Simple Matrix multiplication takes very long

Question

Bug introduced in 10.1.0 and fixed in 10.3.0

I came across some strange behaviour during a computation involving matrices with symbolic values. It is reproduced below.

Multiplying a random 30x30 array with symbolic values with the identity matrix takes 4 seconds! Converting to a SparseArray seems to work, but I don't understand why it should since the matrix size is so small.

Mathematica Version is 10.1

aa = Table[RandomInteger[10] + Sqrt[RandomInteger[10]]/(1 + 
   Sqrt[RandomInteger[10]]), {30}, {30}];

IdentityMatrix[30].aa; // Timing

{3.95225, Null}

SparseArray[IdentityMatrix[30]].aa; // Timing

{0., Null}

This has to be a bug ... In 10.0 it is not slow but in 10.1 it is. I can reproduce it on OS X. Can you report it to WRI? — Szabolcs, Jul 09 '15 at 15:21
already paying attention to it. I've passed it along to the appropriate tester, etc. — rcollyer, Jul 09 '15 at 15:26
Quite relevant is this prior MSE post. It seems someone here (and I mean right here) did not know to leave well enough alone (smack smack). — Daniel Lichtblau, Jul 09 '15 at 15:56
Thank you for reporting it. Makes the 3+ hours to narrow it down, worthwhile. — A. Roy, Jul 09 '15 at 17:24

score 3 · Answer 1 · answered Oct 15 '15 at 23:30

This bug has been fixed as of Mathematica 10.3.

$Version

(* "10.3.0 for Mac OS X x86 (64-bit) (October 9, 2015)" *)

aa = Table[RandomInteger[10] + 
       Sqrt[RandomInteger[10]]/(1 + Sqrt[RandomInteger[10]]), {30}, {30}];

IdentityMatrix[30].aa; // Timing

(* {0.028025, Null} *)

Simple Matrix multiplication takes very long

1 Answers1