Frequency of a substring in a string | Set 2 - GeeksforGeeks

30 Oct, 2020
Given a string str of length N and a substring pattern of length M, the task is to find the frequency of occurrences of pattern as a substring in the given string. If pattern is present in the string str, then print “Yes” with the count of its occurrence. Otherwise, print “No”.
Examples:
Input: str = “geeksforgeeks”, pattern = “geeks”Output: 2Explanation:The occurrence of the string “geeks” in the string “geeksforgeeks” is at index 0 and 8.Therefore, the count is 2.
Input: str = “dhimanman”, pattern = “max”Output: 0
Naive Approach: Refer to the previous post for the simplest approach to solve the problem.Time Complexity: O(N*M)Auxiliary Space: O(1)
Approach using KMP Algorithm: Refer to the previous post of this article to solve the problem using KMP algorithm.Time Complexity: O(N + M)Auxiliary Space: O(M)
Approach using Regular Expression: Follow the steps below to solve the problem:
Form the regular expression of the string pattern using regex() function.
Create a smatch M using function smatch().
Check the presence of the string pattern in the string str using function regex_match() as:
regex_search(str, m, c)where,str is the given string,m is smatch,c is the regular expression of the string pattern.
In the above steps, if the function regex_match() returns True, then print “Yes” and find the occurrence of the string pattern. Otherwise, print “No”.
Create a variable numberOfMatches of data type ptrdiff_t to store the count of occurrence.
Find the numberOfMatches using function regex_iterator() function as:
ptrdiff_t numberOfMatches = std::distance(sregex_iterator(S.begin(), S.end(), c), sregex_iterator())
Print the count of occurrence in the above steps as the result.
Below is the implementation of the above approach:
C++
// C++ program for the above approach  #include <bits/stdc++.h>using namespace std;  // Function to find the frequency of// substring in the given string Svoid find_frequency(string S,                    string pattern){    // Create a regular expression    // of the string pattern    regex c(pattern);      // Determines the matching behavior    smatch m;      // Use member function on 'm'    // regex_search to check if    // string X is present in S or not    if (regex_search(S, m, c) == true) {        cout << "Yes"             << "\n";    }    else {        cout << "No";    }      // Count the number of matches    ptrdiff_t numberOfMatches        = std::distance(            sregex_iterator(S.begin(),                            S.end(), c),            sregex_iterator());      // Print the coun of occurrence    cout << "Frequency of string "         << pattern << " is "         << numberOfMatches;}// Driver codeint32_t main(){    // Given string str and pattern    string str = "geeksforgeeks";    string pattern = "geeks";      // Function Call    find_frequency(str, pattern);      return 0;}
Yes
Frequency of string geeks is 2

Time Complexity: O(N + M)Auxiliary Space: O(M)
frequency-counting
regular-expression
strings
substring
Pattern Searching
Searching
Strings
Searching
Strings
Pattern Searching
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