Subtract column from a matrix
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In matlab it is easy to subtract number from column or row. I want to subtract column [n x 1] from a matrix [n x m]. Is it possible to doit without for loop? When I wrote it just with '-', there was dimension mismatch error. Thanks.
1 个评论
采纳的回答
Image Analyst
2014-9-25
You could use repmat() to create an array of the same size:
out = inputArray - repmat(columnVector, [1, m]);
6 个评论
Mikhail
2014-9-25
Image Analyst
2014-9-25
Not always.
inputArray = rand(100,100);
columnVector = rand(100, 1);
tic
out = bsxfun(@minus, inputArray, columnVector);
toc
tic;
out = inputArray - repmat(columnVector, [1, 100]);
toc
Elapsed time is 0.000508 seconds.
Elapsed time is 0.000357 seconds.
To continue with the one-upping: When it is that fast, unless you are doing it many many times, it doesn't really matter.
Just did a quick test:
ii = [10 100 200 300 400 500 1000 2000 5000 7500 10000 20000]
a = [ii' ii'];
cnt = 0;
for sz = ii
sz
cnt = cnt + 1;
inputArray = rand(sz);
columnVector = rand(sz, 1);
tic
bsxfun(@minus, inputArray, columnVector);
out(1) = toc;
tic;
inputArray - repmat(columnVector, [1, sz]);
out(2) = toc;
a(cnt,:) = out;
end
plot(ii,a);
legend({'bsxfun','repmat'});
ylabel('Elapsed time (s)')
xlabel('Array size')
That results in:
Titus Edelhofer
2014-9-26
Nice comparison!
One aspect has not been mentioned yet: for larger matrices/vectors the memoryfriendliness of bsxfun compared to repmat (which blows up the memory usage significantly).
Titus
Image Analyst
2014-9-26
Good point. This can be done by adding these lines into the loop:
memoryUsed = memory;
fprintf('Memory used by MATLAB = %d bytes.\n', memoryUsed.MemUsedMATLAB)
更多回答（1 个）
Use bsxfun, it will expands singleton dimensions:
n = 10; m = 20;
matrix = rand(n, m);
column = rand(n, 1);
bsxfun(@minus, matrix, column)
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