問題描述

在我的程序中有幾次，我不得不檢查變量是否是眾多選項之一.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 }

我搞砸了OR"，但似乎沒有什么是正確的.我試過了

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.

我想方便地區分幾個組.例如

I'd like to conveniently distinguish between several groups. For example

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

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

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

推薦答案

如果您要檢查的值足夠小，您可以創建您要查找的值的位掩碼，然后檢查要設置的位.

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.

假設您關心幾個組.

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
}

這種方法最適合不超過您的 CPU 喜歡使用的自然大小的值.在現代，這通常是 64，但可能會因環境的具體情況而異.

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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