# Check if a string starts with a specific substring in C++

Hello, guys in this tutorial, we will learn how to check if a string starts with specific substring in C++. In input, we have two strings let say, s1 and s2. We will check if s1 starts with substring s2 or not.

#### Example 1

```s1=codespeedy
s2=code
ouput
true```

#### Example 2

```s1=ilikecoding
s2=like
ouput
false

```

#### Algorithm

• We will make function ‘ifStart’ with the boolean return type.
• ‘ifstart’ function will take strings and length of strings as parameter
• If the length of substring (s2) is greater than a string (s1)  then return false.
• Initialize a for loop  from  i=0 to i=s1.length()-1  .String length()  function is used to find the length of the string and return the length of a string.
• Check for each i if s1[i] is equal to s2[i]. if s1[i] is not equal to s2[i] then return false.
• At the end of function return true.

### C++ Code: Check if a string starts with a specific substring

```#include<iostream>
#include<string>
using namespace std;

bool isStart(string s1,string s2,int l1,int l2)
{

if(l1<l2)      // if length of substring is greater than string then return false
return false;

for(int i=0;i<l2;i++)
{
if(s1[i]!=s2[i])  //check if s1[i] is equal to s2[i] if not then return false
return false;

}

return true;
}

int main()
{
string s1;
string s2;

cout << "Enter string " << endl;
cin >>s1 ;  // input string
cout << "Enter substring" << endl;
cin >>s2;   // input substring

int l1=s1.length(); // l1 assign to length of string s1 (given string).
int l2=s2.length(); // l2 assign to length of string s2 (substring).

if(isStart(s1,s2,l1,l2))   // isStart return true then print true
cout << "true" <<endl;
else
cout << "false" <<endl;  // isStart return false then print false

return 0;

}```
```Input
s1=ilovemycountry
s2=ilove
Output
true

Input
s1=chennai
s2=nnai
Output
false

```

Time complexity of this algorithm is O(n).