Creating a list or array with a fixed size?

Question

I want to create an array or list which only contains 3 entries at any given time. The function should loop and with each loop 1 entry will be added to the list pushing the oldest value out and then a value will be calculated based on the 3 values in the list.

I have tried:

import numpy as np

z = np.ndarray((3,),float)

np.append(z, [12, 14.56, 12.46, 1.56])

which creates a numpy array with only 3 values (afaik) however the array is populated with strange values:

z= ([  1.56889217e-163,   1.01899555e-297,   1.03395110e-297])

anyone know why/what I'm doing wrong or have a better solution for what I want to do?

Bi Rico · Accepted Answer · 2013-05-07 23:49:58Z

5

Use a deque:

from collections import deque
z = deque(maxlen=3)
z.extend([1, 2, 3, 4])
print z
# deque([2, 3, 4], maxlen=3)
z.append(5)
# deque([3, 4, 5], maxlen=3)
print z

You can also appendleft and extendleft on a deque if you need to.

answered May 7, 2013 at 23:49

Bi Rico

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askewchan · Accepted Answer · 2013-05-07 22:29:06Z

2

z is being initialized as an empty array. Those strange values are just random numbers that were in the empty memory slots before z was created. You need to have an initial value.

Start with:

z = np.array([14.56, 12.46, 1.56])

And then modify its values, but don't append (that changes the size of the array).

Then here's an example of a function that will 'roll' the elements of your array back by one (or any number) and then replace the first value with a new value.

def push(a, n):
     a = np.roll(a, 1)
     a[0] = n
     return a

Of course, instead of n, you want to use your function of a. For example, if you want to append the sum of the original array to the beginning:

def push_sum(a):
     a = np.roll(a, 1)
     a[0] = a.sum()
     return a

Then:

In [19]: z = np.array([14.56, 12.46, 1.56])

In [20]: push(z, 14)
Out[20]: array([ 14.  ,  14.56,  12.46])

In [29]: push_sum(z)
Out[29]: array([ 28.58,  14.56,  12.46])

edited May 7, 2013 at 22:29

answered May 7, 2013 at 22:12

askewchan

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3 Comments

Ryan Saxe Over a year ago

way does the output have more than 3 items?

mark mcmurray Over a year ago

It does indeed, that was mainly because I was trying to find out which value it would drop in that case, but this answers my question anyway

askewchan Over a year ago

@mark Trying to assign N values to an array of length ≠ N will just give you a ValueError

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