c++ how to get “one digit exponent” with printf

Question

Is there a way to print in scientific notation less than 3 places for exponent part of number? The 6.1 formatting doesn\'t affect exponent but only the number part:

Answer 1

C/C++ specifies at least two exponent digits with printf("%e",...). To print only 1, and to deal with Visual Studio which, by default, prints at least 3, additional code is needed.

Consider IOStreams @Dietmar Kühl

If C++ code still wants to use printf() style formats:

Adjusting the value of a double before calling printf() too often results in rounding issues, range shorting and general corner case failures like dealing with log10(0.0). Also consider large double just near a power-of-10 where log10() may come up short, -0.0, INF, NAN.

In this case, better to post-process the string.

  double var = 1.23e-9;
  //                    - 1 . x e - EEEEE \0
  #define ExpectedSize (1+1+1+1+1+1+  5 + 1)
  char buf[ExpectedSize + 10];
  snprintf(buf, sizeof buf, "%.1e", var);
  char *e = strchr(buf, 'e');  // lucky 'e' not in "Infinity" nor "NaN"
  if (e) {
    e++;
    int expo = atoi(e);
    snprintf(e, sizeof buf - (e - buf), "%1d", expo);
  }
  printf("'%6s'\n", buf);  // '1.2e-9'

Note: %e is amiable to post-processing as its width is not so unwieldy as "%f". sprintf(buf, "%f", DBL_MAX) could be 1000s of char.