Hi,
Unfortunately, it is a requirement that all iterations of scan return a
tensor of the same shape.
You can pad the initial tensor explicitly by using tensor.join, or
allocating zeros and using set_subtensor.
On Thu, Nov 17, 2016, VM wrote:
> I'm trying to slice a 2D array using coordinates given by another array.
>
> Example :
>
> def example(p, Q):
> a = T.tensor4('a')
> q = T.matrix('q')
>
> b = q[:, 0]
> c = q[:, 1]
>
> b_i = T.cast(b, 'int32')
> c_i = T.cast(c, 'int32')
>
> d, _ = theano.map(lambda pix, x, y : pix[0, x-1:x+2, y-1:y+2],
> sequences=[a, b_i, c_i])
>
>
> f = theano.function([a, q], d, allow_input_downcast=True)
> return f(p, Q)
>
> # test
> p = np.arange(200).reshape((2, 1, 10, 10))
> Q = np.array([[3, 4], [5, 6]])
> print p
> answer = example(p, Q)
> print answer
> =========================================
> Gives :
> [[[[ 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 25 26 27 28 29]
> [ 30 31 32 33 34 35 36 37 38 39]
> [ 40 41 42 43 44 45 46 47 48 49]
> [ 50 51 52 53 54 55 56 57 58 59]
> [ 60 61 62 63 64 65 66 67 68 69]
> [ 70 71 72 73 74 75 76 77 78 79]
> [ 80 81 82 83 84 85 86 87 88 89]
> [ 90 91 92 93 94 95 96 97 98 99]]]
>
>
> [[[100 101 102 103 104 105 106 107 108 109]
> [110 111 112 113 114 115 116 117 118 119]
> [120 121 122 123 124 125 126 127 128 129]
> [130 131 132 133 134 135 136 137 138 139]
> [140 141 142 143 144 145 146 147 148 149]
> [150 151 152 153 154 155 156 157 158 159]
> [160 161 162 163 164 165 166 167 168 169]
> [170 171 172 173 174 175 176 177 178 179]
> [180 181 182 183 184 185 186 187 188 189]
> [190 191 192 193 194 195 196 197 198 199]]]]
> [[[ 23. 24. 25.]
> [ 33. 34. 35.]
> [ 43. 44. 45.]]
>
> [[ 145. 146. 147.]
> [ 155. 156. 157.]
> [ 165. 166. 167.]]]
>
> Which is the correct output.
> However :
> p = np.arange(200).reshape((2, 1, 10, 10))
> Q = np.array([[3, 4], [8, 9]])
> print p
> answer = example5(p, Q)
> print answer
>
> crashes because the coordinate [8, 9] is near the edge of the 10x10 array
> whereas [3, 4] isn't. So the two iterations in scan give outputs of two
> different shapes. So it crashes.
> How can I solve this problem (either by zero-padding somewhere, or wrapping
> around the slice) ? I'm not too bothered about exactness, I just need a
> solution that doesn't crash.
>
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>
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--
Pascal
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