multiply desired indexes in for loop

Question

Othman Alkandri 2022-12-28

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https://ww2.mathworks.cn/matlabcentral/answers/1885612-multiply-desired-indexes-in-for-loop

评论： Voss 2022-12-28

I am trying to multiply the some indxes in my vector, bucuase there is a fctor I need to mutlipy it with the t vector of [ 1, 4,2,4,2,4,2 .........., 1].

frist index keep it
secound index multiply by 4
third index multiply by 2
forth index multiply by 4
fifth index multiply by 2
..
..
..
last index keep it

t = [0.58 , 14.48, 19.91, 21.88, 22.59] 
for i = 2:2:(length(t)-1)
    A = 4*(t(i))+2*(t(i+1))
end 
A = t(1)+A+t(end)

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Answer 1

Voss 2022-12-28

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https://ww2.mathworks.cn/matlabcentral/answers/1885612-multiply-desired-indexes-in-for-loop#answer_1137532

编辑：Voss 2022-12-28

在 MATLAB Online 中打开

I think this is what you're going for:

t = [0.58 , 14.48, 19.91, 21.88, 22.59];
Nt = numel(t);
A = zeros(1,Nt);
for i = 2:2:(Nt-1)
    A([i, i+1]) = [4*t(i), 2*t(i+1)];
end
A([1, end]) = t([1, end]);
disp(A);
    0.5800   57.9200   39.8200   87.5200   22.5900

You can do the same thing like this:

t = [0.58 , 14.48, 19.91, 21.88, 22.59];
f = ones(1,numel(t)); % factors by which to multiply each element of t
f(2:2:end-1) = 4;
f(3:2:end-1) = 2;
A = f.*t;
disp(A);
    0.5800   57.9200   39.8200   87.5200   22.5900

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Othman Alkandri 2022-12-28

Thank you

Voss 2022-12-28

You're welcome!

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Answer 2

Image Analyst 2022-12-28

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https://ww2.mathworks.cn/matlabcentral/answers/1885612-multiply-desired-indexes-in-for-loop#answer_1137537

在 MATLAB Online 中打开

I don't think your for loop does what you asked for in words. Here, try this to multiply a weights vector [1,4,2,4,2,4,......1] by a data vector t:

t = [0.58 , 14.48, 19.91, 21.88, 22.59];
numRepeats = length(t); % Number of times the pair [4,2] repeats.
% Make weights in the form [4,2,4,2,4,2,4,2,......];
weights = repmat([4,2], 1, numRepeats);
% That was too long, so let's crop it so we can multiply the vectors together.
% Also make the first and last element 1.
weights = [1, weights(1:length(t)-2), 1]; % [1,4,2,4,1] for this particular t.
% Now multiply the weights vector by the t vector element-by-element
A = t .* weights
A = 1×5
    0.5800   57.9200   39.8200   87.5200   22.5900