# 5 Effective Ways to Count the Number of Consistent Strings in Python

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π‘ Problem Formulation: Youβre given an array of strings and a string representing a set of allowed characters. The task is to count the number of strings in the array that consist only of the characters in the allowed set. For example, if your allowed set is "ab" and your array is ["ad", "bd", "aaab", "baa", "badab"], the output should be 2 since only "aaab" and "baa" consist solely of characters 'a' and 'b'.

## Method 1: Using Set Operations

This method leverages set operations to determine if the allowed characters cover the characters in each string. It defines a function, count_consistent_strings(allowed, words) that returns the count of consistent strings by comparing set intersections.

Here’s an example:

def count_consistent_strings(allowed, words):
allowed_set = set(allowed)
return sum(1 for word in words if set(word).issubset(allowed_set))

allowed_chars = "ab"
count = count_consistent_strings(allowed_chars, words)
print(count)

Output: 2

This method initializes a set of allowed characters and iterates through the list of words, checking if all characters of the word are in the allowed set. The issubset() method efficiently checks this condition, and the sum function accumulates the count of consistent strings.

## Method 2: Using String Methods

This method uses built-in string methods to count characters that occur only in the allowed set for each word. Defines count_consistent_strings(allowed, words) that iterates through each word and checks for consistency with the allowed characters.

Here’s an example:

def count_consistent_strings(allowed, words):
return sum(all(ch in allowed for ch in word) for word in words)

allowed_chars = "ab"
count = count_consistent_strings(allowed_chars, words)
print(count)

Output: 2

This method goes through each character in each word and checks whether it is present in the allowed characters. The all() function is used to ensure all characters in a word meet the condition, and the number of consistent strings is accumulated using sum.

## Method 3: Filtering and Lambda Function

Combining filter function and lambda, this method filters out inconsistent strings. The function count_consistent_strings(allowed, words) employs a lambda to determine if a string is consistent and filters accordingly.

Here’s an example:

def count_consistent_strings(allowed, words):
return len(list(filter(lambda word: all(ch in allowed for ch in word), words)))

allowed_chars = "ab"
count = count_consistent_strings(allowed_chars, words)
print(count)

Output: 2

The lambda function takes each word and applies the conditional check for all characters, using filter() to keep only the words that pass the check. The len() function then provides the count of consistent strings from the filtered list.

## Method 4: Using List Comprehension

This method uses a list comprehension to achieve the same result. The function count_consistent_strings(allowed, words) uses a succinct list comprehension to evaluate the consistency of strings.

Here’s an example:

def count_consistent_strings(allowed, words):
return len([word for word in words if set(word).issubset(set(allowed))])

allowed_chars = "ab"
count = count_consistent_strings(allowed_chars, words)
print(count)

Output: 2

This method uses a list comprehension to mimic the effect of filtering by set operations. It checks if the set of characters in each word is a subset of the allowed characters, and then counts the number of consistent strings by evaluating the length of the resulting list.

## Bonus One-Liner Method 5: Using List Comprehension and Sum

For a succinct one-liner solution, this method combines list comprehension with Pythonβs sum function to count the consistent strings in a compact fashion.

Here’s an example:

allowed_chars = "ab"
count = sum(set(word).issubset(set(allowed_chars)) for word in words)
print(count)

Output: 2

This one-line approach is elegant and efficient. It constructs a generator that evaluates whether a word’s set is a subset of the allowed characters set for each word and then counts the number of True instances with sum.

## Summary/Discussion

• Method 1: Using Set Operations. Strengths: It’s efficient and easy to read. Weaknesses: Might not be the most intuitive for beginners.
• Method 2: Using String Methods. Strengths: Straightforward for those familiar with string operations. Weaknesses: Possibly less efficient due to multiple in checks.
• Method 3: Filtering and Lambda Function. Strengths: Makes use of functional programming paradigms. Weaknesses: Could be less readable for those not comfortable with lambdas.
• Method 4: Using List Comprehension. Strengths: Compact and pythonic. Weaknesses: Creates an intermediate list, potentially using more memory for large inputs.
• Method 5: Bonus One-Liner. Strengths: Most compact and elegant. Weaknesses: May sacrifice a bit of readability for brevity.