# Mismatched offset when computing cross-correlation using FFT

In MATLAB if I compute the cross-correlation of 2 arbitrary signals using the xcorr function I get exactly the answer I expect. If I then calculate the cross-correlation using element-wise fft multiplication, the resulting signal is incorrect (offset) by 1 element. Can anyone tell me what I'm missing? Here is my MATLAB script:

a = [0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0];
b = [1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0];
elems = length(a);

% Calculate the cross-correlation coefficients using built-in function
[ac_coeffs, lags] = xcorr(a, b);
subplot(2, 1, 1);
plot(lags, ac_coeffs);

% Now do it manually by fft multiplication
a = [a, zeros(1, elems - 1)];
b = [b, zeros(1, elems - 1)];
af = fft(a);
bf = fft(b);
mult = af.* conj(bf);
result = ifft(mult);
subplot(2,1,2);
plot(result);


You're missing an fftshift to get the correct alignment:
plot(lags,fftshift(result));