英文:
How to Get the True Count in Boolean Array in Kotlin
问题
我已阅读了 Kotlin 文档并尝试了 BooleanArray.count(predicate: (Boolean) -> Boolean)，但它仍然返回的是 array 的大小，而不是 true 的数量：
val checks = arrayOf(true, false, true, true, true)
val trueCount = checks.count { true }
我的代码有什么问题？
英文:
I've read the kotlin documentation and tried BooleanArray.count(predicate: (Boolean) -> Boolean), but it still returns the size of the array, not the number of true:
val checks = arrayOf(true, false, true, true, true)
val trueCount = checks.count { true }
What's wrong with my code?
答案1
得分: 15
你在这里传递了一个始终会返回 true 的条件：
val trueCount = checks.count { true }
这将确实返回数组的大小。我预期你的代码应该是：
val trueCount = checks.count { it }
换句话说，对于“使用被称为 it 的项目，表达式 it 返回 true”的项目计数。
另一种可能更易读的替代方案是显式检查相等性：
val trueCount = checks.count { it == true }
就个人而言，我倾向于避免与布尔字面值进行比较，但我可以理解在这种情况下至少可以有理地更清晰。
英文:
You're passing in a predicate that would always return true here:
val trueCount = checks.count { true }
And that would indeed return the size of the array. I'd expect your code to be:
val trueCount = checks.count { it }
In other words, the count of items for which "with the item called it, the expression it returns true".
An alternative you might find easier to read that explicitly checks for equality:
val trueCount = checks.count { it == true }
Personally I tend to avoid comparisons against Boolean literals, but I can see how it's at least arguably clearer in this case.
答案2
得分: 8
使用 count { it } 替代 count { true }
count 函数的参数并不是要在数组中查找的值；它是一个谓词（predicate）：一个接受一个参数并返回 true 或 false 的函数。这个谓词对数组中的每个项调用一次，由谓词返回的值（true 或 false）决定是否对该项进行计数。因此，结果是数组中谓词评估为 true 的项的数量。
在这个上下文中，写 { true } 相当于 { value -> true }，它是一个恒定的函数。无论 value 是什么，它始终返回 true。因此，“计算使 { true } 返回 true 的项的数量” 正好等同于“计算项的数量”。
相反，你想要计算值为 true 的项的数量。你应该使用的谓词是 { value -> value }，它是一个接受单个参数并返回该参数值的函数。由于当你不给参数命名时，单个 lambda 参数隐式命名为 it，因此 { value -> value } 的简写形式就是 { it }。
英文:
Use count { it } instead of count { true }
The argument to count isn't a value to look for in the array; it's a predicate: a function that takes one argument and returns true or false. The predicate is called once for each item in the array, and the value (true or false) returned by that predicate determines whether the item is counted or not. So the result is the number of items in the array for which the predicate evaluated to true.
In this context, writing { true } is shorthand for { value -> true }, and is a constant function. It will always return true, regardless of what the value is. So "count the number of items for which { true } returns true" is exactly the same as "count the number of items".
What you want instead is to count the number of items where the value is true. The predicate you should use is { value -> value }, which is a function that takes a single argument and returns the value of that argument. Since a single lambda parameter is implicitly named it when you don't give it a name, the shorthand for { value -> value } is just { it }.
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