[Array] Count Prefixes of a Given String - Easy

Counting prefixes of a string is a common problem. We want to find out how many strings in a list start with the same prefix as a given string. We can solve this problem easily using arrays. We will go through the list of strings and check which ones match the prefix. By using programming languages like Java, Python, and C++, we can get good performance while counting these prefixes.

In this article, we will look at different ways to count prefixes of a string using arrays in various programming languages. We will talk about the basic method. We will also give examples in Java, Python, and C++. Lastly, we will share better solutions for each language. We will answer some common questions about this topic too. The headers we will cover are:

• Count Prefixes of Given String Using Arrays in Java
• Java Implementation of Count Prefixes of Given String
• Optimized Java Solution for Count Prefixes of Given String
• Count Prefixes of Given String Using Arrays in Python
• Python Implementation of Count Prefixes of Given String
• Optimized Python Solution for Count Prefixes of Given String
• Count Prefixes of Given String Using Arrays in C++
• C++ Implementation of Count Prefixes of Given String
• Optimized C++ Solution for Count Prefixes of Given String
• Frequently Asked Questions

If you want to read more about similar array problems, check these articles: Array Two Sum, Array Best Time to Buy and Sell Stock, and Array Contains Duplicate.

Java Implementation of Count Prefixes of Given String

We can count the prefixes of a given string from a list of strings in Java using a simple way. Our goal is to find how many strings in the list are prefixes of the given string. Here is a simple code for this idea:

public class CountPrefixes {
    public static int countPrefixes(String[] words, String s) {
        int count = 0;
        for (String word : words) {
            if (s.startsWith(word)) {
                count++;
            }
        }
        return count;
    }

    public static void main(String[] args) {
        String[] words = {"a", "b", "c", "ab", "bc", "abc"};
        String s = "abc";
        int result = countPrefixes(words, s);
        System.out.println("Number of prefixes: " + result);
    }
}

Explanation:

We have the countPrefixes method. It goes through each word in the words list. It checks if the string s starts with that word using the startsWith method.

The count variable keeps track of how many prefixes we find. In the end, it returns the total number of prefixes found in the list.

This code runs in O(n * m) time. Here n is the number of words in the list and m is the average length of the words. This makes it good for a normal size of input.

Optimized Java Solution for Count Prefixes of Given String

We want to count how many prefixes of a given string are in an array. To do this fast, we can use a HashSet. It helps us look up things quickly. This method is much faster than using a nested loop.

Java Implementation

Our optimized method has these steps:

1. We store all the prefixes of the string in a HashSet.
2. We go through the array of strings and check if each string is in the HashSet.
3. We count how many matches we find.

Here is the Java code for this method:

import java.util.HashSet;

public class CountPrefixes {
    public static int countPrefixes(String[] words, String s) {
        HashSet<String> prefixes = new HashSet<>();
        
        // Generate all prefixes of the string s
        for (int i = 0; i <= s.length(); i++) {
            prefixes.add(s.substring(0, i));
        }

        int count = 0;
        // Check if each word is a prefix of s
        for (String word : words) {
            if (prefixes.contains(word)) {
                count++;
            }
        }
        
        return count;
    }

    public static void main(String[] args) {
        String[] words = {"a", "b", "ap", "app", "appl", "apply", "apple"};
        String s = "apple";
        System.out.println("Count of prefixes: " + countPrefixes(words, s)); // Output: 5
    }
}

Explanation of Code

• HashSet: We use a HashSet to keep prefixes. It helps us to find them in O(1) average time.
• Prefix Generation: We make prefixes by going from 0 to the length of the string s. We add each part to the HashSet.
• Counting Matches: We check each word in the words array. If a word is in the HashSet, we add one to our count.

This method works in O(n + m) time. Here, n is the number of words and m is the length of the string s. This makes it good for large inputs.

For more topics like this, we can check Count Equal and Divisible Pairs in an Array. It has more ways to work with arrays.

Count Prefixes of Given String Using Arrays in Python

We want to count how many prefixes of a string match any string in an array. We can do this easily in Python. We will look at each string in the array and check if it is a prefix of the given string.

Basic Implementation

Here is a simple way to do this using a loop:

def count_prefixes(arr, s):
    count = 0
    for prefix in arr:
        if s.startswith(prefix):
            count += 1
    return count

# Example usage
arr = ["a", "b", "ba", "ab", "abc"]
s = "abc"
result = count_prefixes(arr, s)
print(result)  # Output: 3

Optimized Implementation

For a better solution, especially with longer strings or bigger arrays, we can use a set. This helps us find prefixes faster:

def count_prefixes_optimized(arr, s):
    prefixes = set(arr)
    count = 0
    for i in range(1, len(s) + 1):
        if s[:i] in prefixes:
            count += 1
    return count

# Example usage
arr = ["a", "ab", "abc", "b", "ba"]
s = "abc"
result = count_prefixes_optimized(arr, s)
print(result)  # Output: 3

Explanation

• Basic Implementation: The function count_prefixes goes through each prefix in the array. It checks if the prefix matches the start of the string s with startswith().
• Optimized Implementation: The count_prefixes_optimized function puts the prefixes in a set. This makes looking them up much faster. It checks each part of s from the start to its full length.

This method works well for larger data and helps us count all valid prefixes quickly.

For more information on array tasks in Python, we can look at articles like Array - Find All Numbers Disappeared in an Array or Array - Remove Duplicates from Sorted Array.

Python Implementation of Count Prefixes of Given String

We want to count how many prefixes of a given string are in an array of words. We can do this easily by looking at each word in the array. We check if it is a prefix of the target string.

Here is a simple Python code for this:

def count_prefixes(words, target):
    count = 0
    for word in words:
        if target.startswith(word):
            count += 1
    return count

# Example usage
words = ["a", "b", "c", "ab", "bc", "abc"]
target = "abc"
result = count_prefixes(words, target)
print(result)  # Output: 3

Optimized Approach

We can make it faster by using a set for lookups. This helps us check prefixes more quickly:

def count_prefixes_optimized(words, target):
    prefixes = set(words)
    count = 0
    for i in range(1, len(target) + 1):
        if target[:i] in prefixes:
            count += 1
    return count

# Example usage
words = ["a", "b", "c", "ab", "bc", "abc"]
target = "abc"
result = count_prefixes_optimized(words, target)
print(result)  # Output: 3

Explanation of Code

In the count_prefixes function, we go through each word in the words array. We check if it is a prefix of the target string using the startswith method.

In the count_prefixes_optimized function, we use a set for words. This helps us check for prefixes faster. We make prefixes of the target string by slicing it up to each index. Then we check if they are in the set.

This method works well and can solve many cases. It is a good way to count prefixes of a given string in Python. For more problems with arrays, we can look at related articles like Array Two Sum and Array Contains Duplicate.

Optimized Python Solution for Count Prefixes of Given String

We can count the number of prefixes of a given string from an array of words in Python by using a Trie. A Trie is also known as a prefix tree. This method helps us insert and search quickly. It is good when we need to check many prefixes.

Implementation Steps

1. Define a Trie Node: Each node has a dictionary of children and a count of words that go through it.
2. Insert Words into the Trie: For every word in the array, we insert it into the Trie.
3. Count Prefixes: For each prefix of the input string, we go through the Trie to count how many prefixes are there.

Python Code

Here is the Python code:

class TrieNode:
    def __init__(self):
        self.children = {}
        self.count = 0

class Trie:
    def __init__(self):
        self.root = TrieNode()
    
    def insert(self, word):
        node = self.root
        for char in word:
            if char not in node.children:
                node.children[char] = TrieNode()
            node = node.children[char]
            node.count += 1
    
    def count_prefixes(self, prefix):
        node = self.root
        for char in prefix:
            if char not in node.children:
                return 0
            node = node.children[char]
        return node.count

def count_prefixes(words, s):
    trie = Trie()
    for word in words:
        trie.insert(word)
    
    count = 0
    for i in range(1, len(s) + 1):
        count += trie.count_prefixes(s[:i])
    
    return count

# Example usage
words = ["a", "b", "ab", "ba", "abc"]
s = "ab"
result = count_prefixes(words, s)
print(result)  # Output: 5

Explanation of the Code

• TrieNode Class: This class shows each node of the Trie. It has a dictionary for children and a count of prefixes.
• Trie Class: This class has methods to insert words and count prefixes.
• count_prefixes Function: This function takes a list of words and a string. It inserts each word into the Trie and counts the prefixes of the string.

This solution works well with a time complexity of O(N * M). Here N is the number of words and M is the length of the longest word. This makes it good for large inputs.

For more problems about arrays, we can look at Count Equal and Divisible Pairs in an Array.

Count Prefixes of Given String Using Arrays in C++

To count how many prefixes of a string match with any string in an array, we can use a simple way with arrays in C++. We want to find out how many strings in the array are prefixes of the given string.

C++ Implementation

Here is a simple C++ code to count prefixes:

#include <iostream>
#include <vector>
#include <string>

using namespace std;

int countPrefixes(vector<string>& words, string s) {
    int count = 0;
    for (const string& word : words) {
        if (s.find(word) == 0) { // Check if 'word' is a prefix of 's'
            count++;
        }
    }
    return count;
}

int main() {
    vector<string> words = {"a", "b", "ba", "ab", "abc"};
    string s = "abc";
    int result = countPrefixes(words, s);
    cout << "Number of prefixes: " << result << endl; // Output: 3
    return 0;
}

Explanation of the Code

• We make a function called countPrefixes. This function takes a list of strings (words) and a target string (s).
• The function goes through each word in the words array.
• It checks if the current word is a prefix of s using the find() method.
• If it is a prefix, we add one to the count.
• In the end, we return the total count of prefixes.

Complexity

• Time Complexity: O(n * m). Here n is the number of words in the array. m is the longest length of the words.
• Space Complexity: O(1) because we use a fixed amount of space.

This code counts the prefixes well and can be used in many cases where we need to match prefixes. For more information on array work, look at the article about Array Two Sum.

C++ Implementation of Count Prefixes of Given String

We want to count how many strings from a list are prefixes of a target string in C++. It is a simple task.

Here is a sample code:

#include <iostream>
#include <vector>
#include <string>

using namespace std;

int countPrefixes(vector<string>& words, string s) {
    int count = 0;
    for (const string& word : words) {
        if (s.find(word) == 0) { // Check if 's' starts with 'word'
            count++;
        }
    }
    return count;
}

int main() {
    vector<string> words = {"a", "b", "c", "ab", "bc", "abc"};
    string target = "abc";
    int result = countPrefixes(words, target);
    cout << "Number of prefixes: " << result << endl; // Output: 3
    return 0;
}

Explanation:

The function countPrefixes takes a list of strings called words and a target string called s.

We start with a counter called count. It is set to zero.

Then we look at each word in the words list. We check if the target string s starts with that word. We do this using s.find(word) == 0.

If it is true, we add one to the counter.

At the end, the function gives back the total number of prefixes found.

This code counts the prefixes in O(n * m) time. Here, n is the number of words and m is the average length of the words. We can change the code to fit our needs and project rules.

For more tips about working with arrays, look at Array Remove Duplicates from Sorted Array.

Optimized C++ Solution for Count Prefixes of Given String

To count how many prefixes of a string match any string in a list of words, we can use a trie. A trie is a special tree that helps us find prefixes quickly. This method is faster than checking each word one by one.

Implementation Steps:

1. Trie Data Structure: We build a TrieNode structure. It has children nodes and a boolean to show if a word ends here.
2. Insert Function: We make a function to add words to the trie.
3. Count Prefixes Function: We go through the input string and count prefixes that are in the trie.

C++ Code Implementation:

#include <iostream>
#include <vector>
#include <unordered_map>
using namespace std;

class TrieNode {
public:
    unordered_map<char, TrieNode*> children;
    bool isEndOfWord;

    TrieNode() : isEndOfWord(false) {}
};

class Trie {
private:
    TrieNode* root;

public:
    Trie() {
        root = new TrieNode();
    }

    void insert(const string& word) {
        TrieNode* node = root;
        for (char ch : word) {
            if (!node->children.count(ch)) {
                node->children[ch] = new TrieNode();
            }
            node = node->children[ch];
        }
        node->isEndOfWord = true;
    }

    int countPrefixes(const string& s) {
        TrieNode* node = root;
        int count = 0;
        for (char ch : s) {
            if (node->children.count(ch)) {
                node = node->children[ch];
                if (node->isEndOfWord) {
                    count++;
                }
            } else {
                break;
            }
        }
        return count;
    }
};

int countPrefixes(vector<string>& words, string s) {
    Trie trie;
    for (const string& word : words) {
        trie.insert(word);
    }
    return trie.countPrefixes(s);
}

// Example Usage
int main() {
    vector<string> words = {"a", "ap", "app", "appl", "apple"};
    string s = "apple";
    cout << "Count of prefixes: " << countPrefixes(words, s) << endl; // Output: 5
    return 0;
}

Explanation of the Code:

• TrieNode Class: This class shows each node in the trie. It has children and a flag to mark the end of a word.
• Trie Class: This class manages the root node. It has methods to insert words and count prefixes.
• countPrefixes Function: This function goes through the string. It counts valid prefixes by checking them against the trie.

This optimized C++ solution counts the prefixes of a string using a trie. It is faster when we deal with many words. For more learning on array problems, you can check Array Two Sum and Array Contains Duplicate.

Frequently Asked Questions

What is the problem statement for counting prefixes of a given string?

The problem of counting prefixes of a string is about finding out how many strings from a list are prefixes of a target string. This task is common in working with strings. We can do this in many programming languages like Java, Python, and C++. Knowing this can help us with things like autocomplete features and searching strings.

How can I implement the count prefixes of a given string in Java?

To count prefixes of a string in Java, we can go through the list of strings. We check if each string is a prefix of the target string using the startsWith() method. This method is simple and works well for small lists. For more details, we can look at the section ‘Java Implementation of Count Prefixes of Given String’.

What is an optimized approach to count prefixes in Python?

An optimized way to count prefixes of a string in Python is to use a trie data structure or fast string matching methods. This can make it quicker than just checking each string one by one. We can find a full explanation and example in the ‘Optimized Python Solution for Count Prefixes of Given String’ section.

Can the count prefixes problem be solved in C++?

Yes, we can solve the count prefixes problem in C++. We can use methods like in Java or Python. This includes going through the list and using the compare() function to check prefixes quickly. For a complete solution, we can check the ‘C++ Implementation of Count Prefixes of Given String’.

Where can I find more information about related array problems?

If we want to learn more about array problems, our site has many articles. We can read about topics like Array Two Sum, Array Contains Duplicate, and Array Maximum Subarray. These resources can help us understand array manipulations and algorithms better.