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POHow to cleanly index numpy arrays with arrays (or anything else that supports addition so that it can be offset)
 

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  1. POHow to cleanly index numpy arrays with arrays (or anything else that supports addition so that it can be offset)
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    <p>The easiest way to explain my question may be with an example, so let me define some arrays:</p> <pre><code>&gt;&gt;&gt; test = arange(25).reshape((5,5)) &gt;&gt;&gt; test array([[ 0, 1, 2, 3, 4], [ 5, 6, 7, 8, 9], [10, 11, 12, 13, 14], [15, 16, 17, 18, 19], [20, 21, 22, 23, 24]]) &gt;&gt;&gt; Xinds = array([1,2,3]) &gt;&gt;&gt; Yinds = array([1,2,3]) </code></pre> <p>Now, if I wanted the elements in rows 1, 2, and 3 and in column 0, I could go:</p> <pre><code>&gt;&gt;&gt; test[Yinds,0] array([ 5, 10, 15]) </code></pre> <p>If I wanted the items in rows 1, 2, and 3 and all columns, I could go:</p> <pre><code>&gt;&gt;&gt; test[Yinds, :] array([[ 5, 6, 7, 8, 9], [10, 11, 12, 13, 14], [15, 16, 17, 18, 19]]) </code></pre> <p>However, if I try to extend this to get the elements in rows 1, 2, and 3 and columns 1, 2, and 3, -- surprise! -- I instead get the elements in (1,1), (2,2), and (3,3)</p> <pre><code>&gt;&gt;&gt; test[Yinds, Xinds] array([ 6, 12, 18]) </code></pre> <p>Instead of what I want:</p> <pre><code>&gt;&gt;&gt; test[Yinds, :][:, Xinds] array([[ 6, 7, 8], [11, 12, 13], [16, 17, 18]]) &gt;&gt;&gt; test[1:4,1:4] array([[ 6, 7, 8], [11, 12, 13], [16, 17, 18]]) </code></pre> <p>I realize I could define a slice, but I want to be able to add an offset to the indices (e.g. Yinds+offset), and that can't be done with slices.</p> <p>I could do something like</p> <pre><code>&gt;&gt;&gt; xStart = 1 &gt;&gt;&gt; xEnd = 4 &gt;&gt;&gt; yStart = 1 &gt;&gt;&gt; yEnd = 4 &gt;&gt;&gt; offset = 1 &gt;&gt;&gt; test[yStart+offset:yEnd+offset, xStart+offset:xEnd+offset] ... </code></pre> <p>or</p> <pre><code>&gt;&gt;&gt; Xinds = array([1,4]) &gt;&gt;&gt; Yinds = array([1,4]) &gt;&gt;&gt; offset = 1 &gt;&gt;&gt; test[slice(*(Yinds+offset)), slice(*(Xinds+offset))] ... </code></pre> <p>But neither is particular clean.</p> <p>Monkey patching the addition operator in to slice doesn't seem to be an option, and inheriting from slice to add the operator doesn't appear to work either; I get the error, "type 'slice' is not an acceptable base type". (*Grumble* This wouldn't be a problem in Ruby *Grumble*)</p> <p>So, my question is, what's the cleanest way to access a (more than 1 dimensional) sub-array with something that can be stored and offset?</p> <p>Options so far:</p> <ul> <li>test[Yinds+offset, :][:, Xinds+offset]</li> <li>test[yStart+offset:yEnd+offset, xStart+offset:xEnd+offset]</li> <li>test[slice(*(Yinds+offset)), slice(*(Xinds+offset))]</li> </ul>
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