How to make an array of alternating 1 and -1?

Question

Samantha Horton 2017-12-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/372072-how-to-make-an-array-of-alternating-1-and-1

编辑： Bill Tubbs 2022-10-31

So far I have

xc = ones(1,12) for n=0:1:11 xc(n) = (-1).^n end

Mathematically, shouldn't it come out to an array of [1,-1,1,-1,1,-1,1,-1,1,-1]? I am getting this error:

"Subscript indices must either be real positive integers or logicals.

Error in test (line 3) xc(n) = (-1).^n"

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Answer 1

Andrei Bobrov 2017-12-9

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out = 2*rem(1:12,2) - 1

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Answer 2

Star Strider 2017-12-9

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MATLAB indexing begins with 1, not 0, so you have to adjust your code slightly:

xc = ones(1,12);
for n=1:12
    xc(n) = (-1).^(n-1);
end
xcv = (-ones(1,12)).^(0:11);    % Vectorised Version

The ‘vectorised version’ is simply to demonstrate how to use MATLAB’s vectorising ability to do the same operation.

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Stephen23 2017-12-9

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Why so complex?:

>> (-1).^(0:11)
ans =
   1  -1   1  -1   1  -1   1  -1   1  -1   1  -1

Zachery Vrbas 2022-10-29

It was so obvious.

Thank you, good sir

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Answer 3

Bill Tubbs 2022-10-31

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https://ww2.mathworks.cn/matlabcentral/answers/372072-how-to-make-an-array-of-alternating-1-and-1#answer_1088068

在 MATLAB Online 中打开

Even easier:

out = cumprod(-ones(1, 12))

out =

-1 1 -1 1 -1 1 -1 1 -1 1 -1 1

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DGM 2022-10-31

编辑：DGM 2022-10-31

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That's faster than I expected, but I guess it makes some sense.

N = 1E6;
timeit(@() f1(N))
ans = 0.0230
timeit(@() f2(N))
ans = 0.0240
timeit(@() f3(N))
ans = 0.0231
timeit(@() f4(N))
ans = 0.0020
timeit(@() f5(N))
ans = 1.7284e-04
timeit(@() f6(N))
ans = 0.0010
function f1(N)
    % slower than i expected
    out = 2*rem(1:N,2) - 1;
end
function f2(N)
    out = (-ones(1,N)).^(0:(N-1));
end
function f3(N)
    out = (-1).^(0:(N-1));
end
function f4(N)
    % faster than i expected
    out = -cumprod(-ones(1,N));
end
function f5(N) 
    % only works if N is even!
    out = reshape(repmat([1;-1],[1 N/2]),1,[]);
end
function f6(N)
    % this is a bit slower and uglier than f5, but works for any N
    out = reshape(repmat([1;-1],[1 ceil(N/2)]),1,[]);
    out = out(1:N);
end