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POWhy don't these two math functions return the same result?
 

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  1. POWhy don't these two math functions return the same result?
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    <p>I'm trying to use fancy indexing instead of looping to speed up a function in Numpy. To the best of my knowledge, I've implemented the fancy indexing version correctly. The problem is that the two functions (loop and fancy-indexed) do not return the same result. I'm not sure why. It's worth pointing out that the functions do return the same result if a smaller array is used (e.g., 20 x 20 x 20).</p> <p>Below I've included everything necessary to reproduce the error. If the functions do return the same result, then the line <code>find_maxdiff(data) - find_maxdiff_fancy(data)</code> should return an array full of zeroes. </p> <pre><code>from numpy import * def rms(data, axis=0): return sqrt(mean(data ** 2, axis)) def find_maxdiff(data): samples, channels, epochs = shape(data) window_size = 50 maxdiff = zeros(epochs) for epoch in xrange(epochs): signal = rms(data[:, :, epoch], axis=1) for t in xrange(window_size, alen(signal) - window_size): amp_a = mean(signal[t-window_size:t], axis=0) amp_b = mean(signal[t:t+window_size], axis=0) the_diff = abs(amp_b - amp_a) if the_diff &gt; maxdiff[epoch]: maxdiff[epoch] = the_diff return maxdiff def find_maxdiff_fancy(data): samples, channels, epochs = shape(data) window_size = 50 maxdiff = zeros(epochs) signal = rms(data, axis=1) for t in xrange(window_size, alen(signal) - window_size): amp_a = mean(signal[t-window_size:t], axis=0) amp_b = mean(signal[t:t+window_size], axis=0) the_diff = abs(amp_b - amp_a) maxdiff[the_diff &gt; maxdiff] = the_diff return maxdiff data = random.random((600, 20, 100)) find_maxdiff(data) - find_maxdiff_fancy(data) data = random.random((20, 20, 20)) find_maxdiff(data) - find_maxdiff_fancy(data) </code></pre>
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    1. PO
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    1. USpealco
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    1. COWhat magnitude of difference is there between the two? This isn't the typical floating point accuracy issue that catches so many people out is it?
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      1. POWhy don't these two math functions return the same result?
      1. USspender
    2. COAt what value between 20x20x20 and 600x20x100 do things start to go wrong? Do things go wrong gradually and more and more, or all at once?
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      1. POWhy don't these two math functions return the same result?
      1. USAakashM
    3. COThe magnitude of the differences is rather large to just be floating point errors.
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      1. POWhy don't these two math functions return the same result?
      1. USpealco
 

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