Expression or statement is incomplete or incorrect

I'm getting the above error when I try and run this piece of code. Any help would be appreciated

LVec = (1:20);
for LVec
    if LVec == 2 or LVec == 4 or LVec == 6 or LVec = 8 or Lvec = 10 or LVec = 12 or LVec = 14 or LVec = 16 or LVec = 18 or Lvec = 20
        LVec = Lvec .* -1
    else LVec = LVec
    end
end

Answer 1

[ES]

you cant use or. Instead use

And your for loop is also incomplete.

May be you wanted to do this..

LVecArray = (1:20);
for loop=1:length(LVecArray)
    LVec = LVecArray(loop);
    if LVec == 2 || LVec == 4 || LVec == 6 || LVec == 8 || LVec == 10 || LVec == 12 || LVec == 14 || LVec == 16 || LVec == 18 || LVec == 20
        LVec = LVec .* -1;
    else
        LVec = LVec;
    end
   LVecArray(loop) = LVec;
end

Another solution is

LVecArray = (1:20);
mulArray = repmat([1,-1],1,10)
LVecArray = LVecArray.*mulArray

Answer 2

[KSSV]

I am not sure what you are trying to do...but this is the code which works:

for LVec = 1:20
    if LVec == 2 || LVec == 4 || LVec == 6 || LVec == 8 || LVec == 10 ||..................
               LVec == 12 || LVec == 14 || LVec == 16 || LVec == 18 || LVec == 20
        LVec = LVec .* -1 ;
    else
        LVec = LVec
    end
end