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Scanf/Printf double variable C

Let's say I have this following bit of code in C:

double var; scanf("%lf", &var); printf("%lf", var); printf("%f", var); 

It reads from stdin variable 'var' and then prints twice in stdout 'var'. I understand that's how you read a double variable from stdin, but my questions are:

  1. Why can you print a double with %lf?
  2. Why can you print a double with %f?
  3. Which one is better and correct to use?
like image 689
Dragos Rizescu Avatar asked Nov 13 '13 11:11

Dragos Rizescu


People also ask

Can I use %F for double in c?

We can print the double value using both %f and %lf format specifier because printf treats both float and double are same. So, we can use both %f and %lf to print a double value.

What is %d for double in c?

%d stands for decimal and it expects an argument of type int (or some smaller signed integer type that then gets promoted). Floating-point types float and double both get passed the same way (promoted to double ) and both of them use %f .

How do I scan a double variable?

To read a double, supply scanf with a format string containing the conversion specification %lf (that's a lower case L, not a one), and include a double variable preceded by an ampersand as the second parameter.


2 Answers

For variable argument functions like printf and scanf, the arguments are promoted, for example, any smaller integer types are promoted to int, float is promoted to double.

scanf takes parameters of pointers, so the promotion rule takes no effect. It must use %f for float* and %lf for double*.

printf will never see a float argument, float is always promoted to double. The format specifier is %f. But C99 also says %lf is the same as %f in printf:

C99 §7.19.6.1 The fprintf function

l (ell) Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long int or unsigned long int argument; that a following n conversion specifier applies to a pointer to a long int argument; that a following c conversion specifier applies to a wint_t argument; that a following s conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a, A, e, E, f, F, g, or G conversion specifier.

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Yu Hao Avatar answered Sep 26 '22 21:09

Yu Hao


When a float is passed to printf, it is automatically converted to a double. This is part of the default argument promotions, which apply to functions that have a variable parameter list (containing ...), largely for historical reasons. Therefore, the “natural” specifier for a float, %f, must work with a double argument. So the %f and %lf specifiers for printf are the same; they both take a double value.

When scanf is called, pointers are passed, not direct values. A pointer to float is not converted to a pointer to double (this could not work since the pointed-to object cannot change when you change the pointer type). So, for scanf, the argument for %f must be a pointer to float, and the argument for %lf must be a pointer to double.

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Eric Postpischil Avatar answered Sep 22 '22 21:09

Eric Postpischil