CODEWITHSUNDEEP
haskell
Jane Doe
Jane Doe

Reputation: 61

List-comprehension output not what I assumed

I assume its simple as its in a past paper of my university but the function is:

[(x,y) | x <- [0..2], y<-[0,x])

and the output 

  [(0,0),(0,0),(1,0), (1,1), (2,0), (2,2)]

The (2,0) confuses me, if y maps to 0 to x whilst x is equal to 1 = (1,1) wouldnt it be

[(0,0),(0,0),(1,0), (1,1), **(2,1)**, (2,2)]

or is it because due to the y using all its numbers in its list [0,1] it reverts back to 0?

Upvotes: 0

Views: 84

Answers (1)

Random Dev
Random Dev

Reputation: 52270

case [(x,y) | x <- [0..2], y<-[0,x]]

[0,x] is:

  • [0,0] for x=0
  • [0,1] for x=1
  • [0,2] for x=2

so if you pair each of those ys up with the corresponding x you get:

   [(0,0),(0,0)] -- x = 0
++ [(1,0),(1,1)] -- x = 1
++ [(2,0),(2,2)] -- x = 2

which yields your given output

note: [0,2] has length 2 and is quite different from [0..2] which has length 3 and contains 1

case [(x,y) | x <- [0..2], y<-[0..x]]

it's not that different - [0..x] is:

  • [0] for x=0
  • [0,1] for x=1
  • [0,1,2] for x=2

and if you pair each of those ys up with the corresponding x you get

   [(0,0))]            -- x = 0
++ [(1,0),(1,1)]       -- x = 1
++ [(2,0),(2,1),(2,2)] -- x = 2

which then would give you the result

[(0,0),(1,0),(1,1),(2,0),(2,1),(2,2)]

Upvotes: 6

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