在C位移位乘法不使用2的幂 [英] bit shift multiplication in c not using powers of 2

问题描述

如何通过使用36位移执行乘法？是不是只能通过2的幂乘？例如：

How can I perform multiplication by 36 using bit-shifting? Isn't it only possible to multiply by powers of 2? For example:

unsigned x = 4; // binary 00000000 00000000 00000000 00001000
unsigned y = x << 3; // multiply by 8, resulting in binary 00000000 ... 00100000

谢谢！

推荐答案

您不能用2的无动力由单独位移繁殖。

You can't multiply by a non-power of 2 by bit shifting alone.

但是你可以使用分解加法和乘法：

But you can break it down using addition and multiplication:

x * 36 = x * (32 + 4)
       = (x * 32) + (x * 4)

由于 32 和 4 是 2 的权力（ 2 ^ 5 和 2 ^ 2 分别），您可以执行这些作为单独的变化和增加的结果。

Since 32 and 4 are powers of 2 (2^5 and 2^2 respectively), you can perform those as separate shifts and add the results.

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