#include <stdio.h>
int main()
{
char s1[100], s2[100], i, j;
printf("Enter first string: ");
scanf("%s", s1);
printf("Enter second string: ");
scanf("%s", s2);
// calculate the length of string s1
// and store it in i
for(i = 0; s1[i] != '\0'; ++i);
for(j = 0; s2[j] != '\0'; ++j, ++i)
{
s1[i] = s2[j];
}
s1[i] = '\0';
printf("After concatenation: %s", s1);
return 0;
}

Output

Enter first string: lol
Enter second string: :)
After concatenation: lol:)

C program to concatenate two strings:

#include <stdio.h>

int main() {
    char str1[100], str2[100], result[200];
    int i, j;

    printf("Enter the first string:\n");
    gets(str1);

    printf("Enter the second string:\n");
    gets(str2);

    i = 0;
    while (str1[i] != '\0') {
        result[i] = str1[i];
        i++;
    }

    j = 0;
    while (str2[j] != '\0') {
        result[i] = str2[j];
        i++;
        j++;
    }

    result[i] = '\0';

    printf("Concatenated string: %s\n", result);

    return 0;
}

Explanation:

• #include <stdio.h> includes the standard input-output header file in our program.
• char str1[100], str2[100], result[200]; declares three-character arrays str1, str2 and result to store the two input strings and the concatenated string respectively.
• int i, j; declares two integer variables i and j.
• The printf() statements are used to prompt the user to input the two strings and the gets() the function is used to input the strings.
• The first while loop starting from while (str1[i] != '\0') copies each character of the first string str1 to the result string result until the end of the string is reached.
• The second while loop starting from while (str2[j] != '\0') copies each character of the second string str2 to the result string result starting from the position after the end of the first string until the end of the second string is reached.
• The statement result[i] = '\0'; adds the null character at the end of the concatenated string to indicate the end of the string.
• The last printf() the statement displays the concatenated string.

When we compile and run this program, it will prompt the user to input two strings and then concatenate them. Finally, it will display the concatenated string.