Scala：关于“ foldLeft”的奇怪行为

Question

Define:

val a = List(1, 2, 3, 4, 5, 6, 7)

Consider the following line with foldLeft :

a.foldLeft(""){case (num, sum) => sum + (num.toString + "-")}

My expectation was that the program is going to do:

((((( "7-" + "6-" ) + "5-" ) + "4-" ) + "3-" ) + "2-" ) + "1-"

which is 7-6-5-4-3-2-1-

But what I get is: 7654321------- . Why is this the case?

Answer 1

You mixed up the parameters to foldLeft . Check the documentation for List.foldLeft . Note that the z "zero" value has the same type as the second parameter in the function argument, not the first.

This should work closer to expected:

a.foldLeft(""){case (sum, num) => sum + (num.toString + "-")}
// res0: String = 1-2-3-4-5-6-7-

However, if you want the numbers in reverse-order, then you might want to use foldRight . Maybe this is actually what you were going for in the first place (notice that the arguments num and sum are in the same order you gave):

a.foldRight(""){case (num, sum) => sum + (num.toString + "-")}
// res1: String = 7-6-5-4-3-2-1-

Answer 2

From your expectation, I expect you expected foldRight behavior:

scala> val a = List(1, 2, 3, 4, 5, 6, 7)
a: List[Int] = List(1, 2, 3, 4, 5, 6, 7)

scala> a.foldRight(""){case (num, sum) => sum + (num.toString + "-")}
res0: String = 7-6-5-4-3-2-1-