Counting the number of occurrences of a string within a string - c++

Question

What's the best way of counting all the occurrences of a substring inside a string?
Example: counting the occurrences of Foo inside FooBarFooBarFoo

Answer 1

One way to do is to use std::string find function:
#include <string>
#include <iostream>
int main()
{
int occurrences = 0;
std::string::size_type pos = 0;
std::string s = "FooBarFooBarFoo";
std::string target = "Foo";
while ((pos = s.find(target, pos )) != std::string::npos) {
++ occurrences;
pos += target.length();
}
std::cout << occurrences << std::endl;
}

Answer 2

#include <iostream>
#include <string>
// returns count of non-overlapping occurrences of 'sub' in 'str'
int countSubstring(const std::string& str, const std::string& sub)
{
if (sub.length() == 0) return 0;
int count = 0;
for (size_t offset = str.find(sub); offset != std::string::npos;
offset = str.find(sub, offset + sub.length()))
{
++count;
}
return count;
}
int main()
{
std::cout << countSubstring("FooBarFooBarFoo", "Foo") << '\n';
return 0;
}

Answer 3

You should use KMP Algorithm for this.
It solves it in O(M+N) time where M and N are the lengths of the two strings.
For more info-
https://www.geeksforgeeks.org/frequency-substring-string/
So what KMP Algorithm does is, it search for string pattern. When a pattern has a sub-pattern appears more than one in the sub-pattern, it uses that property to improve the time complexity, also for in the worst case.
The time complexity of KMP is O(n).
Check this out for detailed algorithm:
https://www.geeksforgeeks.org/kmp-algorithm-for-pattern-searching/

Answer 4

#include <iostream>
#include<string>
using namespace std;
int frequency_Substr(string str,string substr)
{
int count=0;
for (int i = 0; i <str.size()-1; i++)
{
int m = 0;
int n = i;
for (int j = 0; j < substr.size(); j++)
{
if (str[n] == substr[j])
{
m++;
}
n++;
}
if (m == substr.size())
{
count++;
}
}
cout << "total number of time substring occur in string is " << count << endl;
return count;
}
int main()
{
string x, y;
cout << "enter string" << endl;
cin >> x;
cout << "enter substring" << endl;
cin >> y;
frequency_Substr(x, y);
return 0;
}

Answer 5

#include<iostream>
#include<string>
using namespace std;
int main()
{
string s1,s2;
int i=0;
cout<<"enter the string"<<endl;
getline(cin,s1);
cout<<"enter the substring"<<endl;
cin>>s2;
int count=0;
string::iterator it=s1.begin();
while(it!=s1.end())
{
if(*it==s2[0])
{
int x =s1.find(s2);
string subs=s1.substr(x,s2.size());
if(s2==subs)
count++;
}
++it;
}
cout<<count<<endl;
return 0;
}