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Incorrect parallel computation

• To: mathgroup at smc.vnet.net
• Subject: [mg125939] Incorrect parallel computation
• From: Yasha Gindikin <gindikin at gmail.com>
• Date: Sun, 8 Apr 2012 04:16:00 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com

Dear colleagues,

would you please advise me on why the following simple code executes incorrectly when parallelized?

a = Subsets[Range[1, 5], {3}];
L = Length@a;
b = Table[0, {5}];
c = Table[0, {5}];
SetSharedVariable[c];
Do[b[[a[[l, k]]]] += 1, {l, L}, {k, 3}];
ParallelDo[c[[a[[l, k]]]] += 1, {l, L}, {k, 3}];
b===c

False

Thank you!
Best regards,
Yasha Gindikin.