What is the correct format specifier for double
in printf? Is it %f
or is it %lf
? I believe it's %f
, but I am not sure.
#include <stdio.h>
int main()
{
double d = 1.4;
printf("%lf", d); // Is this wrong?
}
This question is related to
c
floating-point
printf
double
format-specifiers
Format %lf
is a perfectly correct printf
format for double
, exactly as you used it. There's nothing wrong with your code.
Format %lf
in printf
was not supported in old (pre-C99) versions of C language, which created superficial "inconsistency" between format specifiers for double
in printf
and scanf
. That superficial inconsistency has been fixed in C99.
You are not required to use %lf
with double
in printf
. You can use %f
as well, if you so prefer (%lf
and %f
are equivalent in printf
). But in modern C it makes perfect sense to prefer to use %f
with float
, %lf
with double
and %Lf
with long double
, consistently in both printf
and scanf
.
Given the C99 standard (namely, the N1256 draft), the rules depend on the function kind: fprintf (printf, sprintf, ...) or scanf.
Here are relevant parts extracted:
Foreword
This second edition cancels and replaces the first edition, ISO/IEC 9899:1990, as amended and corrected by ISO/IEC 9899/COR1:1994, ISO/IEC 9899/AMD1:1995, and ISO/IEC 9899/COR2:1996. Major changes from the previous edition include:
%lf
conversion specifier allowed inprintf
7.19.6.1 The
fprintf
function7 The length modifiers and their meanings are:
l (ell) Specifies that (...) has no effect on a following a, A, e, E, f, F, g, or G conversion specifier.
L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a long double argument.
The same rules specified for fprintf
apply for printf
, sprintf
and similar functions.
7.19.6.2 The
fscanf
function11 The length modifiers and their meanings are:
l (ell) Specifies that (...) that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to double;
L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to long double.
12 The conversion specifiers and their meanings are: a,e,f,g Matches an optionally signed floating-point number, (...)
14 The conversion specifiers A, E, F, G, and X are also valid and behave the same as, respectively, a, e, f, g, and x.
The long story short, for fprintf
the following specifiers and corresponding types are specified:
%f
-> double%Lf
-> long double.and for fscanf
it is:
%f
-> float%lf
-> double%Lf
-> long double.%Lf
(note the capital L
) is the format specifier for long doubles.
For plain doubles
, either %e
, %E
, %f
, %g
or %G
will do.
It can be %f
, %g
or %e
depending on how you want the number to be formatted. See here for more details. The l
modifier is required in scanf
with double
, but not in printf
.
Source: Stackoverflow.com