiming // First
murf = Abs[s]^2; // AbsoluteTiming // First
(*
  0.022909
  0.063441
*)

foo == murf
(*  True  *)
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    $\begingroup$ Ah yes precisely what I was looking for, many thanks Michael! Is there a repository of such tricks? $\endgroup$
    – Roman
    May 10, 2019 at 14:25
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    $\begingroup$ @Roman I was just looking. I thought there was a post about useful Internal` functions, but I couldn't find it just now. The context contains some useful numerical functions like Log1p and Expm1. Statistics`Library` also contains some nice, well-programmed functions. $\endgroup$
    – Michael E2
    May 10, 2019 at 14:31
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    $\begingroup$ @MichaelE2 mathematica.stackexchange.com/questions/805/… $\endgroup$
    – Chris K
    May 10, 2019 at 14:31
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    $\begingroup$ @ChrisK That must be it! Thanks. $\endgroup$
    – Michael E2
    May 10, 2019 at 14:32
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    $\begingroup$ @CATrevillian I would have thought it was in the MKL (Intel Math Kernel Library), but I didn't find it there. I guess I don't know. $\endgroup$
    – Michael E2
    May 11, 2019 at 3:10
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for v5.2, s Conjugate[s] is fast too, ref the pic:

enter image description here

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    $\begingroup$ On my computer, Re[s*Conjugate[s]] is about five to ten times slower than Internal`AbsSquare[s]. What is your $Version and what CPU do you have? $\endgroup$
    – Roman
    Jul 8, 2020 at 12:27
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    $\begingroup$ Hi, people here generally like users to post code as Mathematica code instead of just images or TeX, so they can copy-paste it. It makes it convenient for them and more likely they will engage with your posts. You may find this meta Q&A helpful. -- BTW, have you seen RandomVariate[NormalDistribution[], {10^7, 2}]? It's much faster on my machine. Ditto for RandomComplex[]. $\endgroup$
    – Michael E2
    Jul 8, 2020 at 12:48
  • $\begingroup$ @Roman Re[] is unnecessary, though it's very fast. My version is very old, it's v5.2. So there's no Internal`AbsSquare[]. $\endgroup$
    – infoage
    Jul 8, 2020 at 18:14
  • $\begingroup$ @MichaelE2 Thanks, man. My version is v5.2. This code is so simple that I had no motivation to paste text version at that moment. Sorry. $\endgroup$
    – infoage
    Jul 8, 2020 at 18:18
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    $\begingroup$ Maybe it's worth adding the version info to your answer. It turns out that I don't have the Statistics`NormalDistribution package (in V12.1.1), I suppose because it's been replaced by top-level statistics functions some versions ago. $\endgroup$
    – Michael E2
    Jul 8, 2020 at 19:08

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