Circular indexing of an array

I have run into this a number of times in a recent project I'm working on and was wondering what good solutions are out there. What happens is I often have an array representing positions on a line.

X = 1:10;

I want to create a function that indexes the code and circularly wraps around such that when I go through the first iteration of my loop idx-1 = 0 is interpreted as a 10 instead.

for i = 1:10
     (X(i-1)+X(i))/2
end

I've tried appending the end element of the array on to the beginning.

X   = [10 1:10]
for i = 2:11
...

Seems kind of like an ugly solution though. Any other suggestions?

Thanks!

Answer 1

[Walter Roberson]

wrapN = @(x, N) (1 + mod(x-1, n));
N = length(X);
for i = 1:10
   (X(wrapN(i-1,N))+X(wrapN(i,N)))/2
end

[Abhinav Gupta]

Just a minor typo in the wrapN function. It should be:

wrapN = @(x, n) (1 + mod(x-1, n));

[Walter Roberson]

Ah, right. Or to be consistent with my naming,

wrapN = @(x, N) (1 + mod(x-1, N));

Answer 2

[Oleg Komarov]

I have something different, a vectorized approach:

X     = 1:10;

shift = 1;

B1    = (circshift(X,[1 shift]) + X)/2 
B2    = (X([end-shift+1 1:end-shift]) + X)/2 %equivalent

isequal(B1,B2) % 1

Answer 3

[Teja Muppirala]

X = 1:10;
ifft(fft(X).*fft([1 1]/2,numel(X)))

Answer 4

[bym]

I don't know if it will help but you can use X as the index variable in the loop

X = [10 1:10]
for X
...
end

alternatively, if inside the loop is what you posted you can do

y = conv(X,[.5 .5]);
y([1 end]) = [];

without a loop

Answer 5

[Andrei Bobrov]

in this case

X = 1:10;
conv(X([end,1:end]),[1 1]/2,'valid');

Answer 6

[Wade]

Thanks all!

Answer 7

[amir moin]

Taking you need a circle of length N=10; then you can set the array X(100) to the value of circular index Index(k)

N=10;
Index = @(i) mod(i,N)+(mod(i,N)==0).*N;

X  = Index(1:100);
end

[Walter Roberson]

With that code, Index(1) would be 1, Index(9) would be 9, Index(10) would be 1, index(11) would be 1 as well. Only 9 out of the 10 locations would be used, and location 1 would be used twice.

https://www.mathworks.com/matlabcentral/answers/44993-circular-indexing-of-an-array#comment_450173 has the correct code.

[amir moin]

Sorry! I had made a mistake. Now the corrected solution is there. Thank you for your comment.

[Walter Roberson]

Seems complicated compared to my

wrapN = @(x, N) (1 + mod(x-1, N));
X = wrapN(1:100, 10);