How to find all occurrences of multiple elements in a list?

Question

I have a list:

['a','b','b','c']

to find all occurrences of an element I use:

incd=['a','b','b','c']
indeces=[i for i, x in enumerate(incd) if x == 'b']

how can I search for two elements and all their positions?

w1='a'
w2='b'
indeces=[i for i, x in enumerate(incd) if x == w1|w2]

returns

TypeError: unsupported operand type(s) for |: 'str' and 'str'

and

indeces=[i for i, x in enumerate(incd) if x == 'a|b']

returns

[]

both fails

I would like to have returned

[0, 1, 2]

Answer 1

IIUC,

s=pd.Series(incd)
s[s.eq(w1)|s.eq(w2)].index
#Int64Index([0, 1, 2], dtype='int64')

Answer 2

you do that: you have to use the condition 'or'

incd = ['a', 'b', 'b', 'c']
w1 = 'a'
w2 = 'b'
indeces = [i for i, x in enumerate(incd) if x == w1 or x== w2]

if you have lot of data to test: use a list

w = ['a', 'b' ,'d',...]
indeces = [i for i, x in enumerate(incd) if x in w]

Answer 3

I suggest going through the Operators in Python .

Replace this :

if x == w1|w2   

With this :

if x == w1 or x == w2

enumerate over the list, and check if the element is equal to w1 or w2 :

s = ['a','b','b','c']

w1 = 'a'
w2 = 'b'

for indx, elem in enumerate(s):
   if elem == w1 or elem == w2:
      print("Elem: {} at Index {}".format(elem, indx))

OUTPUT :

Elem: a at Index 0
Elem: b at Index 1
Elem: b at Index 2

Shorter-version :

print([i for i, e in enumerate(s) if e == w1 or e == w2])   # to have a tuple of both elem and indx replace i with (e,i)

OUTPUT :

[0, 1, 2]

Answer 4

Since you tag pandas

l=['a','b','b','c']

s=pd.Series(range(len(l)),index=l)
s.get(['a','b'])
Out[893]: 
a    0
b    1
b    2
dtype: int64

Answer 5

Using a set for more speed and a dynamic number of elements to search for:

find = {2, 3} # The elements we want to find
a = [1,2,2,3,4] # our list
x = [ind for ind, val in enumerate(a) if val in find]
print(x)

Answer 6

You can use in operator to expression logical OR relation.

incd = list('abcd')
w1, w2 = 'a', 'b'

indeces = [i for i, x in enumerate(incd) if x in [w1, w2]]

It results correct indeces as desired

indeces = [0, 1]

Answer 7

indeces=[i for i, x in enumerate(incd) if x == w1 or x == w2]

Answer 8

Use a defaultdict with list as its default

from collections import defaultdict
d = defaultdict(list)
for n,e in enumerate(incd): 
    d[e].append(n)

What's in d at this point?

>>> d
defaultdict(<class 'list'>, {'a': [0], 'b': [1, 2], 'c': [3]})

and to get positions of 'a' or 'b' (and prove it works for a key 'foo' not in incd )

print( d['a']+d['b']+d['foo'] )
# gives [0,1,2]