I want to know actually how .indices works and what is the main difference is between this two for loops.
for (arg in args)
println(arg)
or
for (i in args.indices)
println(args[i])
那么withIndex()函数有什么用处呢？
for ((index, value) in array.withIndex()) {
println("the element at $index is $value")
}
这些只是迭代数组的不同方式，具体取决于您在for循环的主体中需要访问的内容:当前元素(第一个 case )、当前索引(第二个 case )或两者(第三个 case ).
如果您想知道它们在幕后是如何工作的，只需跳到Kotlin运行时的代码(IntelliJ中的Ctrl+B)，就能找到答案.
For indices specifically, this is pretty simple, it's implemented as an extension property that returns an IntRange which a for loop can then iterate over:
/**
* Returns the range of valid indices for the array.
*/
public val <T> Array<out T>.indices: IntRange
get() = IntRange(0, lastIndex)