Copy values from numpy array to another array depending on a third value

Question

im using numpy 2d arrays. I want to copy values from one array to another array, but only if an expression is true.

e.g.

for row in range(len(anImage1)):
  for col in range(len(anImage1[0])):
    if col > 100:
      anImage1[row][col] = min(anImage1[row][col], anImage2[row][col])

or

for row in range(len(anImage1)):
  for col in range(len(anImage1[0])):
    if anImageMask[row][col] == 255:
      anImage1[row][col] = anImage2[row][col]

I know this is a very poor and slow solution... Can somebody explain me how to speed up the code?

Jaime · Accepted Answer · 2014-07-22 12:02:05Z

2

Assume that the anImage1 and anImage12 are 256 by 256.

anImage1 = np.random.randint(0, 255, (256, 256))
anImage2 = np.random.randint(0, 255, (256, 256))

Just, the first condition can be replaced by (updated min to minimum. see @jaime's comment)

anImage1[:, 101:] = np.minimum(anImage1[:, 101:], anImage2[:, 101:], axis=0)

and the second condition is:

cond = anImage1 == 255
anImage1[cond] = anImage2[cond]

Avoid loop is important to optimize numpy performance.

edited Jul 22, 2014 at 12:02

Jaime

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answered Jul 22, 2014 at 7:54

emesday

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2 Comments

Jaime Over a year ago

Your use of np.min is inefficient, as it has to create an intermediate array stacking both images. You can get the same result efficiently with np.minimum.

emesday Over a year ago

@Jaime I didn't know about that. Thanks for let us know!

dirkster · Accepted Answer · 2014-07-22 08:12:06Z

0

Depends. If you have no information about where in the array your condition is true, you have no choice but to check the condition for each element in the array. In that case writing code like

for i in xrange(len(A)):
  for j in xrange(len(A[0])):
    if condition(A[i][j])
      B[i][j] = A[i][j]

is unavoidable. If you find yourself doing this repeatedly with the same condition, you might use a trick like storing the indices for which the condition is true, e.g.

source = ["a", "b", "c", "a"]
dest = [None]*len(source)
condition = lambda x : x == "a"
indices = [i for i in xrange(len(source)) if condition(source[i])]
for index in indices:
    dest[index] = source[index]

Hope this helps.

answered Jul 22, 2014 at 8:12

dirkster

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