Divide table takes a lot of calculation time

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Jesse
Jesse 2018-11-15
评论： Guillaume 2018-11-16
In my script i want to devide a table, it works for now, but it takes a lot of time.
How can i do this easier or faster?
reactionforce{3:end,:} = reactionforce{3:end,:}/1e10

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Guillaume
Guillaume 2018-11-15
I doubt you're using any feature of a table with a table that large so why not store your data in a matrix instead. Matrix operations will be slightly faster. Saying that, with your example table, the division takes 76 milliseconds on my computer. I would hardly call that a lot of time.
reactionforce = table2array(reactionforce); %convert table to matrix since you don't use the features of tables
%...
reactionforce(3:end, :) = reactionforce(3:end, :) / 1e10;
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Jesse
Jesse 2018-11-16
Thanks, on my computer it took about 20 seconds, so maybe that could also be the problem ;)
With this method it is much faster, about 1 second.
Thanks!
Guillaume
Guillaume 2018-11-16
The contents of a table is always held as a cell array, with one table variable (column) per cell. Therefore, when you write reactionforce{3:end, :} = reactionforce{3;end, :}/1e10, matlab has to:
  • concatenate all these cells into one big matrix. This involves copying the content of each variable into a new memory location
  • extract the rows of the matrix that you asked for
  • perform the division
  • convert the result back into a cell array of column
So this is always going to take more time than just performing a division on a matrix. It's surprising that it's so slow on your machine though.

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