How to Write a code to implement strstr function in C

Strstr is the inbuilt function given in C language. This function is used to find the substring in other string. Before to write our own code to implement the strstr function in C, I briefly introduce you inbuilt strstr function with its syntax.

Syntax Declaration:

int strstr(char*, char*);

This function is very easy to use in c language. It has two arguments as char strings. First string contents base string in which we want to search sub-string. The Second argument is sub-string that need to be searched in base string.

If it founds substring in the base string, it returns 1. Otherwise, it returns 0.

Now here, instead of using an inbuilt strstr function, we want to implement a strstr function in C. This function will work same like as inbuilt strstr() function.

I have been asked this question in IBM placement interview on campus.

Code to implement strstr function in C

#include<stdio.h>

int fStrStr(char* str, char* strSub)
{
	int i=0, j=0;
	int nTemp = i;
	int nStrLen = strlen(str);
	int nStrSubLen = strlen(strSub);
	for(i=0; i<nStrLen-nStrSubLen; i++)
	{
		nTemp = i;
		for(j=0; j<nStrSubLen; j++)
		{
			
			if(str[nTemp]==strSub[j])
			{
				if(j==nStrSubLen-1)
					return 1;
				nTemp++;
			}
			else 
				break;
		}
	}
	return 0;
}


int main()
{
	char str[] = "CSEStack";
	char strSub[] = "SES";

	if(fStrStr(str, strSub))
		printf("Sub-string found.");
	else
		printf("Sub-string not found.");

		
}

Outout

Case 1:

char str[] = "CSEStack";
char strSub[] = "SES";
=>Sub-string found.

Case 2:

char str[] = "CSEStack";
char strSub[] = "SDS";
=>Sub-string not found.

Note: Here, I am using fstrstr() function that will work as inbuild function strstr(). Even you can use the same function name as strstr(). If there is inbuilt as well as user-defined function with the same name, the user-defined function will get the call.

Time Complexity: O(n^2)

As in the worst case, we are comparing every letter in the base string with every letter in the substring, it’s complexity is O(n^2).

Difficulty Level: Medium