Preallocating table makes it slower?

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Christian Tieber
Christian Tieber 2020-5-5
评论： Star Strider 2020-5-5
I have to add data to a table with a loop. I can estimate how large the table approx will be at the end. But i dont know the exact size. I also dont know the exact size of the data added with every iteration. I notized that the needed time grows nearly exponetialy when using vertcat. So i tried to preallocate the table. It made things worse!
Can some one help me out?
thanks in advance
Christian
Wrote some code to demostrate the problem:
rows = 1e6;
columns = 6;
aproxRows = rows * 1.5;
Table= table();
preallTable = array2table(zeros(aproxRows, columns));
preallTime = [];
preallTime(1) = 0;
Time = [];
Time(1) = 0;
start = 1;
for i = 1:1e3
i
addTable = array2table(rand(1000, columns)); %size of table is unknown in reality for every loop pass
End = start-1 + height(addTable);
preallTic = tic;
preallTable(start:End, :) = addTable;
preallTime(end+1) = preallTime(end) + toc(preallTic);
start = End+1;
Tic = tic;
Table = vertcat(Table,addTable);
Time(end+1) = Time(end) + toc(Tic);
end
%cut off unused space
preallTable(start:end, :) = [];
%plot Time
figure
hold on
plot(preallTime)
plot(Time)
legend({'preallTime', 'Time'})

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采纳的回答

Star Strider
Star Strider 2020-5-5
I would preallocate the array, then do the array2table call at the end, after the array was created and is stable (nothing more to be added to it).
  2 个评论
Christian Tieber
Christian Tieber 2020-5-5
The data to add, comes as a table. Not shure if a can change that.
Star Strider
Star Strider 2020-5-5
Youi can easily change it to an array using the table2array function. If the variable names in the table are important, you can extract them with:
VarNames = T.Properties.VariableNames;
where ‘T’ is the original table name. When you have finished changing the array, you can the re-define the variable names in the new table (created with array2table) using ‘VarNames’ as the cell array value for the 'VariableNames' name-value pair (providing that the number of the variable names has not changed from the original table).

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