Question

How can I make this print properly without using two printf calls?

char* second = "Second%d";
printf("First%d"second,1,2);

Answer 1

The code you showed us is syntactically invalid, but I presume you want to do something that has the same effect as:

printf("First%dSecond%d", 1, 2);

As you know, the first argument to printf is the format string. It doesn’t have to be a literal; you can build it any way you like.

Here’s an example:

#include <stdio.h>
#include <string.h>
int main(void)
{
    char *second = "Second%d";
    char format[100];
    strcpy(format, "First%d");
    strcat(format, second);

    printf(format, 1, 2);

    putchar('\n');
    return 0;
}

Some notes:

I’ve added a newline after the output. Output text should (almost) always be terminated by a newline.

I’ve set an arbitrary size of 100 bytes for the format string. More generally, you could declare

char *format;

and initialize it with a call to malloc(), allocating the size you actually need (and checking that malloc() didn’t signal failure by returning a null pointer); you’d then want to call free(format); after you’re done with it.

As templatetypedef says in a comment, this kind of thing can be potentially dangerous if the format string comes from an uncontrolled source.

(Or you could just call printf twice; it’s not that much more expensive than calling it once.)