C++：“float”的 printf() 格式规范是什么？
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C++: What is the printf() format spec for "float"?
提问by Michael Fitzpatrick
C++: What is the printf()format spec for float? (Visual C++)
C++：什么是printf()格式规范float？(Visual C++)
It used to be that I used %gfor floatand %lgfor double.
它曾经是我用%g的float和%lg为double。
It looks like the spec changed and floatis undefined and doubleis %g.
看起来规范已更改并且float未定义并且double是%g.
I have bits in memory that I am printing out so casting is not an option.
我的内存中有一些要打印出来，所以不能选择强制转换。
Is there a way that I can print out floatvalues using printf()?
有没有办法可以float使用打印出值printf()？
Update:
更新：
This code was written for unit testing generic C++ libs used on an embedded system.
Here's what I had to do to get the floatto work.
The code is in a template function:
此代码是为在嵌入式系统上使用的通用 C++ 库进行单元测试而编写的。这是我必须做的事情float才能正常工作。代码在模板函数中：
template <typename T,typename TTYP,typename Ttyp,int bits,bool IsSigned>
Error testMatrixT()
{ ...
Here is a code snip:
这是一个代码片段：
if (typeid(Ttyp) == typeid(float)) {
float64 c = *(float32*)&Tp(row,col);
float64 a1 = *(float32*)&Arg1(row,col);
float64 a2 = *(float32*)&Arg2(row,col);
float64 e = *(float32*)&Exp(row,col);
m_b = (c == e);
_snprintf(m_acDiag, sizeof(m_acDiag)-1
, "add(Arg1,Arg2): arg1=%g, arg2=%g, Expected=%g, Actual=%g, Result: %s"
, a1, a2, e, c, BOOL_PF(m_b));
} else {
...
Pretty ugly isn't it? Using floats as args give bad output. Maybe due to using _snprintf()?
Years ago I would use %lgand it would be OK. Not anymore.
很丑是不是？使用浮点数作为参数会产生错误的输出。可能是因为使用_snprintf()？几年前我会使用%lg它会没事的。不再。
回答by Stephen Canon
double and float use the same format specifiers with printf(%a, %e, %f, and %g). This is because printfis a variadic function. Any float arguments are implicitly promoted to double before the call; you can't actually pass a float to printf.
双和浮子使用相同的格式说明与printf（%a，%e，%f，和%g）。这是因为printf是一个可变参数函数。在调用之前，任何浮点参数都被隐式提升为双精度；您实际上无法将浮点数传递给printf.
回答by eq-
To the question in title: There is none / "%f"
对于标题中的问题：没有/“%f”
To the question in message body: Yes and no.
对于消息正文中的问题：是和否。
printfis a variadic function. All floatarguments are automatically promoted to doubles. You print a float by passing it to printf, but no float actually reaches the printf function.
printf是一个可变参数函数。所有float参数都会自动提升为doubles。您通过将浮点数传递给 printf 来打印浮点数，但实际上没有浮点数到达 printf 函数。
回答by Keith Thompson
%ghas always been the correct format for either float or double (since float arguments are promoted to double for variadic functions like printf).
%g一直是 float 或 double 的正确格式（因为 float 参数被提升为 double 之类的可变参数函数printf）。
%lgwas added as a synonym for %g(in C99, I don't know when or whether C++ adopted it), probably for better symmetry with the *scanffamily.
%lg被添加为%g（在 C99 中，我不知道 C++ 何时或是否采用它）的同义词，可能是为了更好地与*scanf家庭对称。
$Lgis for long double.
$Lg是长双。
You can replace gwith for ein the above, depending on what output format you want.
您可以替换g为f或e在上面，具体取决于您想要的输出格式。
回答by Kiley Naro
From: http://www.cplusplus.com/reference/clibrary/cstdio/printf/
来自：http: //www.cplusplus.com/reference/clibrary/cstdio/printf/
%f Decimal floating point 392.65
%f 十进制浮点数 392.65
回答by Praetorian
Others have pointed out the format for printing float using printf, but, since you're using C++, my suggestion is to avoid this completely.
其他人已经指出了使用 打印浮点数的格式printf，但是，由于您使用的是 C++，我的建议是完全避免这种情况。
Use C++ streams instead, you don't have to worry about format specifiers then. It is also type safe, while printfisn't.
改用 C++ 流，然后您不必担心格式说明符。它也是类型安全的，而printf不是。
#include <iostream>
int main()
{
float f = 1.234;
std::cout << f << std::endl;
}
If you have an array of bytes containing a float, memcpyit into a float variable and use the above code.
如果您有一个包含浮点数的字节数组，memcpy则将其转换为浮点数变量并使用上面的代码。
回答by SSteve
printf("float: %f", floatValue);
or if you want exponential notation:
或者如果你想要指数表示法：
printf("float: %e", floatValue);
