Question

getting positions for set of characters in a python string

set of character :

    string="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    charPositionToFind=A,D,V,Y

expected Output

    postions=[0,3,21,24]

i do it this way

 def find_all(string,char):
     return [i - 1 for i in range(len(string)) if string.startswith(char, i - 1)]

 string="ABCDEYYFGHIAAJKVLMNOPDCQRSTAAVVVUVWXYZ"
 charPositionToFind=['A','D','V','Y']
 position=[]

 for char in charPositionToFind:
    s = find_all(string,char)
    position.extend(s)
 print sorted(position)

  output:
       [0, 3, 5, 6, 11, 12, 15, 21, 27, 28, 29, 30, 31, 33, 36]

But i want the best way to do this

Answer 1

string.index would be nice to use, but there are two issues with it.
1) It only finds the first occurence of the character, and
2) It raise an error if the character is not found, necessitating a check for existence before using index().

Looking at the problem simplistically, these are two easy approaches to the problem:

Method 1:

for character in the string:
    for target in charPositionToFind:
        test if character == target

Method 2:

for target in charPositionToFind:
    for character in the string:
        test if character == target

Runtime wise, the two methods have the same worst case of O(N x M), where N is the size of string and M is the size of charPositionToFind. However, using method 1 allows you to remove the inner loop by using a set. It also avoids having to do a sort at the end, since you are iterating through the characters of string in order. So, using list comprehension to avoid for loops:

string = "ABCDEYYFGHIAAJKVLMNOPDCQRSTAAVVVUVWXYZ"
charPositionToFind = 'ADVY'
target_set = set(charPositionToFind)
position = [index for index, char in enumerate(string) if char in target_set]