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?
}

Tetrabasic

If you're stuck with a C89 library, "%lf" is undefined; in C99 and C11 libraries it is defined to be the same as "%f". – Goer

Your variant is as correct as it ever gets. %lf is the correct format specifier for double. But it became so in C99. Before that one had to use %f. – Uptodate

Answer 1

"%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.}

Diskin

Note that g++ rejects %lf when compiling with -Wall -Werror -pedantic: error: ISO C++ does not support the ‘%lf’ gnu_printf format – Regan

@kynan: If so (at leas assuming a current version of g++), that's a bug in g++. For C89/90 and C++98/03, allowing l was an extension. The C99/11 and C++11 standards require the implementation to allow it. – Diskin

Seems only g++ 4.7 chokes on it, it works on 4.6. It's triggered by the format warning i.e. -Wno-format is enough to make it compile. – Regan

Curiously, scanf does want doubles represented by %lf: it complains that it expected float * and found double * with just %f. – Sanfo

@JerryCoffin g++ still defaults to g++98 mode – Druid

@EricDand That's because scanf takes pointers to where to store what it reads, so needs to know how big the space being pointed-at is, whereas printf takes the values themselves, and "default argument promotions" mean both end up as doubles, so the l is essentially optional. – Candelaria

A pointer is a type. Specifically, there requires the value of a pointer type, instead of the value of the object being pointed to. – Garret

Answer 2

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 in printf

7.19.6.1 The fprintf function

7 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 function

11 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.

Hosea

Answer 3

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.

Ivers

-1: l (lowercase) modifier is for integer types (cplusplus.com/reference/clibrary/cstdio/printf), and L is for floating point types. Additionally, the L modifier expects a long double, not a plain double. – Terrilyn

user470379: So where is the contradiction with my answer? Haven't I said that l is not required in printf for double. – Ivers

For printf, the length modifier l is ignored in the case of floating point types because float arguments are always promoted to double. Jerry Coffin's answer has more information. – Foot

Answer 4

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.

Uptodate

With scanf(), "%f", "%lf" match a float *, double *, not float, double as implied by the last line. – Brickle

Answer 5

%Lf (note the capital L) is the format specifier for long doubles.

For plain doubles, either %e, %E, %f, %g or %G will do.

Hubbub

What's the difference between %g and %G ? – Faires

@yanpas, lowercase / uppercase for the exponent symbol, respectively. – Inelegancy

sorry, %g and %G do output E symbol. Also they output INF and inf in different cases – Faires