How vectorize this loop

Question

Luca Re 2023-7-23

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1999623-how-vectorize-this-loop

评论： Image Analyst 2023-7-23

采纳的回答： Bruno Luong

x=8

a=magic(x)

b=[15 7 3 4 9 0 9 4] ====> 0 is important to test it

1 method with loop

c=a;

tic

for i=1:height(a)

for j=1:width(a)

if b(j)~=0

c(i,j)=min(a(i,j),b(j));

end

toc

tic

2 method with loop

z=a.*(~b>0)+min(a,b);

toc

isequal(c,z)

%cù

hi...I'm learning how to vectorize matrices...is it correct or can it be improved?

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Answer 1

Bruno Luong 2023-7-23

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1999623-how-vectorize-this-loop#answer_1277448

编辑：Bruno Luong 2023-7-23

在 MATLAB Online 中打开

repmat is not needed (thanks to auto expension)

columns = 8;
a = magic(columns);
b = [15, 7, 3, 4, 9, 0, 9, 4];  % ====> 0 is important to test it
b2 = b;
b2(b2 == 0) = inf;
% Find the minimum of a or b2
minValues = min(a, b2) % What you call "c"
minValues = 8×8
    15     2     3     4     9     6     7     4
     9     7     3     4     9    51     9     4
    15     7     3     4     9    43     9     4
    15     7     3     4     9    30     9     4
    15     7     3     4     9    38     9     4
    15     7     3     4     9    19     9     4
    15     7     3     4     9    11     9     4
     8     7     3     4     4    62     9     1

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Image Analyst 2023-7-23

@Luca Re forgot to enter his release. I don't know when auto expansion was introduced (and it's hard to find out that year in the help documentation) so I gave the version with manual expansion just to make sure he had something that worked. Of course if your version works for him, then he has a recent version and this is even more vectorized than mine.

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Answer 2

Image Analyst 2023-7-23

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1999623-how-vectorize-this-loop#answer_1277438

在 MATLAB Online 中打开

Sounds a lot like a homework assignment, but I'm going to assume it's not and give one possible solution:

columns = 8;
a = magic(columns);
b = [15, 7, 3, 4, 9, 0, 9, 4];  % ====> 0 is important to test it
% Fill out b so that it's a matrix the same size as a
b2 = repmat(b, [columns, 1]);
% Get rid of zeros
b2(b2 == 0) = inf
b2 = 8×8
    15     7     3     4     9   Inf     9     4
    15     7     3     4     9   Inf     9     4
    15     7     3     4     9   Inf     9     4
    15     7     3     4     9   Inf     9     4
    15     7     3     4     9   Inf     9     4
    15     7     3     4     9   Inf     9     4
    15     7     3     4     9   Inf     9     4
    15     7     3     4     9   Inf     9     4
% Find the minimum of a or b2
minValues = min(a, b2) % What you call "c"
minValues = 8×8
    15     2     3     4     9     6     7     4
     9     7     3     4     9    51     9     4
    15     7     3     4     9    43     9     4
    15     7     3     4     9    30     9     4
    15     7     3     4     9    38     9     4
    15     7     3     4     9    19     9     4
    15     7     3     4     9    11     9     4
     8     7     3     4     4    62     9     1