printf 中 double 的正确格式说明符
printf 中 double 的正确格式说明符是什么？是%f 还是%lf？我相信它是 %f，但我不确定。
代码示例
#include <stdio.h>
int main()
{
double d = 1.4;
printf("%lf", d); // Is this wrong?
}
What is the correct format specifier for double in printf? Is it %f or is it %lf? I believe it's %f, but I am not sure.
Code sample
#include <stdio.h>
int main()
{
double d = 1.4;
printf("%lf", d); // Is this wrong?
}
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"%f"是（或至少一种）双精度数的正确格式。 没有float的格式，因为如果您尝试将float传递给printf，它会在printf接收到1之前，将其提升为double。"%lf"在当前标准下也是可接受的 - 如果后跟f转换说明符，则l被指定为无效（除其他外）。请注意，这是
printf格式字符串与scanf（和fscanf等）格式字符串显着不同的地方。对于输出，您将传递一个值，当作为可变参数传递时，该值将从float提升为double。对于输入，您要传递一个指针，该指针不会被提升，因此您必须告诉scanf您是否想要读取float或adouble，因此对于scanf，%f表示您要读取float和%lf< /code> 表示您想要读取double（并且，对于long double来说，您可以使用%Lf来读取printf或scanf）。<子>
1. C99，§6.5.2.2/6：“如果表示被调用函数的表达式具有不包含原型的类型，则对每个参数执行整数提升，并且具有 float 类型的参数将提升为 double。这些被称为默认参数提升。”在 C++ 中，措辞有些不同（例如，它不使用“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经历默认提升。
"%f"is the (or at least one) correct format for a double. There is no format for afloat, because if you attempt to pass afloattoprintf, it'll be promoted todoublebeforeprintfreceives it1."%lf"is also acceptable under the current standard -- thelis specified as having no effect if followed by thefconversion specifier (among others).Note that this is one place that
printfformat strings differ substantially fromscanf(andfscanf, etc.) format strings. For output, you're passing a value, which will be promoted fromfloattodoublewhen passed as a variadic parameter. For input you're passing a pointer, which is not promoted, so you have to tellscanfwhether you want to read afloator adouble, so forscanf,%fmeans you want to read afloatand%lfmeans you want to read adouble(and, for what it's worth, for along double, you use%Lffor eitherprintforscanf).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 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.
鉴于C99标准（即N1256草案），规则取决于
函数种类：fprintf (printf, sprintf, ...) 或 scanf。
以下是摘录的相关部分：
为
fprintf指定的相同规则也适用于printf、sprintf和类似函数。长话短说，对于
fprintf，指定了以下说明符和相应的类型：%f->双%Lf->长双.对于
fscanf来说，它是：%f->浮动%lf->双%Lf->长双.Given the C99 standard (namely, the N1256 draft), the rules depend on the
function kind: fprintf (printf, sprintf, ...) or scanf.
Here are relevant parts extracted:
The same rules specified for
fprintfapply forprintf,sprintfand similar functions.The long story short, for
fprintfthe following specifiers and corresponding types are specified:%f-> double%Lf-> long double.and for
fscanfit is:%f-> float%lf-> double%Lf-> long double.它可以是
%f、%g或%e，具体取决于您希望数字的格式。请参阅此处了解更多详细信息。l修饰符在scanf和double中是必需的，但在printf中不需要。It can be
%f,%gor%edepending on how you want the number to be formatted. See here for more details. Thelmodifier is required inscanfwithdouble, but not inprintf.格式
%lf是double的完全正确的printf格式，与您使用的完全一样。你的代码没有任何问题。旧版本（C99 之前的）C 语言不支持
printf中的格式%lf，这在doubleprintf和scanf中的 code>。这种表面上的不一致已在 C99 中得到修复。您不需要在
printf中将%lf与double一起使用。如果您愿意，也可以使用%f（%lf和%f在printf中是等效的）。但在现代 C 语言中，更喜欢将%f与float一起使用，将%lf与double一起使用，%Lf与long double，在printf和scanf中保持一致。Format
%lfis a perfectly correctprintfformat fordouble, exactly as you used it. There's nothing wrong with your code.Format
%lfinprintfwas not supported in old (pre-C99) versions of C language, which created superficial "inconsistency" between format specifiers fordoubleinprintfandscanf. That superficial inconsistency has been fixed in C99.You are not required to use
%lfwithdoubleinprintf. You can use%fas well, if you so prefer (%lfand%fare equivalent inprintf). But in modern C it makes perfect sense to prefer to use%fwithfloat,%lfwithdoubleand%Lfwithlong double, consistently in bothprintfandscanf.%Lf（注意大写的L）是长双精度数的“noreferrer">格式说明符。对于普通的
双精度，可以是%e、%E、%f、%g或%G即可。%Lf(note the capitalL) is the format specifier for long doubles.For plain
doubles, either%e,%E,%f,%gor%Gwill do.