％g printf说明符到底是什么意思? [英] What precisely does the %g printf specifier mean?

问题描述

%g说明符的行为似乎不像大多数来源记录的那样.

The %g specifier doesn't seem to behave in the way that most sources document it as behaving.

根据我发现的大多数资料，在使用printf指示符的多种语言中，%g指示符应等效于%f或%e-两者对于所提供的内容都会产生较短的输出价值.例如，在撰写此问题时， cplusplus.com 说，g说明符的意思是:

According to most sources I've found, across multiple languages that use printf specifiers, the %g specifier is supposed to be equivalent to either %f or %e - whichever would produce shorter output for the provided value. For instance, at the time of writing this question, cplusplus.com says that the g specifier means:

使用最短的表示形式:%e或%f

PHP手册说，它表示:

g-％e 和％f 中的较短者.

和这是一个堆栈溢出答案，其中声称

%g使用最短的表示形式.

%g uses the shortest representation.

和一个Quora答案声称:

%g以这两种表示形式中的最短形式打印数字

%g prints the number in the shortest of these two representations

但是这种行为不是我在现实中看到的.如果我编译并运行此程序(作为C或C ++，这是一个有效的程序，两者的行为相同):

But this behaviour isn't what I see in reality. If I compile and run this program (as C or C++ - it's a valid program with the same behaviour in both):

#include <stdio.h>

int main(void) {
    double x = 123456.0;
    printf("%e\n", x);
    printf("%f\n", x);
    printf("%g\n", x);
    printf("\n");

    double y = 1234567.0;
    printf("%e\n", y);
    printf("%f\n", y);
    printf("%g\n", y);
    return 0;
}

...然后我看到此输出:

... then I see this output:

1.234560e+05
123456.000000
123456

1.234567e+06
1234567.000000
1.23457e+06

很明显，对于上述x或y，%g输出与 %e或%f输出都不完全匹配.而且，%g看起来也不是使输出长度最小.如果y像x一样以科学计数法未印刷 ，则其格式可能更简洁.

Clearly, the %g output doesn't quite match either the %e or %f output for either x or y above. What's more, it doesn't look like %g is minimising the output length either; y could've been formatted more succinctly if, like x, it had not been printed in scientific notation.

我上面引用的所有消息来源对我来说都是骗人的吗?

Are all of the sources I've quoted above lying to me?

在支持这些格式说明符的其他语言中，我看到了相同或相似的行为，可能是因为它们在幕后呼唤了C函数的printf系列.例如，我在Python中看到以下输出:

I see identical or similar behaviour in other languages that support these format specifiers, perhaps because under the hood they call out to the printf family of C functions. For instance, I see this output in Python:

>>> print('%g' % 123456.0)
123456
>>> print('%g' % 1234567.0)
1.23457e+06

在PHP中:

php > printf('%g', 123456.0);
123456
php > printf('%g', 1234567.0);
1.23457e+6

在Ruby中:

irb(main):024:0* printf("%g\n", 123456.0)
123456
=> nil
irb(main):025:0> printf("%g\n", 1234567.0)
1.23457e+06
=> nil

控制此输出的逻辑是什么?

What's the logic that governs this output?

推荐答案

这是C11标准中的g/G说明符的完整描述:

This is the full description of the g/G specifier in the C11 standard:

表示浮点数的 double 参数是 转换为f或e样式(对于G，则转换为F或E样式 转换说明符)，具体取决于转换后的值和 精确.如果 P 为非零，则等于精度；如果精度为0，则等于6. 省略；如果精度为零，则为1.然后，如果 样式E的指数为 X :

A double argument representing a floating-point number is converted in style f or e (or in style F or E in the case of a G conversion specifier), depending on the value converted and the precision. Let P equal the precision if nonzero, 6 if the precision is omitted, or 1 if the precision is zero. Then, if a conversion with style E would have an exponent of X:

   如果 P > X ≥−4，则转换为 样式为f(或F)，精度为 P −(X + 1).
   否则， 转换采用样式e(或E)和精度 P − 1.

     if P > X ≥ −4, the conversion is with style f (or F) and precision P − (X + 1).
     otherwise, the conversion is with style e (or E) and precision P − 1.

最后，除非 使用#标志，将从分数中删除所有尾随零 结果的一部分和小数点字符将被删除，如果 没有剩余的小数部分.

Finally, unless the # flag is used, any trailing zeros are removed from the fractional portion of the result and the decimal-point character is removed if there is no fractional portion remaining.

一个 double 参数 表示无穷大或NaN的样式转换为f或F 转换说明符.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

此行为有点类似于仅使用%f和%e中的最短表示形式，但不等效.有两个重要的区别:

This behaviour is somewhat similar to simply using the shortest representation out of %f and %e, but not equivalent. There are two important differences:

• 使用%g时，尾随零(可能还有小数点)会被剥夺，这可能导致%g说明符的输出与 %f或%e将会产生.
• 是否使用%f样式或%e样式格式的决定完全基于%e样式表示法所需的指数大小，并且 not 直接取决于哪种表示形式会更短.在多种情况下，此规则会导致%g选择更长的表示形式，例如在问题中所示的情况，其中%g使用科学计数法，即使这会使输出的4个字符超出所需的长度.

• Trailing zeros (and, potentially, the decimal point) get stripped when using %g, which can cause the output of a %g specifier to not exactly match what either %f or %e would've produced.
• The decision about whether to use %f-style or %e-style formatting is made based purely upon the size of the exponent that would be needed in %e-style notation, and does not directly depend on which representation would be shorter. There are several scenarios in which this rule results in %g selecting the longer representation, like the one shown in the question where %g uses scientific notation even though this makes the output 4 characters longer than it needs to be.

如果难以解析C标准的措辞，则 Python文档提供了相同行为的另一种描述:

In case the C standard's wording is hard to parse, the Python documentation provides another description of the same behaviour:

常规格式.对于给定的精度p >= 1 ， 这会将数字四舍五入为p 有效数字，并且 然后将结果格式化为定点格式 或以科学计数法表示，具体取决于其大小.

General format. For a given precision p >= 1, this rounds the number to p significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude.

精确的规则如下:假设 结果格式为演示文稿类型'e' 和 精度p-1 将具有指数exp .然后 如果-4 <= exp < p ，则数字已格式化 演示文稿类型'f' 和精度 p-1-exp .否则，数字将被格式化 演示文稿类型为'e' 且精度为p-1 . 在这两种情况下，都删除了不重要的尾随零 从有效位开始，小数点也是 如果后面没有剩余数字，则将其删除.

The precise rules are as follows: suppose that the result formatted with presentation type 'e' and precision p-1 would have exponent exp. Then if -4 <= exp < p, the number is formatted with presentation type 'f' and precision p-1-exp. Otherwise, the number is formatted with presentation type 'e' and precision p-1. In both cases insignificant trailing zeros are removed from the significand, and the decimal point is also removed if there are no remaining digits following it.

正负无穷大，正负无穷大 零和nans的格式为inf ，-inf ， 分别0 ，-0 和nan 精度.

Positive and negative infinity, positive and negative zero, and nans, are formatted as inf, -inf, 0, -0 and nan respectively, regardless of the precision.

互联网上许多声称%g只是从%e和%f中选出最短的消息来源都是错误的.

The many sources on the internet that claim that %g just picks the shortest out of %e and %f are simply wrong.

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