Building up an array in numpy/scipy by iteration in Python?


Often, I am building an array by iterating through some data, e.g.:

my_array = []
for n in range(1000):
  # do operation, get value 
# cast to array
my_array = array(my_array)

I find that I have to first build a list and then cast it (using “array”) to an array. Is there a way around these? All these casting calls clutter the code… how can I iteratively build up “my_array”, with it being an array from the start?

Asked By: user248237



The recommended way to do this is to preallocate before the loop and use slicing and indexing to insert

my_array = numpy.zeros(1,1000)
for i in xrange(1000):
    #for 1D array
    my_array[i] = functionToGetValue(i)
    #OR to fill an entire row
    my_array[i:] = functionToGetValue(i)
    #or to fill an entire column
    my_array[:,i] = functionToGetValue(i)

numpy does provide an array.resize() method, but this will be far slower due to the cost of reallocating memory inside a loop. If you must have flexibility, then I’m afraid the only way is to create an array from a list.

EDIT: If you are worried that you’re allocating too much memory for your data, I’d use the method above to over-allocate and then when the loop is done, lop off the unused bits of the array using array.resize(). This will be far, far faster than constantly reallocating the array inside the loop.

EDIT: In response to @user248237’s comment, assuming you know any one dimension of the array (for simplicity’s sake):

my_array = numpy.array(10000, SOMECONSTANT)

for i in xrange(someVariable):
    if i >= my_array.shape[0]:
        my_array.resize((my_array.shape[0]*2, SOMECONSTANT))

    my_array[i:] = someFunction()

#lop off extra bits with resize() here

The general principle is “allocate more than you think you’ll need, and if things change, resize the array as few times as possible”. Doubling the size could be thought of as excessive, but in fact this is the method used by several data structures in several standard libraries in other languages (java.util.Vector does this by default for example. I think several implementations of std::vector in C++ do this as well).

Answered By: Chinmay Kanchi

If i understand your question correctly, this should do what you want:

# the array passed into your function
ax = NP.random.randint(10, 99, 20).reshape(5, 4)

# just define a function to operate on some data
fnx = lambda x : NP.sum(x)**2

# apply the function directly to the numpy array
new_row = NP.apply_along_axis(func1d=fnx, axis=0, arr=ax)

# 'append' the new values to the original array
new_row = new_row.reshape(1,4)
ax = NP.vstack((ax, new_row))
Answered By: doug

NumPy provides a ‘fromiter’ method:

def myfunc(n):
    for i in range(n):
        yield i**2

np.fromiter(myfunc(5), dtype=int)

which yields

array([ 0,  1,  4,  9, 16])
Answered By: Stefan van der Walt

Building up the array using list.append() seems to be much faster than any kind of dynamic resizing of a Numpy array:

import numpy as np
import timeit

class ndarray_builder:
  def __init__(self, capacity_step, column_count):
    self.capacity_step = capacity_step
    self.column_count = column_count
    self.arr = np.empty((self.capacity_step, self.column_count))
    self.row_pointer = 0

  def __enter__(self):
    return self

  def __exit__(self, type, value, traceback):
  def append(self, row):
    if self.row_pointer == self.arr.shape[0]:
      self.arr.resize((self.arr.shape[0] + self.capacity_step, self.column_count))
    self.arr[self.row_pointer] = row
    self.row_pointer += 1
  def close(self):
    self.arr.resize((self.row_pointer, self.column_count))

def with_builder():
  with ndarray_builder(1000, 2) as b:
    for i in range(10000):
      b.append((1, 2))
      b.append((3, 4))
  return b.arr

def without_builder():
  b = []
  for i in range(10000):
    b.append((1, 2))
    b.append((3, 4))
  return np.array(b)

print(f'without_builder: {timeit.timeit(without_builder, number=1000)}')
print(f'with_builder: {timeit.timeit(with_builder, number=1000)}')

without_builder: 3.4763141250000444
with_builder: 7.960973499999909
Answered By: youurayy
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