Torch7 Tensor slicing

------------------------------------------------------------------------ slicing.lua-- -- This script demonstrates tensor slicing / manipulation.-- To run this script, simply do:-- $ th A_slicing.lua-- and then press 'y' or 'return' at each step, to keep going.-- little function to pause execution, and request user inputfunction next()   local answer = nil   while answer ~= '' and answer ~= 'y' and answer ~= 'Y' and neverstall ~= true do      io.write("continue ([y]/n/!)? ")      io.flush()      answer=io.read()      if answer == '!' then         neverstall = true      end      if answer == 'n' then         print('exiting...')         os.exit()      end   end   print ''endprint '----------------------------------------------------------------------'print 'creating a few tensors't1 = torch.range(1,75):resize(3,5,5)print 't1 = torch.range(1,75):resize(3,5,5)'print 't1 = 'print(t1)t2 = torch.range(1,25):resize(5,5)print 't2 = torch.range(1,25):resize(5,5)'print 't2 = 'print(t2)print 'done.'print ''next()print '----------------------------------------------------------------------'print 'the most basic slicing is done using the [] operator'print ''print 't1 ='print( t1 )print 't1[2] ='print( t1[2] )next()print '----------------------------------------------------------------------'print 't1_1 is a view in the existing t1 tensor: changing the values'print 'in t1_1 directly affects t1:'print ''t1[2]:fill(7)print 't1[2]:fill(7)'print 't1[2] ='print( t1[2] )print 't1 ='print( t1 )next()print '----------------------------------------------------------------------'print 'more complex slicing can be done using the [{}] operator'print 'this operator lets you specify one list/number per dimension'print 'for example, t2 is a 2-dimensional tensor, therefore'print 'we should pass 2 lists/numbers to the [{}] operator:'print ''t2_slice1 = t2[{ {},2 }]t2_slice2 = t2[{ 2,{} }]      -- equivalent to t2[2]t2_slice3 = t2[{ {2},{} }]t2_slice4 = t2[{ {1,3},{3,4} }]t2_slice5 = t2[{ {3},{4} }]t2_slice6 = t2[{ 3,4 }]print 't2 = 'print(t2)print 't2[{ {},2 }] ='print(t2_slice1)print 't2[{ 2,{} }] ='print(t2_slice2)print 't2[{ {2},{} }] ='print(t2_slice3)print 't2[{ {1,3},{3,4} }] ='print(t2_slice4)print 't2[{ {3},{4} }] ='print(t2_slice5)print 't2[{ 3,4 }] ='print(t2_slice6)next()print '----------------------------------------------------------------------'print 'negative indexes can also be used:'print ''t2_slice7 = t2[{ {},{2,-2} }]t2_slice8 = t2[{ -1,-1 }]print 't2[{ {},{2,-2} }] ='print(t2_slice7)print 't2[{ -1,-1 }] ='print(t2_slice8)next()print '----------------------------------------------------------------------'print 'in basic Lua, the = operator cannot be overloaded (that speeds up the language parser'print 'a lot...), but you can use the [{}] operator to copy tensors, and subtensors:'print ''print 't3 = torch.Tensor(5)'print 't3[{}] = t2[{ {},1 }]'t3 = torch.Tensor(5)t3[{}] = t2[{ {},1 }]print 't3 ='print(t3)next()print '----------------------------------------------------------------------'print 'if you need to slice arbitrary subtensors, you will need to do it in steps:'print ''t4 = torch.Tensor(5,2)t4[{ {},1 }] = t2[{ {},2 }]t4[{ {},2 }] = t2[{ {},5 }]print [[t4 = torch.Tensor(5,2)t4[{ {},1 }] = t2[{ {},2 }]t4[{ {},2 }] = t2[{ {},5 }]]]print 't4 ='print(t4)