C Program to Concatenate Two Strings

In this article, we will learn about how to concatenate two strings with and without using library functions. Let's first start with concatenating strings using a library function.

Concatenate a string in C using the library function

To concatenate or append one string to another string in C programming, you have to ask the user to enter any two strings and concatenate both the strings using the strcat() function, as shown here in the program given below. The function accepts two arguments: strcat(str1, str2). Here, the string value of str2 will be appended at the end of string str1.

#include<stdio.h>
#include<conio.h>
#include<string.h>
int main()
{
    char str1[50], str2[50];
    printf("Enter first string: ");
    gets(str1);
    printf("Enter second string: ");
    gets(str2);
    strcat(str1, str2);
    printf("\nString after concatenation is:\n%s", str1);
    getch();
    return 0;
}

The program was written in the Code::Blocks IDE; therefore, after a successful build and run, you will get the following output:

Now supply the value of the first string, say codes, and press the ENTER key. Then supply the string value for the second string, say cracker, and finally press the ENTER key to see the concatenated string as shown in the snapshot given here:

The value of the second string cracker (in this case) gets copied into the first string codes, and the concatenated string will become fresherearth as shown in the output. If we use the function strcat() as strcat(str2, str1), then the string value of str1 will get copied to str2. Therefore, if we supply codes for str1 and cracker for str2. Then strcat(str2, str1) will output the concatenated string as crackercodes.

Concatenate a string in C without using the strcat() function

Now let's create another program that will also concatenate strings, but this time without using any library functions.

#include<stdio.h>
#include<conio.h>
int main()
{
    char strOne[50], strTwo[50], i, count=0;
    printf("Enter first string: ");
    gets(strOne);
    printf("Enter second string: ");
    gets(strTwo);
    for(i=0; strOne[i]!='\0'; i++)
        count++;
    for(i=0; strTwo[i]!='\0'; i++)
    {
        strOne[count] = strTwo[i];
        count++;
    }
    strOne[count] = '\0';
    printf("\nString after concatenation is:\n%s", strOne);
    getch();
    return 0;
}

The program is created in such a way that the value of the second string (strTwo) will get concatenated into the first string (strOne). And the program will print the value of the concatenated string as output, as shown in the sample run given below:

Program Explained

• Using the gets() function, obtain two string values from the user.
• The first string, say codes, is initialized to strOne, and second string, say cracker, is initialized to strTwo.
• Create a for() loop to get the length of the first string, strOne.
• If the string reaches the null-terminated character ('\0'), it means that there is nothing left inside it.
• After exiting from the for() loop, we will have a count variable that contains the length of the string strOne.
• We have calculated the length of the first string to append the content of the second string into the first from the place where the first string ends.
• Create another for() loop to append the content of the second string, strTwo into the first string, strOne.
• After appending all of strTwo's characters, append a null-terminated character ('\0') to the end of strOne.
• Print the final value of strOne, which will be the concatenated string.

Concatenate two strings into a third

Another program that will read any two strings from the user and concatenate them into a third string without using any library functions is as follows:

#include<stdio.h>
#include<conio.h>
int main()
{
    char strOne[50], strTwo[50], strCon[100], i, count=0;
    printf("Enter first string: ");
    gets(strOne);
    printf("Enter second string: ");
    gets(strTwo);
    for(i=0; strOne[i]!='\0'; i++)
    {
        strCon[i] = strOne[i];
        count++;
    }
    for(i=0; strTwo[i]!='\0'; i++)
    {
        strCon[count] = strTwo[i];
        count++;
    }
    strCon[count] = '\0';
    printf("\nString 1 = %s", strOne);
    printf("\nString 2 = %s", strTwo);
    printf("\nConcatenated String = %s", strCon);
    getch();
    return 0;
}

After a successful build and run, here is the first snapshot of the sample run:

Provide any two strings, say codes and cracker, then press the ENTER key to see the three strings. The first and second strings are the strings the user has provided, and the third string will be the concatenated string. Here is the second snapshot of the sample run:

The same program in different languages

• C++ Concatenate Two Strings
• Java Concatenate Two Strings
• Python Concatenate Two Strings

C Quiz