torch.argmax() fails to find a maximum value in a tensor containing data
I have a tensor of shape [batch_size, channel, depth, height, width]:

torch.Size([1, 1, 32, 64, 64])

with data:

tensor([[[[[-1.8540, -2.8068, -2.7348,  ..., -1.9074, -1.8227, -1.4540],
           [-2.7012, -4.2785, -3.7421,  ..., -3.1961, -2.7786, -1.8042],
           [-2.1924, -4.2202, -4.4361,  ..., -3.1203, -2.9282, -2.3800],
           ...,
           [-2.7429, -4.3133, -4.4029,  ..., -4.4971, -5.3288, -2.8659],
           [-3.0169, -4.0198, -3.6886,  ..., -3.7542, -4.5010, -2.4040],
           [-1.6174, -2.5340, -2.3974,  ..., -1.9249, -2.4107, -1.2664]],

          [[-2.7840, -3.2442, -3.6118,  ..., -3.1365, -2.8342, -1.9516],
           [-3.5764, -4.9253, -5.9196,  ..., -4.8373, -4.2233, -3.3809],
           [-3.1701, -5.0826, -5.6424,  ..., -5.2955, -4.6438, -3.4820],
           ...,
           [-4.0111, -6.1946, -5.6582,  ..., -6.7947, -6.5305, -4.2866],
           [-4.2103, -6.6177, -6.0420,  ..., -5.8076, -6.2128, -3.2093],
           [-2.3174, -4.1081, -3.7369,  ..., -3.5552, -3.1871, -1.9736]],

          [[-2.8441, -4.1575, -3.8233,  ..., -3.5065, -3.4313, -2.3030],
           [-4.0076, -5.4939, -6.2451,  ..., -4.6663, -4.9835, -3.1530],
           [-3.4737, -5.6347, -6.0232,  ..., -5.6191, -5.2626, -3.6109],
           ...,
           [-3.8026, -5.3676, -6.1460,  ..., -7.6695, -6.7640, -4.1681],
           [-4.4012, -6.1293, -6.1859,  ..., -6.0011, -6.1012, -3.5307],
           [-2.7917, -4.2264, -4.1388,  ..., -4.2080, -3.5555, -1.6384]],

          ...,

          [[-2.2204, -3.5705, -4.3114,  ..., -4.2249, -3.9628, -2.9190],
           [-3.6343, -5.3445, -6.1638,  ..., -6.3998, -6.7561, -4.8491],
           [-3.4870, -5.5835, -5.6436,  ..., -6.8527, -7.2536, -4.8143],
           ...,
           [-2.4492, -3.7896, -5.4344,  ..., -6.2853, -6.0766, -3.7538],
           [-2.4723, -3.8393, -4.8480,  ..., -5.6503, -5.0375, -3.5580],
           [-1.6161, -2.9843, -3.2865,  ..., -3.2627, -3.2887, -2.5750]],

          [[-2.1509, -3.8303, -4.2807,  ..., -3.7945, -3.7561, -3.0863],
           [-3.1012, -5.1321, -6.1387,  ..., -6.5191, -6.3268, -4.4283],
           [-2.8346, -5.0640, -5.4868,  ..., -6.6515, -6.5529, -4.3672],
           ...,
           [-2.7278, -4.2538, -4.9776,  ..., -6.4153, -6.0100, -3.9929],
           [-2.8002, -4.0473, -4.7455,  ..., -5.4203, -4.7286, -3.4111],
           [-1.7964, -3.2307, -3.6329,  ..., -3.2750, -2.3952, -1.9714]],

          [[-1.4447, -2.1572, -2.4487,  ..., -2.3859, -2.9540, -1.8451],
           [-1.8075, -2.8380, -3.5621,  ..., -3.8641, -3.5828, -2.7304],
           [-1.7862, -2.9849, -3.8364,  ..., -4.3380, -4.4745, -2.8476],
           ...,
           [-1.8043, -2.5662, -2.7296,  ..., -4.2772, -3.9882, -2.8654],
           [-1.2364, -2.5228, -2.7190,  ..., -4.1142, -3.6160, -2.2325],
           [-1.0395, -1.7621, -2.5738,  ..., -2.0349, -1.5140, -1.1625]]]]]


Now to get the prediction from this I use 

torch.argmax(data, 1) 

which should give me the location of maximum values in the channel dimension, but instead I get a tensor containing only zeros. Even max(torch.argmax()) produces 0.

How can this be, the tensor is only a single dimension and a single batch, how can it return a 0? 

To get rid of the negative values I applied torch.nn.Sigmoid() on it, but still argmax failed to find a maximum value. Which I dont understand, how can there not be a maximum value?

numpy.argmax(output.detach().numpy(), 1) gives the same output, all 0.

Am I not using argmax correctly? 

On this page everything is confusing for argmax. 
The example they selected is 4x4 so you cannot spot the difference 

a = torch.randn(5, 3)
print(a)
print(torch.argmax(a, dim=0))
print(torch.argmax(a, dim=1))


Out

tensor([[-1.0329,  0.2042,  2.5499],
        [ 0.9893,  0.3913,  0.5096],
        [ 0.4951,  0.2260, -0.3810],
        [-1.8953, -0.6823,  0.8349],
        [-0.6217,  0.4068, -1.0846]])
tensor([1, 4, 0])
tensor([2, 0, 0, 2, 1])


See how in dim=0 we have 3 values. This is the dimension of columns.
So it is telling you the element with index 1 in the first column is the max in that column.
The other dim=1 is the dimension of rows, so we have 5 values.

For your example you can calculate the shape of result for argmax:

for i in range(data.dim()):
    print("dim", i)
    r =torch.argmax(data,i)
    print(r.shape)

dim 0
torch.Size([1, 32, 64, 64])
dim 1
torch.Size([1, 32, 64, 64])
dim 2
torch.Size([1, 1, 64, 64])
dim 3
torch.Size([1, 1, 32, 64])
dim 4
torch.Size([1, 1, 32, 64])


And for dim=0 and dim=0 you should have all 0 sine the dim is 1 (index = 0).




  I tried that, but then how do I extract the maximum values from argmax and which dimension should it look against? 


data = torch.randn(32, 64, 64)

values, indices = data.max(0)
print(values, indices)

values, indices = values.max(0)
print(values, indices)

values, indices = values.max(0)
print(values, indices

)

tensor([[1.9918, 1.6041, 2.6535,  ..., 1.5768, 1.7320, 1.8234],
        [1.6700, 2.4574, 1.8548,  ..., 1.8770, 1.7674, 1.6194],
        [1.8361, 1.6800, 1.8982,  ..., 1.7983, 2.7109, 2.2166],
        ...,
        [2.7439, 1.6215, 2.9740,  ..., 1.7031, 1.4445, 1.6681],
        [1.9437, 1.4507, 1.8551,  ..., 2.5853, 1.9753, 2.4046],
        [1.4198, 2.5250, 1.8949,  ..., 3.2618, 2.8547, 2.0487]]) tensor([[ 4,  7, 21,  ..., 27, 28, 17],
        [16, 27, 18,  ..., 29, 30, 19],
        [ 6, 16, 14,  ..., 22, 24, 29],
        ...,
        [16, 16,  8,  ..., 21, 27, 22],
        [15,  0,  0,  ...,  9, 12,  3],
        [30, 14,  9,  ..., 23, 20, 14]])
tensor([3.2089, 4.1386, 3.2650, 3.3497, 4.4210, 3.0439, 3.5144, 3.2356, 3.3058,
        3.2702, 2.9981, 3.6997, 3.1719, 3.4962, 3.0889, 3.6220, 3.9256, 4.1314,
        3.0804, 3.3636, 3.5517, 3.2052, 3.6548, 3.7064, 3.6531, 4.5144, 3.1287,
        4.1465, 3.1906, 3.1493, 3.1996, 3.6754, 3.7610, 3.5968, 3.2109, 3.6037,
        3.2799, 3.0069, 3.0386, 3.0240, 3.5372, 3.6539, 3.5571, 3.2047, 3.1218,
        4.2479, 3.1230, 3.0372, 3.0258, 3.8679, 3.6409, 3.0938, 3.1246, 2.9426,
        4.0824, 3.8124, 3.4226, 3.3459, 4.1600, 3.6566, 3.0351, 3.3969, 3.5842,
        3.0997]) tensor([17, 21, 30, 62, 62, 63, 43, 31, 45, 63, 20,  4, 58, 23, 22, 43, 54, 30,
        15, 28, 13,  4,  4, 28,  6, 52, 53, 19, 33, 20,  3,  1, 14, 40,  0,  0,
        46, 62, 58, 45, 28, 50,  4, 55, 25,  5, 21, 16, 27, 32, 10, 19, 38, 30,
        48, 27, 20,  9,  2, 39, 55, 58, 32,  6])
tensor(4.5144) tensor(25)


This was by dimension, or simple 

m = values.max() 


will give you the max value.

a = torch.argmax(values)
idx = np.unravel_index(a, values.shape)


will give you the index.
