c - 正确的格式说明符用于printf中的double(Correct format specifier for double in printf)

Question

What is the correct format specifier for double in printf?

(printf中double的正确格式说明符是什么？)

Is it %f or is it %lf ?

(是%f还是%lf ？)

I believe it's %f , but I am not sure.

(我相信是%f ，但我不确定。)

Code sample (代码样例)

#include <stdio.h>

int main()
{
   double d = 1.4;
   printf("%lf", d); // Is this wrong?
}

  ask by Leopard translate from so

Answer 1

"%f" is the (or at least one) correct format for a double.

("%f"是双精度的（或至少一种）正确格式。)

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 it ¹ .

(有一个无格式的float ，因为如果你试图传递一个float到printf ，它会被提升到double之前printf接受它^1。)

"%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).

(在当前标准下， "%lf"也是可以接受的-如果l后面跟随f转换说明符（除其他外），则l被指定为无效。)

Note that this is one place that printf format strings differ substantially from scanf (and fscanf , etc.) format strings.

(请注意，这是printf格式字符串与scanf （和fscanf等）格式字符串大不相同的地方。)

For output, you're passing a value , which will be promoted from float to double when passed as a variadic parameter.

(对于输出，您正在传递一个value ，当作为可变参数传递时，该值将从float提升为double 。)

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 ).

(对于输入，您要传递未提升的指针 ，因此必须告诉scanf是要读取float还是double ，因此对于scanf ， %f表示要读取float ， %lf表示要读取float读取double （对于一个long double %Lf ，它的价值是，对于printf或scanf使用%Lf ）。)

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_{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."}

(_{1. C99，第6.5.2.2/6节：“如果表示被调用函数的表达式的类型不包含原型，则对每个参数执行整数提升，而将类型为float的参数提升为双精度。这些称为默认参数提升。”})

_{In C++ the wording is somewhat different (eg, 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.}

(_{在C ++中，措词有些不同（例如，它不使用单词“ prototype”），但是效果是相同的：所有可变参数在被函数接收之前都经过默认提升。})