How to use C/C++ format specifiers to achieve the same as "g" but with "1." instead of "1.0"

I'd like to print floats via printf as follows:

• (1) right-align into a string of given width (here 8)
• (2) show relevant decimal digits if possible, but no unneccesary trailing 0s after .
• (3) also do this for rounded values, i.e. format 1.0 as "1."

With g I cannot achieve (3), with f I cannot achieve (2,3).

From the docs it seems that # would do the trick for (3)

> Used with a, A, e, E, f, F, g or G it forces the written output to contain a decimal point even if no more digits follow. By default, if no digits follow, no decimal point is written.

So #g can achieve (3), but it unfortunately does more than is written there: it also breaks the feature (2) by removing also relevant decimal digits (see below).

Here are some examples I tried, the last line shows what I am looking for:

Can somebody help how to achieve the wanted output in the last line?

Suggestions:

Modify goal

Use "%8g" as that is standard and closest to OP's goal.

Post process output

"#" in "%g" modifies 2 things: a '.' is always printed and trailing zeroes are not discarded. OP seems to want just the first feature.

Use "%#*.*g", n, n-1, ... and post-process the string.

This is tricky and deserves extensive testing.

Do not

• Pre-process the float before printing. Edge cases will catch you.

• Add text in post processing. Edge cases will catch you.

• "%e" does not provide non-exponential form.

• "%f" can make for long output with values like -FLT_MAX.

Sample code. Simplifications exists.

#include <stdio.h>
int main() {
  float val[] = {1.0, -1.0, 1.9f, -999.1f, 1.000001f};
  size_t v_n = sizeof val / sizeof val[0];
  puts("????        1.             -1.             1.9          -999.1        1.000001");
  fputs("           ", stdout);
  for (size_t v_i = 0; v_i < v_n; v_i++) {
    double v = val[v_i];
    char s[100];
    int n = 8;
    snprintf(s, sizeof s, "%#*.*g", n, n - 1, v);
    char *dot = strchr(s, '.');
    if (dot) {
      char *begin = dot + 1;
      begin = begin + strspn(begin, "0123456789");
      char *end = begin;
      while (begin[-1] == '0')
        begin--;
      if (begin < end) {
        //printf("xx: <%s> <%s>\n", begin, end);
        size_t len = strlen(end);
        memmove(begin, end, len + 1);
      }
    }
    printf(" %-13s", s);
  }
}

Output

????        1.             -1.             1.9          -999.1        1.000001
           1.           -1.          1.9          -999.1       1.000001     

Some test for for anyone to use.
It may help this investigation.

#include <stdio.h>
int main() {
  float val[] = {1.0, -1.0, 1.9f, -999.1f, 1.000001f};
  size_t v_n = sizeof val / sizeof val[0];
  const char *format[] = {"%8.2g", "%8.g", "%8g", "%#8.2g", "%#8.g", "%#8g",
      "%8.2f", "%8.f", "%8f", "%#8.2f", "%#8.f", "%#8f", };
  size_t f_n = sizeof format / sizeof format[0];
  for (size_t f_i = 0; f_i < f_n; f_i++) {
    char f[100];
    snprintf(f, sizeof f, "<%s>", format[f_i]);
    printf("%-13s, ", f);
    for (size_t v_i = 0; v_i < v_n; v_i++) {
      double v = val[v_i];
      char s[100];
      snprintf(s, sizeof s, f, v);
      printf("%-13s", s);
    }
    puts("");
  }
}

Output

<%8.2g>      , <       1>   <      -1>   <     1.9>   <  -1e+03>   <       1>   
<%8.g>       , <       1>   <      -1>   <       2>   <  -1e+03>   <       1>   
<%8g>        , <       1>   <      -1>   <     1.9>   <  -999.1>   <       1>   
<%#8.2g>     , <     1.0>   <    -1.0>   <     1.9>   <-1.0e+03>   <     1.0>   
<%#8.g>      , <      1.>   <     -1.>   <      2.>   < -1.e+03>   <      1.>   
<%#8g>       , < 1.00000>   <-1.00000>   < 1.90000>   <-999.100>   < 1.00000>   
<%8.2f>      , <    1.00>   <   -1.00>   <    1.90>   < -999.10>   <    1.00>   
<%8.f>       , <       1>   <      -1>   <       2>   <    -999>   <       1>   
<%8f>        , <1.000000>   <-1.000000>  <1.900000>   <-999.099976><1.000001>   
<%#8.2f>     , <    1.00>   <   -1.00>   <    1.90>   < -999.10>   <    1.00>   
<%#8.f>      , <      1.>   <     -1.>   <      2.>   <   -999.>   <      1.>   
<%#8f>       , <1.000000>   <-1.000000>  <1.900000>   <-999.099976><1.000001>   

You cannot obtain the desired output with printf alone, but you can use snprintf and adjust the output this way:

int format_number(double v) {
    char buf[500];
    int len = snprintf(buf, sizeof buf, "%f", v);
    while (len > 0 && buf[len - 1] = '0')
        buf[--len] = '
int format_number(double v) {
char buf[500];
int len = snprintf(buf, sizeof buf, "%f", v);
while (len > 0 && buf[len - 1] = '0')
buf[--len] = '\0';
return printf("%8s", buf);
}
';
    return printf("%8s", buf);
}