Correct format specifier for double in printf
Correct format specifier for double in printf
Question
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.
Code sample
#include <stdio.h>
int main()
{
double d = 1.4;
printf("%lf", d); // Is this wrong?
}
Accepted Answer
"%f" is the (or at least one) correct format for a double. There is no format for a float, because if you attempt to pass a float to printf, it'll be promoted to double before printf receives it1. "%lf" is also acceptable under the current standard -- the l is specified as having no effect if followed by the f conversion specifier (among others).
Note that this is one place that printf format strings differ substantially from scanf (and fscanf, etc.) format strings. For output, you're passing a value, which will be promoted from float to double when passed as a variadic parameter. For input you're passing a pointer, which is not promoted, so you have to tell scanf whether you want to read a float or a double, so for scanf, %f means you want to read a float and %lf means you want to read a double (and, for what it's worth, for a long double, you use %Lf for either printf or scanf).
1. C99, §6.5.2.2/6: "If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions." In C++ the wording is somewhat different (e.g., it doesn't use the word "prototype") but the effect is the same: all the variadic parameters undergo default promotions before they're received by the function.
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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:
%lfconversion specifier allowed inprintf7.19.6.1 The
fprintffunction7 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
fscanffunction11 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.
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.
%Lf (note the capital L) is the format specifier for long doubles.
For plain doubles, either %e, %E, %f, %g or %G will do.