printf doubles with fixed overall length

Question

I would like to printf doubles such that the overall length is always the same and the number is rounded if too long.

For example, with overall length 7:

double a = 1.23456789;
double b = 12.3456789;
double c = 123.456789;
printf("a: %f\n", a);
printf("b: %f\n", b);
printf("c: %f\n", c);

would print:

a: 1.23457
b: 12.3457
c: 123.457

Is there a simple way to achieve this?

Answer 1

Yes, there is. See the specifications for printf conversion specifiers. Use a # to select the alternative form to keep the trailing zeros. Use the g to obtain a representation with a fixed total number of digits. The .6 specifies the exact number of digits.

printf("a: %#.6g\n", a);
printf("b: %#.6g\n", b);
printf("c: %#.6g\n", c);

A precision of 6 digits happens to be the default, so this works as well

printf("a: %#g\n", a);

Answer 2

printf("a: %.6g\nb: %.6g\nc: %.6g\n", a, b, c);

See ideone