printf中正确的格式说明符为double [英] Correct format specifier for double in printf

问题描述

在printf中 double 的格式说明符是什么？是％f 还是％lf ？我相信它的％f 但我不确定。

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.

代码示例

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

推荐答案

％f是双重格式（或至少一个）正确的格式。 printf ，将在 printf 之前升格为 double 收到它 ¹ 。 ％lf也符合现行标准 - l 被指定为无效果 f 转换说明符（等等）。

"%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).

请注意，这是 printf 格式字符串与 scanf （和 fscanf 等）的格式字符串大不相同。对于输出，您传递一个值，它将从 float 升级到 double 作为可变参数传递。对于输入，您传递的一个指针不会被提升，所以你必须告诉 scanf ，无论你想读一个 float 或 double ，所以对于 scanf ， ％f 表示您想阅读 float 和％lf 表示您想阅读一个 double （而且，对于一个 long double 而言，您使用％ Lf 为 printf 或 scanf ）。

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：如果表示被调用函数的表达式的类型不包括一个原型，对每个参数执行整数促销，并且类型为float的参数被提升为双倍，这被称为默认参数促销。在C ++中，措辞有些不同（例如，它使用原型一词），但效果是相同的：所有可变参数在被函数接收之前都会进行默认升级。}

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