How to convert for loop to parfor?

Asked by Pasquale on 9 Feb 2018

Latest activity Commented on by Pasquale on 12 Feb 2018

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I need to make my code faster and I am trying to convert for loops into parfor. Often I see this message: "the entire array or structure is a broadcast variable. This might result in unnecessary communication overhead". The code runs, but takes the same time as with for loops. How could I convert my code below? The message appears for variables 'elements' and 'nodes' (here I wrote only the first 20 rows for them).

nodes=load('nodes.txt');
elements=load('elements.txt');
nel=size(elements,1);
parfor i=1:nel
      nodes_el=elements(i,3:5);
      coor_el=nodes(nodes_el,2:3);
      X=coor_el(:,1);
      Y=coor_el(:,2);
      trian=polyshape(X,Y);
      [bx,by]=centroid(trian);
      cx(i)=bx;
      cy(i)=by;
      Area(i)=polyarea(X,Y);
  end
elements.txt
1  2  3  63
1  4  5  64
1  6  7  65
1  8  9  66
1  9  10  67
1  10  11  68
1  12  13  69
1  14  15  70
1  16  17  18
1  18  19  71
1  20  21  72
1  22  23  73
1  24  25  74
1  26  27  28
1  28  29  75
1  30  31  76
1  32  33  77
1  34  35  78
1  36  37  38
1  38  39  79
nodes.txt.
55.0000000  50.0000000
56.8750000  50.0000000
58.7500000  50.0000000
60.6250000  50.0000000
62.5000000  50.0000000
64.3750000  50.0000000
66.2500000  50.0000000
68.1250000  50.0000000
70.0000000  50.0000000
73.7500000  50.0000000
77.5000000  50.0000000
81.2500000  50.0000000
85.0000000  50.0000000
88.7500000  50.0000000
92.5000000  50.0000000
96.2500000  50.0000000
100.0000000  50.0000000
100.0000000  55.0000000
100.0000000  60.0000000
100.0000000  65.0000000

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1 Answer

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Answer by Shashank on 12 Feb 2018

Accepted Answer

Hi pasquale,

I think in your case there will not be any speed up if you use 'parfor' since the statements in the 'for' loop are dependent on the previous one.

For example,

coor_el=nodes(nodes_el,2:3);

is dependent on calculating 'nodes_el' which is done in the previous statement:

nodes_el=elements(i,3:5);

For more information on this, refer to the following documentation link

Convert for-Loops Into parfor-Loops

- Shashank

1 Comment

Pasquale on 12 Feb 2018

Ok, thanks. Can I ask you if another problem can be solved? When I try to convert for to parfor loop, I see also this message: "the temporary variable is used after the parfor loop, but its value is not available after the loop".

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