Reassign with index

clc
clear
mat_size = 15000;
idx_length = 1e4;
replications = 5;
execute_time = zeros(replications, 6);

for k = 1:replications
    A = randn(mat_size, mat_size);
    B = rand(mat_size, mat_size);
    idx = randi(mat_size, idx_length, 1);

%% method 1
    t = tic;
    for i = 2:length(idx)
        A(idx(i), idx(1:i-1)) = B(idx(i), idx(1:i-1));
    end
    execute_time(k, 1) = toc(t);

%% method 2
    t = tic;
    idx2 = arrayfun(@(x) sub2ind(size(A), idx(x)*ones(x-1, 1), idx(1:x-1)), 2:length(idx), 'UniformOutput', false);
    idx2 = cat(1, idx2{:});
    A(idx2) = B(idx2);
    execute_time(k, 2) = toc(t);

%% method 3
    t = tic;
    idx3 = bsxfun(@plus, idx', (idx-1) * size(A,1));
    idx3 = idx3(triu(true(length(idx), length(idx)),1));
    % disp(isequal(idx3, idx2)) % 1
    A(idx3) = B(idx3);
    execute_time(k, 3) = toc(t);

%% method 4 (repelem is introduced in matlab 2015a)
    t = tic;
    tmp = repmat(1:length(idx)', 1, length(idx));
    tmp = tmp(triu(true(length(idx), length(idx)), 1));
    idx4 = repelem(idx(2:length(idx)), 1:length(idx)-1) + (idx(tmp)-1) * size(A, 1);
    % disp(isequal(idx4, idx2)) % 1
    A(idx4) = B(idx4);
    execute_time(k, 4) = toc(t);

%% method 5 use mex
    % mex('-v', '-largeArrayDims', 'indexAssign.cpp')
    t = tic;
    A2 = indexAssign(A, B, idx);
    execute_time(k, 5) = toc(t);

%% method 6 use mex with inplaced modification
    % mex('-v', '-largeArrayDims', 'indexAssign_inplace.cpp')
    t = tic;
    indexAssign_inplace(A, B, idx);
    execute_time(k, 6) = toc(t);

    clear A A2 B idx idx2 idx3 idx4
end
disp(mean(execute_time))
% matrix size method 1  method 2  method 3  method 4  method 5  method 6
%        5000   1.8311    3.1876    2.2596    2.8247    1.2955    1.2534
%       15000   2.2537    3.2950    2.5996    3.1878    2.2237    1.8598