c - C 中的 %f 和 %lf 有什么区别？

Question

我在C书的C例子中看到这两个参数，但是作者没有详细说明两者有什么区别。我知道 %f 指定一个 float 应该取代它。我试着查找这个但很难找到这个 w 符号。 %lf 呢？

Answer 1

简短的回答是它对 printf 没有影响, 并表示使用 float或 double在scanf .

对于 printf , float 类型的参数被提升为double所以两者都是%f和 %lf用于double .对于 scanf , 你应该使用 %f对于 float和 %lf对于 double .

下面我们中的语言律师的更多详细信息:

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%f之间没有区别和 %lf在printf家庭。 ISO C 标准(其中的所有引用均来自 C11)，第 7.21.6.1 The fprintf function 节,款/7状态，对于 l修饰符(我的重点):

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long int or unsigned long int argument; that a following n conversion specifier applies to a pointer to a long int argument; that a following c conversion specifier applies to a wint_t argument; that a following s conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a, A, e, E, f, F, g, or G conversion specifier.

它不需要修改 f 的原因说明符是因为该说明符已经表示double ，来自段落 /8在列出 %f 类型的同一部分说明符:

A double argument representing a floating-point number is converted to decimal notation

这与函数原型(prototype)中椭圆后面的参数按照 6.5.2.2 Function calls 部分的默认参数提升有关。 ,款/7 :

The ellipsis notation in a function prototype declarator causes argument type conversion to stop after the last declared parameter. The default argument promotions are performed on trailing arguments.

自 printf (以及 printf 类函数的整个族)声明为 int printf(const char * restrict format, ...);对于省略号，该规则适用于此处。 6.5.2.2 Function calls 部分介绍了默认参数提升,款/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.

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对于 scanf家庭，它要求使用 double而不是 float .科7.21.6.2 The fscanf function /11 :

Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to long int or unsigned long int; that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to double; or that a following c, s, or [ conversion specifier applies to an argument with type pointer to wchar_t.

这修改了 /12该部分的段落说明，对于 %f :

Matches an optionally signed floating-point number, infinity, or NaN, whose format is the same as expected for the subject sequence of the strtod function. The corresponding argument shall be a pointer to floating.