I've the following two arrays:
array1 = [0, 1, 1, 0]
array2 = ['foo', 'bar', 'hello', 'bye']
I want to save into an array the values of array2 that has the index 1in array1.
On the example above, the desired result should be result_array = ['bar', 'hello'].
I've tried something like this, but it's not working.
for i in array1:
if i = 1:
result_array.append(array2[i])
Thanks in advance
The problem with your code is that you're using = in if condition, replace it with ==. And secondly to get the index as well as item you need to use enumerate, currently you're appending array[i], so your code will end up appending 'bar' two times.
>>> result_array = []
>>> for i, x in enumerate(array1):
if x == 1:
result_array.append(array2[i])
...
>>> result_array
['bar', 'hello']
Another better way to do this is to use zip and a list comprehension:
>>> [b for a, b in zip(array1, array2) if a==1]
['bar', 'hello']
And fastest way to do this is to use itertools.compress:
>>> from itertools import compress
>>> list(compress(array2, array1))
['bar', 'hello']
If you're interested in only the 1s in array 1, then you can use zip to iterate through multiple list/array at the same time, see http://docs.python.org/2/library/functions.html#zip (note that zip will ONLY iterate until the length of the shorter list):
>>> array1 = [0, 1, 1, 0]
>>> array2 = ['foo', 'bar', 'hello', 'bye']
>>> [j for i,j in zip(array1, array2) if i == 1]
['bar', 'hello']
If you want an index of the two arrays using array1 as key in a defaultdict, see http://docs.python.org/2/library/collections.html#collections.defaultdict:
>>> from collections import defaultdict
>>> array1 = [0, 1, 1, 0]
>>> array2 = ['foo', 'bar', 'hello', 'bye']
>>> array3 = defaultdict(list)
>>> for i,j in zip(array1,array2):
... array3[i].append(j)
...
>>> array3[1]
['bar', 'hello']
>>> array3[0]
['foo', 'bye']
You can use the built-in zip function and list comprehensions:
print [y for x, y in zip(array1, array2) if x == 1]
Or, if you're into numpy:
>>> import numpy as np
>>> array1 = np.array([0, 1, 1, 0])
>>> array2 = np.array(['foo', 'bar', 'hello', 'bye'])
>>> # np.nonzero returns indices of array1 that are, well, non-zero
>>> array2[np.nonzero(array1)]
array(['bar', 'hello'],
dtype='|S5')
The modified version of your code which will work is:
for i in range(len(array1)):
if array1[i]:
result_array.append(array2[i])
The problems in your code are:
1) You use for i in array1, where i is the temp iterator created to represent the values in the list. So it will always be either 0 or 1, depending on what value of the list it represents
2) if i=1 is wrong. = is assignment operator and == should be used to compare values. Since you said the list contains only 0s and 1s, if i: us enough
from itertools import izip
res = [v2 for v1, v2 in izip(array1, array2) if v1==1]
You may use simple zip instead of izip for small lists.
if i = 1:is a typo?lists.