String Concatenation

strcat(first_string,second_string);

#include<stdio.h>
int main()
{
	 char s1[100];
	char s2[100];
	puts("Enter s1");
	gets(s1);
	puts("Enter s2");
	gets(s2);
	strcat(s1,s2);
	puts("After concatenation s1 contains");
	puts(s1);
	return 0;
}

Enter s1
sjit
Enter s2
students
After concatenation s1 contains
Sjitstudents

#include<stdio.h>
int main()
{
	 char s1[100];
	char s2[100];
	int i=0,j=0;
	puts("Enter s1");
	gets(s1);
	puts("Enter s2");
	gets(s2);
	while(s1[i] != '\0')
	{
		i++;
	}
	while(s2[j] !='\0')
	{
		s1[i] = s2[j];
		i++;
		j++;
	}
	s1[i] = '\0';
	puts("After concatenation s1 contains");
	puts(s1);
	return 0;
}

Enter s1
sjit
Enter s2
students
After concatenation s1 contains
sjitstudents