Leopard Leopard - 4 months ago 28
C Question

Correct format specifier for double in printf

What is the correct format specifier for

double
in printf? Is it
%f
or is it
%lf
? I believe its
%f
but I am not sure.

Code sample

#include <stdio.h>
int main()
{
double d =1.4;
printf("%lf", d); //is this wrong?
}

Answer

"%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 does 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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