I need help making two things happening at once

I m trying to make it so that my velocity based movement has friction

this is my code

if v > 0 then v-=1 end if v < 0 then v+=1 end

For realistic friction, I recommend something like the following:

trunc = function(x)
  if floor(x) == x then
    return x
  end
  if x > 0 then
    return floor(x)
  end
  return ceil(x)
end

update = function()
  v = trunc(v * 0.8)
end

thanks