How vectorize this loop

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

end

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?

Answer 1

[Bruno Luong]

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

[Image Analyst]

@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.

Answer 2

[Image Analyst]

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

[Luca Re]

My sokution is not correct?

[Stephen23]

"My sokution is not correct?"

That depends: are negative values within scope?

[Luca Re]

In matrix "a" yes...in array "b" no Ok i understand...thank