Find maximum index from a vector source

Question

I am trying to create a block that compare 2 values from a vector source and return the maximum index. In general, my vector source will be a set of values [0,2,0,0,0,0,0,0,0,0,0,0,0,0,0,0]. I will compare the first two values. The expected result should be 1. When i tried to run the code, it returns with index error: index 1 is out of bound for axis 0 with size 1. Where does this error come from?

Is there any way to check the array size within GNUradio? Thanks in advance

class blk(gr.sync_block):  # other base classes are basic_block, decim_block, interp_block

    def __init__(self, FFT_size = 32, subchannel1 = 0, subchannel2 = 1):  # only default arguments here
        """arguments to this function show up as parameters in GRC"""
        gr.sync_block.__init__(
            self,
            name='Peak Detection',   # will show up in GRC
            in_sig=[(np.float32, FFT_size)],
            out_sig=[np.float32]
        )
        # if an attribute with the same name as a parameter is found,
        # a callback is registered (properties work, too).
        self.FFT_size = FFT_size
        self.subchannel1 = subchannel1
        self.subchannel2 = subchannel2

    def work(self, input_items, output_items):
      for portIndex in range(len(input_items)):
            for vectorIndex in range(len(input_items[0])):
                  channel1 = input_items[0][self.subchannel1]
                  channel2 = input_items[0][self.subchannel2]
                  for sampleIndex in range(len(input_items[0][vectorIndex])):
                  array = [channel1[sampleIndex], channel2[sampleIndex]]
                  output_items[0][:] = np.argmax(array)
      return len(output_items[0])