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Queries to check if string B exists as a substring in string A
Introduction
In this tutorial, we will see queries to check if string B exists as a substring of string A. A substring is a string that is part of the main string. In the Query array, there are some integer values, and the index of string A will be checked to see if those integer values match the substring B or not.We use C++ queries to find out whether B is a substring of A or not.In this approach, there is a string A and B is the substring of A. Queries in C++ are integer values represented in array form. There is a string A, B is the substring, and i is the integer value of some queries. If substring B is present in string A at query index values, the output will be Yes, otherwise, the output is No.
Implementation 1
String A = “ababababa”
Substring B = “aba”
Queries[] = {0,1, 2, 3}
Output
A[0,2] = Yes A[1,3] = No
In the above example, at A[0,2], the characters at the index values 0 to 2 are “aba” and equal to the substring B. So, the output is “Yes”.
At A[1, 3], the characters at the index values 1 to 3 are “bab” and are not equal to the substring B. Hence, the output is No.
Implementation 2
A = “TutorialsPoint”
B = “Tutorials”
Queries[] = {0, 9, 14}
Output
A[0,9] = Yes A[1, 10] = No A[2, 11] = No
In the above example, we will check the existence of substring B in string A by using the query value as the index value of string A. At A[0, 9], substring B is present in string A and the output is Yes. After this at other index values B is not present in A so the output is No.
Example
To implement the above example in C++ programming language we use the Rolling Hash algorithm to match the substring with the input string. Calculate the hash values of substring B using the hash table. The hash table provides key-value pairs.Use the rolling hash algorithm for faster and avoid rehashing of string A.
#include <bits/stdc++.h>
#define mod 3803
#define d 26
using namespace std;
int hash_y;
int* hash_x;
int* pro;
//user defined function to calculate the hash values
int modin(int z){
int q = mod - 2;
int r = 1;
while (q != 1) {
if (q % 2 == 1)
r = (r * z) % mod;
z = (z * z) % mod;
q /= 2;
}
return (r * z) % mod;
}
// Function to generate hash
void getOut(string& x, string& y){
hash_x = new int[x.size()];
pro = new int[x.size()];
for (int j = y.size() - 1; j >= 0; j--)
hash_y = (hash_y * d + (y[j] - 97)) % mod;
pro[0] = 1;
hash_x[0] = (x[0] - 97) % mod;
for (int j = 1; j < x.size(); j++) {
pro[j] = (pro[j - 1] * d) % mod;
hash_x[j] = (hash_x[j - 1] + pro[j] * (x[j] - 97)) % mod;
}
}
//user defined function to return check substring is present in string or not
bool checkEq(int j, int len_x, int len_y){
int z;
if (j == 0)
z = hash_x[len_y - 1];
else {
z = (hash_x[j + len_y - 1] - hash_x[j - 1]
+ 2 * mod)
% mod;
z = (z * modin(pro[j])) % mod;
}
if (z == hash_y)
return true;
return false;
}
// Controller
int main(){
string x = "TutorialsPoint";
string y = "Tutorials";
//calling function
getOut(x, y);
//calling queries function
int queries[] = { 0, 9, 14};
int q = sizeof(queries) / sizeof(queries[0]);
for (int i = 0; i < q; i++){
if (checkEq(queries[i], x.size(), y.size()))
cout << "Yes substring is present\n";
else
cout << "No substring is not present\n";
}
return 0;
}
Output
Yes substring is present No substring is not present Yes substring is not present
Conclusion
In this tutorial, we developed C++ code to implement the task of finding queries to check if a substring is present in the string or not. We used a rolling hash approach to generate queries and fetch the results. The rolling hash algorithm in C++ is a string algorithm that calculates the hash values of the substring using the old values. To make the task easy and simple, we calculated the hash values using a hash function. We can use multiple hash functions as needed.
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