CSC 347 - Concepts of Programming Languages

Map, Filter, and Fold Examples

Instructor: Stefan Mitsch

Exercise: Process a List of Numbers

val xs = List(1,2,3,4)
val ys = List(2,3,4,5)

Which snippets return true?

1. ys == xs.map(x => x+1)
2. ys == xs.map(_ + 1)
3. ys == xs.filter(x => x % 2 == 0)
4. ys == xs.fold(0)((x,y) => x+y)

• Snippets 1 and 2

Exercise: Process a List of Numbers

val xs = List(1,2,3,4)
val ys = List(2,4)

Which snippets return true?

1. ys == xs.map(x => x+1)
2. ys == xs.map(_ + 1)
3. ys == xs.filter(x => x % 2 == 0)
4. ys == xs.filter(_ % 2 == 0)
5. ys == xs.fold(0)((x,y) => x+y)

• Snippets 3 and 4

Exercise: Process a List of Numbers

val xs = List(1,2,3,4)
val ys = 10

Which snippets return true?

1. ys == xs.map(x => x+1)
2. ys == xs.map(_ + 1)
3. ys == xs.filter(x => x % 2 == 0)
4. ys == xs.filter(_ % 2 == 0)
5. ys == xs.fold(0)((x,y) => x+y)

• Snippet 5

Exercise: Process a List of Numbers

val xs = List(1,2,3,4)
val ys = "01234"

Which snippets return true?

1. ys == xs.map(x => x+1)
2. ys == xs.map(_ + 1)
3. ys == xs.filter(x => x % 2 == 0)
4. ys == xs.filter(_ % 2 == 0)
5. ys == xs.fold("0")((x,y) => x.toString+y)

• Snippet 5

Exercise: Process a List of Numbers

val xs = List(1,2,3,4)
val ys = "1,2,3,4,"

Which snippets return true?

1. ys == xs.map(x => x+1)
2. ys == xs.map(_ + 1)
3. ys == xs.filter(x => x % 2)
4. ys == xs.filter(_ % 2 == 0)
5. ys == xs.fold(",")((x,y) => x.toString+y)
6. ys == xs.map(x => x.toString + ",").fold("")((x,y) => x+y)

• Snippet 5

Exercise: Process a List of Numbers

val xs = List(1,2,3,4)
val ys = ",2,4,"

Which snippets return true?

1. ys == xs.map(x => x+1)
2. ys == xs.map(_ + 1)
3. ys == xs.filter(x => x % 2)
4. ys == xs.filter(_ % 2 == 0)
5. ys == xs.fold(",")((x,y) => x.toString+y)
6. ys == xs.filter(_ % 2 == 0).map(x => x.toString + ",").fold(",")((x,y) => x+y)
7. ys == xs.filter(_ % 2 == 0).map(_.toString + ",").fold(",")(_ + _)

• Snippets 5 and 6