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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? }
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  • 22
    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". Commented Apr 29, 2012 at 0:06
  • 2
    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. Commented May 7, 2016 at 19:59
  • The full list of the specifiers is here: stackoverflow.com/questions/79601134/… Commented Jul 2 at 3:21

5 Answers 5

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810

"%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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7 Comments

Note that g++ rejects %lf when compiling with -Wall -Werror -pedantic: error: ISO C++ does not support the ‘%lf’ gnu_printf format
@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.
Curiously, scanf does want doubles represented by %lf: it complains that it expected float * and found double * with just %f.
@JerryCoffin g++ still defaults to g++98 mode
@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.
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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:

  • %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.
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Comments

37

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.

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3 Comments

-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.
user470379: So where is the contradiction with my answer? Haven't I said that l is not required in printf for double.
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.
20

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.

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1 Comment

With scanf(), "%f", "%lf" match a float *, double *, not float, double as implied by the last line.
13

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

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

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3 Comments

What's the difference between %g and %G ?
@yanpas, lowercase / uppercase for the exponent symbol, respectively.
sorry, %g and %G do output E symbol. Also they output INF and inf in different cases

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