C ++将变量与多个值进行比较的最有效的方法？ [英] C++ Most efficient way to compare a variable to multiple values?

问题描述

在我的程序中几次，我不得不检查一个变量是否是许多选项之一。例如

A few times in my program, I've had to check if a variable was one of many options. For example

if (num = (<1 or 2 or 3>)) { DO STUFF }

我搞错了'OR's，但没有什么似乎是对的。我尝试了

I've messed around with 'OR's, but nothing seems to be right. I've tried

if (num == (1 || 2 || 3))

但它什么都不做。请帮忙！
提前感谢。

but it does nothing. Please help! Thanks in advance.

我需要区分几个组。例如...

P.S. I need to distinguish between several groups. For example...

if (num = (1,2,3))

else if (num = (4,5,6))

else if (num = (7,8,9))

推荐答案

如果您想要检查的值足够小，您可以创建一个您想要的值的位掩码，然后检查

If the values you want to check are sufficiently small, you could create a bit mask of the values that you seek and then check for that bit to be set.

假设您关心几个群组。

static const unsigned values_group_1 = (1 << 1) | (1 << 2) | (1 << 3);
static const unsigned values_group_2 = (1 << 4) | (1 << 5) | (1 << 6);
static const unsigned values_group_3 = (1 << 7) | (1 << 8) | (1 << 9);    
if ((1 << value_to_check) & values_group_1) {
  // You found a match for group 1
}
if ((1 << value_to_check) & values_group_2) {
  // You found a match for group 2
}
if ((1 << value_to_check) & values_group_3) {
  // You found a match for group 3
}

超过您的CPU喜欢使用的自然大小。这在现代通常为64，但可能会根据您的环境的具体情况而有所不同。

This approach works best for values that don't exceed the natural size your CPU likes to work with. This would typically be 64 in modern times, but may vary depending upon the specifics of your environment.

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