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  lt stdio h gt   int main        double d   1 4     printf  quot  lf quot   d      Is this wrong

User · Answer

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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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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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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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

User · Answer

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

[c] Correct format specifier for double in printf

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