A string is a sequence of characters. The characters can be the alphabet, symbols, number, alphanumeric, or a combination of all. In this article, I will show you how to **check substring for a given circular string**. Whether a string is present in the given circular string. *Sometimes we tend to think of so complex algorithm for a simple given problem but later we realize it and can be solved easily.* So it is normal if you are also facing a similar issue. I will try to solve this problem in a very simple way.

## What is a circular string?

Basically,* a circular string is a string that does not have a start and endpoint*. For example, let a string be **“Bhutan”** then its circular string will be **“B->h->u->t->a->n->B”**. I have written the letter** ‘B’** at the end of the string mainly to show that after the letter **‘n’** it will be connected to the letter **‘B’.**

Let’s show the possible substring for a given circular string. If a circular string is **“code”**, given below are some of the possible substrings:

- deco, co, de, ode, dec, eco, etc…

## Algorithm to check whether a substring is present in the given circular string.

For a string to be circular, it can be represented using a sequence of characters in a given string. For example, if a string is *“code”*, it can be represented as *“codecod”*, because the longest substring that can be formed using string *“code”* is *“ecod”* as the same length of the given string “code”. In this new string, we will iterate over and see if the given substring can be formed.

### Pseudocode to check substring for a given circular string.

For writing the code, let’s define what are the input and output.

**Input:** string x and substring y.

**Output:** display -1 if y is not present in x else the index of the first letter substring y in the circular string x.

```
//String x for an input string, n for the size of x.
//String y is substring to check, m for the size of y
Algorithm isSubstring(x, y, n, m){
for i=0 to n-2 do
x = x + x[i]
j=0
//size is length of the new updated string x
for i=0 to size-1 do
if j == m then
break
else if x[i] == y[j] then
j++
else then
j=0
if j < m then
return -1
else then
return i - j
}
```

### Source code to check substring for the given circular string

The source code given below is written in c++ programming language. The same logic can be implemented using any choice of your programming language.

```
#include<iostream>
using namespace std;
int isSubstring(string x, string y, int n, int m){
for(int i=0; i<n-1; i++){
x.push_back(x[i]);
}
int j = 0, size = x.length(), i;
for(i=0; i<size; i++){
if(j == m){
break;
}else if(x[i] == y[j]){
j++;
}else{
j = 0;
}
}
if(j < m){
return -1;
}else{
return (i - j);
}
}
int main(){
string x, y;
getline(cin, x);
getline(cin, y);
cout << isSubstring(x,y, x.length(), y.length()) << endl;
return 0;
}
```

### Run time complexity of the algorithm

If the size of the string is** ‘n’** then the circular substring representation will be** n+(n-1)**. This is approximately 2 times n(i.e, 2n). So the run time is linear.

Therefore the run time to check the substring for the given circular string is **O(n)**.

## Conclusion

When we solve the programming question, there will be many ways to solve it. Sometimes we won’t be able to find a simple way to solve it. The reason can be many such as time constraints to solve it. It is okay, given sufficient time to think, you can always find a simple and better way to solve it. Keep practicing to solve more questions, it will improve your analytical thinking, problem-solving, and programming skills. *If you like the article, do share, comment, and follow this website.*

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