Why does scanf need lf for doubles when printf is okay with just f

Question

Why is it that scanf   needs the l in   lf  when reading a double  when printf   can use   f  regardless of whether its argument is a double or a float   Example code   double d  scanf   lf    amp d   printf   f   d

User · Accepted Answer

Because C will promote floats to doubles for functions that take variable arguments  Pointers aren t promoted to anything  so you should be using  lf   lg or  le  or  la in C99  to read in doubles

User · Answer

Because otherwise scanf will think you are passing a pointer to a float which is a smaller size than a double  and it will return an incorrect value

User · Answer

Using either a float or a double value in a C expression will result in a value that is a double anyway  so printf can t tell the difference   Whereas a pointer to a double has to be explicitly signalled to scanf as distinct from a pointer to float  because what the pointer points to is what matters

User · Answer

scanf needs to know the size of the data being pointed at by  amp d to fill it properly  whereas variadic functions promote floats to doubles  not entirely sure why   so printf is always getting a double

User · Answer

scanf needs to know the size of the data being pointed at by  amp d to fill it properly  whereas variadic functions promote floats to doubles  not entirely sure why   so printf is always getting a double

User · Answer

Using either a float or a double value in a C expression will result in a value that is a double anyway  so printf can t tell the difference   Whereas a pointer to a double has to be explicitly signalled to scanf as distinct from a pointer to float  because what the pointer points to is what matters

User · Answer

Because otherwise scanf will think you are passing a pointer to a float which is a smaller size than a double  and it will return an incorrect value

User · Answer

Because otherwise scanf will think you are passing a pointer to a float which is a smaller size than a double  and it will return an incorrect value

User · Answer

scanf needs to know the size of the data being pointed at by  amp d to fill it properly  whereas variadic functions promote floats to doubles  not entirely sure why   so printf is always getting a double

User · Answer

Since  99 the matching between format specifiers and floating-point argument types in C is consistent between printf and scanf  It is     f for float  lf for double  Lf for long double   It just so happens that when arguments of type float are passed as variadic parameters  such arguments are implicitly converted to type double  This is the reason why in printf format specifiers  f and  lf are equivalent and interchangeable  In printf you can  cross-use   lf with float or  f with double   But there s no reason to actually do it in practice  Don t use  f to printf arguments of type double  It is a widespread habit born back in C89 90 times  but it is a bad habit  Use  lf in printf for double and keep  f reserved for float arguments

User · Answer

Since  99 the matching between format specifiers and floating-point argument types in C is consistent between printf and scanf  It is     f for float  lf for double  Lf for long double   It just so happens that when arguments of type float are passed as variadic parameters  such arguments are implicitly converted to type double  This is the reason why in printf format specifiers  f and  lf are equivalent and interchangeable  In printf you can  cross-use   lf with float or  f with double   But there s no reason to actually do it in practice  Don t use  f to printf arguments of type double  It is a widespread habit born back in C89 90 times  but it is a bad habit  Use  lf in printf for double and keep  f reserved for float arguments

User · Answer

Because otherwise scanf will think you are passing a pointer to a float which is a smaller size than a double  and it will return an incorrect value

User · Answer

scanf needs to know the size of the data being pointed at by  amp d to fill it properly  whereas variadic functions promote floats to doubles  not entirely sure why   so printf is always getting a double

User · Answer

Using either a float or a double value in a C expression will result in a value that is a double anyway  so printf can t tell the difference   Whereas a pointer to a double has to be explicitly signalled to scanf as distinct from a pointer to float  because what the pointer points to is what matters

User · Answer

Using either a float or a double value in a C expression will result in a value that is a double anyway  so printf can t tell the difference   Whereas a pointer to a double has to be explicitly signalled to scanf as distinct from a pointer to float  because what the pointer points to is what matters

[c] Why does scanf() need "%lf" for doubles, when printf() is okay with just "%f"?

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