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

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"

Answer 1

[Andrei Bobrov]

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

Answer 2

[Star Strider]

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.

[Stephen23]

Why so complex?:

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

[Zachery Vrbas]

It was so obvious.

Thank you, good sir

Answer 3

[Bill Tubbs]

Even easier:

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

out =

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

[DGM]

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

[Bill Tubbs]

Nice! Thanks for pointing out the simplification (f3). I will update my comment.

Answer 4

[Bill Tubbs]

Here's another way. A geometric series:

out = (-1).^(0:11)

out =

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