CODEWITHSUNDEEP
scala
Daniel
Daniel

Reputation: 6039

Scala: On strange behavior of `foldLeft`

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?

Upvotes: 0

Views: 87

Answers (2)

IgnisErus
IgnisErus

Reputation: 396

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-

Upvotes: 4

som-snytt
som-snytt

Reputation: 39587

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-

Upvotes: 1

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