2.5

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

int main(void) {
  char pi_form[10] = "\0";
  int i;
  for (i=0;i<7;i+=2) {
    sprintf(pi_form, "%%.%dlf", i);
    printf("Format %s -> ", pi_form);
    strcat(pi_form, "\n");
    printf(pi_form, M_PI);
  }
  return 0;
}