在这个使用 reduce() 内部迭代器 function 的示例中，为什么累加器不确认这个数字？

Question

I'm kind of baffled here. I'm simply trying to use the reduce function to create a String representing the elements of a list in numbered order. Here is the code:

val names = listOf("John", "Billy", "Tom", "Joe", "Eric", "Jerry") 

val result = names.reduce { accum, name -> "$accum ${names.indexOf(name) + 1}. $name" }

println(result)     // John 2. Billy 3. Tom 4. Joe 5. Eric 6. Jerry

                   // ^ missing 1.  

I'm expecting the value of the accumulator to accumulate as such after each iteration:

"1. John"

"1. John 2. Billy"

"1. John 2. Billy 3. Tom"

"1. John 2. Billy 3. Tom 4. Joe"

"1. John 2. Billy 3. Tom 4. Joe 5. Eric"

"1. John 2. Billy 3. Tom 4. Joe 5. Eric 6. Jerry"

When I run the code, it prints: John 2. Billy 3. Tom 4. Joe 5. Eric 6. Jerry

I don't understand why the "1." is missing,

Answer 1

Reduce function uses your first value as a starting accumulator.

So operation is only applied between (((1 -> 2) -> 3) -> 4) pairs.

You can achieve expected behaviour with fold function, which takes initial value:

val result = names.fold("") { accum, name -> "$accum ${names.indexOf(name) + 1}. $name" }