How to find subscripts of each element inside many rectangular regions of a matrix?

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JAI PRAKASH
JAI PRAKASH 2018-11-25
回答： JAI PRAKASH 2018-11-25
rowStart = [2; 4; 7]; % a column, may be of more than thousand element
rowEnd = [3; 4; 9]; % a column, may be of more than thousand element
colStart = [1; 1; 2]; % a column, may be of more than thousand element
colEnd = [3; 1; 3]; % a column, may be of more than thousand element
% for obvious reason size of all above vectors would be same
function [subRow subCol] = elementwisesubscript(rowStart, rowEnd, colStart, colEnd)
%% interesested function
% I tried 'arrayfun', its working but is slow
% I tried for loop, 1.5x faster than arrayfun. But thinking there could be a smart optimized way
% needed a code, that is matlab coder compatible.
end
Thanks a lot for consideration.
Attached image further examplifies.

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回答（1 个）

JAI PRAKASH
JAI PRAKASH 2018-11-25
This is 10x times than arrayfun, still i think can be optimsed.
Because no pre-allocation in below code and many other potential improvements.
function [subRow, subCol] = elementwisesubscript(rowStart, rowEnd, colStart, colEnd)
%% subRow identification
rowLength = rowEnd-rowStart+1;
maxRowLength = max(rowLength);
subRow = repmat(rowStart, 1, maxRowLength) + repmat(0:maxRowLength-1, length(rowStart),1);
colLength = colEnd-colStart+1;
for i =2:maxRowLength
subRow(rowLength<i, i) = 0;
end
subRow = subRow';
subRow = subRow(:);
subRow = repelem(subRow, repelem(colLength, maxRowLength));
subRow(subRow==0)=[];
%% subCol identification
maxColLength = max(colLength);
subCol = repmat(colStart, 1, maxColLength) + repmat(0:maxColLength-1, length(colStart),1);
for i =2:maxColLength
subCol(colLength<i, i) = 0;
end
subCol = subCol(:);
colLengthMat = repmat(rowLength,1,maxColLength);
colLengthMat = colLengthMat(:);
subCol = repelem(subCol, colLengthMat);
subCol = reshape(subCol, [], maxColLength);
subCol = subCol';
subCol = subCol(:);
subCol(subCol==0)=[];
end

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