1.0乘法precision和int浮动转换 [英] Precision of multiplication by 1.0 and int to float conversion

问题描述

它是安全的假设条件（INT）（我* 1.0F）== I 适用于任何整数 I ？

Is it safe to assume that the condition (int)(i * 1.0f) == i is true for any integer i?

推荐答案

没有。

如果 I 足够大， INT（浮点（I））！= I （假设浮动是IEEE -754单precision， I = 0x1000001 足以显示出这一点），那么这是假的，因为乘以 1.0F 强制转换为浮动，它改变了价值，即使随后的乘法没有。

If i is sufficiently large that int(float(i)) != i (assuming float is IEEE-754 single precision, i = 0x1000001 suffices to exhibit this) then this is false, because multiplication by 1.0f forces a conversion to float, which changes the value even though the subsequent multiplication does not.

但是，如果 I 是一个32位整数和双击是IEEE-754双，那么它是的事实， INT（我* 1.0）== I 。

However, if i is a 32-bit integer and double is IEEE-754 double, then it is true that int(i*1.0) == i.

只是要完全清楚，由 1.0F 乘法的是的准确。这是一个从 INT 到浮动转换这可能不是。

Just to be totally clear, multiplication by 1.0f is exact. It's the conversion from int to float that may not be.

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