How do you build strings concatenated with int variables in C?

Question

String manipulation in C is not something I'm used to, coming from higher level languages. In this situation, I want to be able to make a string of the form fooN where N is a variable representing a number. In Java, it would look something like

for (int N = 0; N < 5; n++) {
    System.out.println("foo"+N);
}

which will output

foo0
foo1
foo2
foo3
foo4

However, I don't know a straighforward way to do that in C. I could probably hack something out where I make a char[] and figure out how to fill it up, but what I have in mind doesn't seem elegant. Is there a simple way that the C language allows concatenating strings and int variables?

Thanks

Answer 1

Is there a simple way that the C language allows concatenating strings and int variables?

Yes, use s*printf(). The trick is memory management.

1. Use a fixed sized buffer.

   // Coarse calculation of maximum memory needed to string-ify an int
   #define INT_STRING_MAX (sizeof(int)*CHAR_BIT/3 + 3)
   #define FOO_SZ (3 + INT_STRING_MAX)
   
   char buf[FOO_SZ];
   sprintf(buf, "foo%d", n);
   println(buf);
   

2. Allocate memory. Could use snprintf(NULL, 0, ... to calculate memory needs.

   int sz = snprintf(NULL, 0, "foo%d", n);
   char *buf = malloc(sz + 1);
   if (buf) {
     sprintf(buf, "foo%d", n);
     println(buf);
     free(buf);
   }
   

3. Estimate and check memory needs with snprintf().

   // Assume 64-bit or narrower int and so needs at most 21 bytes
   #define FOO_SZ (3 + 21)
   
   char buf[FOO_SZ];
   int cnt = snprintf(buf, sizeof buf, "foo%d", n);
   if (cnt >= 0 && (unsigned) cnt < sizeof buf) {
     println(buf);
   } else {
     // wrong estimate
   }
   

The best choice depends on how simple, efficient & portable you want.