How to use a variable format in `sprintf`

Question

How should I make a variable format sequence for sprintf?.

int i=2,d=9;
String sf;
sprintf(sf,"%%0%d",i);
fprintf(sf,d); 

While expecting the output.

09

The compilation fails, with several errors, according the failing change I apply.

Invalid cast from type 'String' to type 'char*'

Answer 1

1. sprintf cannot be used to write data into String objects. More precisely, it is possible to come up with a "hackish" way to write the result directly into a String object, but the latter is definitely not designed for such use.

   The target buffer should be an array of char, not a String. Which is what the compiler is telling you.

   char buffer[64];
   sprintf(buffer, "%%0%d", i);
   

   or better

   char buffer[64];
   snprintf(buffer, sizeof buffer, "%%0%d", i);
   

2. The format string you used in your sprintf will generate %02 result (since i contains 2 at that moment). Why you are expecting 09 is not clear. Why are you expecting the % character to disappear is not clear either, considering that the format string seems to be deliberately designed to include it.

3. A String object cannot be used as the first parameter of fprintf. It is not clear what that fprintf is doing in your code.

---

Apparently you are trying to use sprintf to generate another format string at run time (using i as field width), which is later used to output the d variable. In that case that would be something like

char format[16];
snprintf(format, sizeof format, "%%0%dd", i);
fprintf(file, format, d);

That fprintf will indeed output 09.