Most efficient way to vertically concatenate numeric data?

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SA-W 2025-4-1

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https://ww2.mathworks.cn/matlabcentral/answers/2175884-most-efficient-way-to-vertically-concatenate-numeric-data

评论： Walter Roberson 2025-4-3

In the profiled output picture below, mergedDataPerRank is a cell array storing 53 double matrices with size (6300, 33). Using vertcat, it takes approx. 0.5 seconds (20 calls) to vertically concatenate the data. Is there a more efficient way to do this or is 0.5 seconds already fairly good?

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Konstantin 2025-4-3

编辑：Konstantin 2025-4-3

But what is "Merge_filesAcrossRanks"? It looks like it is some your own custom function to read a lot of files in a given directory. If it is so (and if the size of all tables is quaranteed a priori), then I would recommend to combine reading and merging: just preallocate the whole final matrix (53*6300 tall, 33 wide), create a "current position (line)" variable, and then read files into this table while advancing the "current position".

Walter Roberson 2025-4-3

在 MATLAB Online 中打开

@Konstantin

That approach turns out to be slower.

nmat = 53;
nrow = 6300;
ncol = 33;
rng(12345)
tic
C = cell(nmat, 1);
for K = 1 : nmat; C{K} = rand(nrow, ncol); end
R1 = vertcat(C{:});
t1 = toc;
rng(12345);
R2 = zeros(nmat*nrow,ncol);
counter = 1;
for K = 1 : nmat; R2(counter:counter+nrow-1,:) = rand(nrow, ncol); counter = counter + nrow; end
t2 = toc;
format long g
[t1, t2]
ans = 1×2
                  0.159601                  0.273057
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