Find Unique Characters in a File

Question

I have a file with 450,000+ rows of entries. Each entry is about 7 characters in length. What I want to know is the unique characters of this file.

For instance, if my f

Answer 1

Python without using a set.

file = open('location', 'r')

letters = []
for line in file:
    for character in line:
        if character not in letters:
            letters.append(character)

print(letters)