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In C++, the strncat() is a string manipulation function defined in the cstring header file, which works on a string which is stored in a c-style char array, which is used to add the first n number of characters of a string to the end of another string.

Signature of strncat() function

char * strncat ( char * destination, const char * source, size_t num )

• This function appends the first num, number of characters of source string at the end of the characters present in the destination string.
• The function returns a char[] array pointer, pointing to the destination string containing the addition of both string values.

Example of strncat() function

In the upcoming code, we are calling strncat() function, to add first 2 characters of one string named arr2 at the end of characters present in another string named arr1.

//  Program to append n number of characters of first string at the end of another string.

#include<iostream>
#include<cstring>

using namespace std;

int main()
{
char arr1[10] = "Have a ";
char arr2[10] = "good day!";

cout<< "The content of first string is : " << arr1 << "\n";
cout<< "The content of second string is : " << arr2 << "\n";

strncat(arr1,arr2,2); // only first two characters from arr2 will be appended to the end of arr1 

cout<< "The addition of two string is : " << arr1 << "\n";
cout<< "The content of second string : " << arr2;

return 0;
}

Output is -

The content of first string is : Have a
The content of second string is : good day!
The addition of two string in the first string : Have a go
The content of second string : good day!

• The first char[] array i.e. arr1 is initialized with a string value Have a and the second char[] array i.e. arr2 is initialized with the value good day!.
• The function strncat() appends first 2 characters of one string value in the arr1 char[] array at the end of characters present in the second char[] array i.e. arr2.

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Adding two strings without using strncat()

Let's create a program appends the first n number of characters of one string at the end of another string without using strncat() function.

// Program to append n, number of characters of one string at the end of another string without strcat() function 


#include<iostream>
#include<cstring>

using namespace std;


int main()
{
char dest[20] = "Hello";
char src[10] = "World";

cout<< "The value in dest string : " << dest << "\n";

cout<< "The second in src string : " << src << "\n";

int len1 = strlen(dest);
int len2 = strlen(src);

cout<< "Length of characters in dest string : " << len1 << "\n";

int num; 
cout<< "How many characters of the first string you to append : ";
cin>>num;

for(int i=0;i<num; i++)
{
	dest[len1] = src[i];
	len1 = len1 + 1 ;
}
dest[len1]='\0';

cout<< "Concatenated strings : " << dest;

return 0;
}

Output is :

The value in dest string : Hello
The second in src string : World
Length of characters in dest string : 5
Length of characters in src string : 5
How many characters of the first string you to append : 5
Concatenated strings : HelloWorld

• The first char[] array i.e. dest is initialized with a string value Hello and the second char[] array i.e. dest is initialized with the value World.
• Next, we have calculated the total length of characters actually present in the first char[] array, by using strlen() function.
• Next, we ask the user to enter the number of characters of the first string, which we manually append to the end of dest char[].

Note: - The function strlen returns the total number of characters actually present in the char[] and not the total number of character it can hold.

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