如何实现Kotlin数组最大元素索引（空数组返回null）及代码咨询

Let's fix your indexOfMax function step by step—there are a few key issues in your current code that are preventing it from working as expected:

1. No handling for empty arrays: Your function needs to return null if the input array is empty, but your current code doesn't check this at all.
2. Incorrect loop syntax: The loop should be for (i in 0..a.lastIndex) (you missed in and the a. prefix for lastIndex).
3. Wrong initialization for max value: Initializing max to 0 will fail if all elements in the array are negative. You should start with the first element of the array as the initial max value.
4. Swapped assignment in comparison: You have e = max when you should be updating max to e when a larger element is found.
5. Returning the max value instead of its index: Your function is supposed to return the index of the maximum element, not the element itself.

Here's the corrected version of the function:

fun indexOfMax(a: IntArray): Int? {
    // Handle empty array case first
    if (a.isEmpty()) return null
    
    var maxIndex = 0
    var maxValue = a[0]
    
    // Iterate from the second element to the end
    for (i in 1..a.lastIndex) {
        val currentValue = a[i]
        if (currentValue > maxValue) {
            maxValue = currentValue
            maxIndex = i
        }
    }
    
    return maxIndex
}

Explanation: #

• We first check if the array is empty and return null immediately if it is.
• We initialize maxIndex to 0 (the index of the first element) and maxValue to the first element's value.
• We loop starting from index 1 (since we already considered index 0) and compare each element to the current maxValue.
• Whenever we find a larger element, we update both maxValue to this new element and maxIndex to its position.
• Finally, we return maxIndex, which holds the position of the largest element in the array.

If you prefer a more idiomatic Kotlin approach using standard library functions, you could also write it like this:

fun indexOfMax(a: IntArray): Int? {
    return if (a.isEmpty()) null else a.indices.maxByOrNull { a[it] }
}

This uses maxByOrNull on the array's indices, comparing each index by the corresponding element value, which gives us the index of the maximum element directly.

内容的提问来源于stack exchange，提问作者AbdElrahman Usama