IF loop to select values superior of a threshold in a matrix

Question

Louise 2020-12-11

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/690520-if-loop-to-select-values-superior-of-a-threshold-in-a-matrix

评论： Louise 2020-12-11

采纳的回答： Jan

A.mat

在 MATLAB Online 中打开

Hi all,

For the matrix A (see attached), my goal is to calculate the mean of each row in the fifth column with the following conditions :

If for the column 1 to 4, values are superior to 0,84 , calculate the mean of the four columns.

If for the column 1 to 4, values are not superior to 0,84 (for instance if column 3 = 0,76), calculate only the mean of the first, second and four columns.

I reach the loop for the first case but I'm not able to do that for the second case....

Here are the few lines I've started to write :

for i=1:100
   if A(i,1:4)>0.85
     A(i,5)=mean(A(i,1:4));
   else if   (I have a doubt for this condition)
     A(i,1) or A(i,2) or A(i,3) or A(i,4) < 0,85 (I have a doubt for this line)
     ................................................................
   end
end

Thanks in advance for your help,

Louise

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Answer 1

Jan 2020-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/690520-if-loop-to-select-values-superior-of-a-threshold-in-a-matrix#answer_572140

编辑：Jan 2020-12-11

在 MATLAB Online 中打开

for k = 1:100
    match   = (A(k, 1:4) > 0.85);
    A(k, 5) = sum(A(k, match)) / sum(match);
end

This works faster without a loop:

match   = (A(:, 1:4) > 0.85);
A(:, 5) = sum(A(:, 1:4) .* match, 2) ./ sum(match);  % Auto-expand, >= Matlab R2016b

Note that "if A(i,1:4)>0.85" might not do, what you expect. The condition of a if statement must be a scalar. So Matlab inserts an all() command implicitely.