Fast value swapping in numpy array

Question

So, this is something, that should be pretty easy, but it seems to take an enormous amount of time for me: I have a numpy array with only two values (example 0 and 255) and I want to invert the matrix in that way, that all values swap (0 becomes 255 and vice versa). The matrices are about 2000³ entries big, so this is serious work! I first tried the numpy.invert method, which is not exactly what I expected. So I tried to do that myself by "storing" the values and then override them:

for i in range(array.length):
            array[i][array[i]==255]=1
            array[i][array[i]==0]=255
            array[i][array[i]==1]=0

which is behaving as expected, but taking a long time (I guess due to the for loop?). Would that be faster if I implement that as a multithreaded calculation, where every thread "inverts" a smaller sub-array? Or is there another way of doing that more conveniently?

Answer 1

In addition to @JanneKarila's and @EOL's excellent suggestions, it's worthwhile to show a more efficient approach to using a mask to do the swap.

Using a boolean mask is more generally useful if you have a more complex comparison than simply swapping two values, but your example uses it in a sub-optimal way.

Currently, you're making multiple temporary copies of the boolean "mask" array (e.g. array[i] == blah) in your example above and performing multiple assignments. You can avoid this by just making the "mask" boolean array once and the inverting it.

If you have enough ram for a temporary copy (of bool dtype), try something like this:

mask = (data == 255)
data[mask] = 0
data[~mask] = 255

Alternately (and equivalently) you could use numpy.where:

data = numpy.where(data == 255, 0, 255)

If you were using a loop to avoid making a full temporary copy, and need to conserve ram, adjust your loop to be something more like this:

for i in range(len(array)):
     mask = (array[i] == 255)
     array[mask] = 0
     array[~mask] = 255

All that having been said, either subtraction or XOR is the way to go in this case, especially if you preform the operation in-place!

Answer 2

To swap 0 and 255, you can use XOR if the data type is one of the integer types.

array ^= 255