C Program to Concatenate Two Strings

Before writing a C program to concatenate two strings. Fist we should have clear understanding that what is a string. A string is a data type in C and it is a collection of characters grouped together. So it is a array of characters. A string declaration is shown as : char stringName [ stringSize ] .

For example : char name[25] ;

In C , compiler put a null ( ‘\0’) character at the end of string. So strings are always null terminated.

Now come to the point how to write C program to concatenate two strings . We can achieve our goals by two methods:

1 ) By using inbuilt C function strcat()

2) Without using strcat()

1 . Program to concatenate Two string without using strcat()

Solution 1:

#include <stdio.h>
int main()
{
 char str1[150];
 char str2[150];
// to hold final ouptput
 char str3[150];
int i = 0, j = 0;

 printf("Enter First string: ");
 scanf("%s",str1);

 printf("Enter Second string: " );
 scanf("%s",str2);

// Inserting first string to str3
 while (str1 [i] != '\0')
 {
    str3[j] = str1[i];
    i++;
    j++;
 }
 
// inserting str2 to str3 

 i = 0;
 while (str2[i] != '\0') 
 {
   str3[j] = str2[i];
   i++;
   j++;
 }
   str3[j] = '\0';
 
// to print final concatenated string
   printf("\nConcatenated string is : %s", str3);
   return 0;
}

Output :

Solution 2:

/******************************************************************************

 Concatenate two strings in C without using strcat function

*******************************************************************************/
#include <stdio.h>
int main()
{
   char s1[100], s2[100], i, j;
   printf("\nEnter first string: ");
   scanf("%s",s1);
   printf("\nEnter second string: ");
   scanf("%s",s2);
   /* count lenght of s1 and store in variable i
    */
   for(i=0; s1[i]!='\0'; ++i); 
 
   /* 
      concatenate string s2 to s1
    */
   for(j=0; s2[j]!='\0'; ++j, ++i)
   {
      s1[i]=s2[j];
   }
   // \0 represents end of string
   s1[i]='\0';
   printf("\nOutput: %s",s1);
   
   return 0;
}

Output :

2 .Program to concatenate Two string using strcat()

Solution :

/******************************************************************************

 Concatenate two strings in C without using strcat function

*******************************************************************************/
#include <stdio.h>
#include <string.h>
int main()
{
   char s1[100], s2[100], i, j;
   printf("\nEnter first string: ");
   scanf("%s",s1);
   
   printf("\nEnter second string: ");
   scanf("%s",s2);
   
   strcat(s1,s2);
    printf("Concatenated String: %s\n", s1);
    return 0;
}

output :