Python program to check if a string contains all unique characters

In this article, we will learn different approaches to check if a string contains all unique characters. This is a common programming problem that tests our understanding of data structures and string manipulation.

Problem Statement

Given a string input, we need to determine whether all characters in the string are unique (no character appears more than once).

Using Boolean Array

We can create an array of boolean values to track which characters we have seen. If we encounter a character that we've already seen, we return False immediately.

Example

def isUniqueChars(st):
    if len(st) > 256:
        return False
    
    # Initialize boolean array for ASCII characters
    char_set = [False] * 128
    
    for i in range(len(st)):
        # Get ASCII value of character
        val = ord(st[i])
        
        # If character already seen, return False
        if char_set[val]:
            return False
        
        # Mark character as seen
        char_set[val] = True
    
    return True

# Test with different strings
print("tutorialspoint:", isUniqueChars("tutorialspoint"))
print("python:", isUniqueChars("python"))
print("abc:", isUniqueChars("abc"))

tutorialspoint: False
python: False
abc: True

Using Python Set

A more Pythonic approach is to use a set data structure, which automatically handles uniqueness.

Example

def isUniqueCharsSet(st):
    # Compare string length with set length
    return len(st) == len(set(st))

# Test with different strings
print("tutorialspoint:", isUniqueCharsSet("tutorialspoint"))
print("python:", isUniqueCharsSet("python"))
print("abc:", isUniqueCharsSet("abc"))
print("hello:", isUniqueCharsSet("hello"))

tutorialspoint: False
python: False
abc: True
hello: False

Using Dictionary for Character Counting

Another approach is to count character frequencies and check if any character appears more than once.

Example

def isUniqueCharsDict(st):
    char_count = {}
    
    for char in st:
        if char in char_count:
            return False
        char_count[char] = 1
    
    return True

# Test with different strings
print("tutorialspoint:", isUniqueCharsDict("tutorialspoint"))
print("python:", isUniqueCharsDict("python"))
print("world:", isUniqueCharsDict("world"))

tutorialspoint: False
python: False
world: True

Comparison

Conclusion

The set-based approach is most Pythonic and readable. Use the boolean array method for performance-critical applications with ASCII strings. All methods have O(n) time complexity but differ in space usage and readability.