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

问题描述

什么是正确的格式说明符双击 printf中？它是％F 还是％LF ？我相信它的％F ，但我不知道。

code样品

 的#include＆LT;＆stdio.h中GT;
诠释的main（）
{
   双D = 1.4;
   的printf（％LF，D）; //这是错的？
}
 

解决方案

％F是（或至少一个），纠正了双格式。有是的一个浮法没有格式，因为如果你试图传递一个浮动来的printf ，它会被提升到双击在的printf 接收它 ¹ 。 ％LF也是目前的标准下可以接受的 - 在→指定为如果遵循由˚F转换符（等等）。

请注意，这是一个地方，的printf 格式字符串从 scanf函数大为不同（和的fscanf 等）格式字符串。对于输出，你传递的值的，将从浮动晋升为双击当作为可变参数参数传递。对于输入你传递一个的指针的，这是不松口，所以你必须要告诉 scanf函数是否要读浮动或双击，所以 scanf函数， ％F 意味着你要读浮动和％LF 意味着你要阅读一个双击（并且，对于它的价值，为长双，使用％ LF 对于任何一个的printf 或 scanf函数）。

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<子>
1. C99，§6.5.2.2/ 6：如果这表示调用的函数的前pression有一个类型不包括样机，整数促销活动是在每个参数执行，有float类型参数是提升一倍。这些被称为默认参数促销活动。在C ++中的措辞有所不同（例如，它用的是原型），但效果是一样的：所有的可变参数的参数进行默认促销他们由函数接收之前。


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?
}

解决方案

"%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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