patch - Scala: subtract index of odd values -

for practical exercise need define function changes index of every value in odd index in list, this:

changepairs(list(1,2,3,4,5,6,7,8,9,10,11))  //> res62: list[int] = list(2, 1, 4, 3, 6, 5, 8, 7, 10, 9, 11)  changepairs(list(2,2,30,4,50,6,7,80,9,100))  //> res63: list[int] = list(2, 2, 4, 30, 6, 50, 80, 7, 100, 9) 

so need swap places of each odd-even pair, , in case i'm left single odd element @ last index (11 in first example), leave is.

i have it's not working, , i'm not sure why.

def changepairs(a: list[int]) = a.zipwithindex.map {  case (s,i) => if (i % 2 != 0) a.patch(i,seq(s),1); a.patch(i-2,seq(s),0); }  

here's 1 way:

def changepairs(a: list[int]) = a.grouped(2).flatmap {   case list(a, b) => list(b, a)   case => }.tolist  changepairs(list(1, 2, 3, 4, 5, 6, 7)) // list(2, 1, 4, 3, 6, 5, 7) 

main idea gets going once think of grouping list sublists of 2 elements, grouped(2) does. on it's easy ride - describe 2 cases, 1 2 elements (in case flip them) , 1 1 element, such 7 in example, in case leave be. use flatmap flatten resulting list of 2-element lists 1 big list, , .tolist out of iterator got grouped.

edit:

i saw a.grouped(2).map(_.reverse).flatten.tolist in comments. yeah, works too, it's same less verbose since instead of "manually" swapping elements, reverse on each sublist.