how could we merge a string into another string within specified position??

Answer 1

When you say "merge" do you mean to append (at the end) or to copy?
If you mean to copy you can try strncpy()
http://www.cplusplus.com/reference/cstring/strncpy/

If you mean to append, then use strncat()
http://www.cplusplus.com/reference/cstring/strncat/

An example using strncpy() function:

#include <stdio.h>
#include <string.h>

int main() {
char str1[25] = "SoloStudy";
int length = (int)strlen(str1);

printf("Before: %s (%d characters)\n", str1, length);
// Copy the string "Learn" into str1
// starting from offset of str1 + 4
// 5 characters will be copied over.
strncpy(str1 + 4, "Learn", 5);
length = (int)strlen(str1);
printf("After: %s (%d characters)\n", str1, length);

return 0;
}

Hth, cmiiw

Answer 2

but this truncated the study part
i am asking
let "hel there " be the string
and we want to add "lo" after 3rd character such that string becomes"hello there"