c++ - 为什么 1.0 乘以 5 得到输出 5 而不是 5.0？

Question

cout<<5*1.0<<endl;
cout<<(float)5<<endl;

在这两种情况下，我的答案都是 5 而不是 5.0。请帮助...

Answer 1

需要指定floatingPoint输出的格式和精度，如:

std::cout << std::fixed << std::setprecision(1) << 5*1.0;

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来自 [The.C++.Programming.Language.Special.Edition] 21.4.3 浮点输出 [io.out.float]:

Floatingpoint output is controlled by a format and a precision:

– Thegeneral format lets the implementation choose a format that presents avalue in the style that best preserves the value in the spaceavailable. The precision specifies the maximum number of digits. Itcorresponds to printf()’s %g (§21.8).

– The scientific format presentsa value with one digit before a decimal point and an exponent. Theprecision specifies the maximum number of digits after the decimalpoint. It corresponds to printf()’s %e .

– The fixed format presents avalue as an integer part followed by a decimal point and a fractionalpart. The precision specifies the maximum number of digits after thedecimal point. It corresponds to printf()’s %f .