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How know the size of matrix after delete raws

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asma khaled
asma khaled 2018-6-22
评论： dpb 2018-6-23
What function do you use to know the size of the array after one or more rows are removed after a condition is achieved within a loop

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回答（4 个）

dpb
dpb 2018-6-22
编辑：dpb 2018-6-22
"I want to delete all rows that start with an element greater than 5 in the matrix"
In Matlab, use "logical addressing" for such problems...
>> s(s(:,1)>5,:)=[]
s =
2 98 52 41 63
4 5 2 6 9
>>
Shweta Singh
Shweta Singh 2018-6-22
You can use the 'size' function as follows:
matrix_example = rand(7,8); %matrix of size 7x8
[matrix_x, matrix_y] = size(matrix_example);
% Delete a row
matrix_example(3,:) = [];
% Get the updated dimensions
[matrix_x, matrix_y] = size(matrix_example);
Hope this helps!
asma khaled
asma khaled 2018-6-22
But if I place this code inside a loop for 1 to the number of rows of matrix, a error message will appear because the size of the array has changed.
asma khaled
asma khaled 2018-6-22
s=[10 20 30 50 60;80 90 40 10 60;2 98 52 41 63;4 5 2 6 9];
[count,count1]=size(s);
for i=1:count
if (s(i,1)>5)
s(i,:)=[];
[count,count1]=size(s);
end
end
error:Index exceeds matrix dimensions.
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dpb
dpb 2018-6-22
编辑：dpb 2018-6-23
Because when you got to the loop index with i = 4, you've already removed more than that number of rows so when you write
s(i,1)
where i==4 and there are only three rows left, the array addressing is outside the array dimensions and that causes the error.
To do something like this in a loop, you would want to start at the end of the array and work backwards so that the loop index is decreasing at at least the same rate as the size of the array--
nr=size(s,1); % initial number rows in s
for i=nr:-1:1 % start from end, work to beginning...
if s(i,1)>5, s(i,:)=[]; end % eliminate if desired.
end
By going backwards, you always have at least as many rows left in the array as the index value because you've removed only those of higher initial position previously for the next iteration.
BTW, don't feel badly; this is a general precept that every beginning programmer has had to learn the hard way first... :)
dpb
dpb 2018-6-23
Again, move to Comment...-dpb
Thank you so much for your help

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