Python program to Find the first non-repeating character from a stream of characters?

In this article, we will find the first non-repeating character from a stream of characters. A non-repeating character is one that appears exactly once in the string. Let's say the following is our input ?

thisisit

The following should be our output displaying first non-repeating character ?

h

Using While Loop

We will find the first non-repeating character by comparing each character with others using a while loop. This approach removes all occurrences of each character and checks if only one was removed ?

# String
my_str = "thisisit"

# Looping
while my_str != "":
    original_len = len(my_str)
    char = my_str[0]
    my_str = my_str.replace(char, "")
    new_len = len(my_str)
    
    if new_len == original_len - 1:
        print("First non-repeating character =", char)
        break
else:
    print("No unique character found!")

First non-repeating character = h

Using a Custom Function with Dictionary

We can create a custom function that uses a dictionary to count character frequencies and a list to maintain the order of first occurrence ?

# Custom function
def find_first_non_repeating(my_str):
    char_order = []
    counts = {}
    
    # Count frequencies and track order
    for char in my_str:
        if char in counts:
            counts[char] += 1
        else:
            counts[char] = 1
            char_order.append(char)
    
    # Find first character with count 1
    for char in char_order:
        if counts[char] == 1:
            return char
    return None

result = find_first_non_repeating('thisisit')
print("First non-repeating character =", result)

First non-repeating character = h

Using Counter from Collections Module

The Counter from the collections module provides a convenient way to count character frequencies. This approach is more concise and efficient ?

from collections import Counter

def find_non_repeating_counter(my_str):
    # Count character frequencies
    freq = Counter(my_str)
    
    # Find first character with frequency 1
    for char in my_str:
        if freq[char] == 1:
            return char
    return None

# Driver code
my_str = "thisisit"
result = find_non_repeating_counter(my_str)
print("First non-repeating character =", result)

h

Comparison of Methods

Conclusion

The Counter method is most efficient and readable for finding the first non-repeating character. The custom function approach provides good control over the process, while the while loop method is educational but less efficient for larger strings.