alternateColumns

pict = makePicture(pickAFile())
width = getWidth(pict)
height = getHeight(pict)
newPict = makeEmptyPicture(width,height)

def alternateColumns(w,pict,newPict):
#@param:
#  w: int, column width, must be a divisor of width;
#  pict: picture;
#  newPict: an empty picture that has the same size of "pict";
  for i in range(0,width,width/(width/(2*w))):
    moveColumnFromAToB(i,w)
    moveColumnFromBToA(i,w)
  show(newPict)

def moveColumnFromAToB(i,w):
#@param:
#  i: int; first pixel on the x axis
#  w: int, column width, must be a multiple of width;
  for y in range(0,height):
      for x in range(i,i+w):
        pix = getPixel(pict,min(width-1,x),y)
        nPix = getPixel(newPict,min(width-1,x+w),y)
        nc = getColor(nPix)
        c = getColor(pix)
        setColor(nPix,c)

def moveColumnFromBToA(i,w):
#@param:
#  i: int; first pixel on the x axis
#  w: int, column width, must be a multiple of width;
    for y in range(0,height):
      for x in range(i+w,i+2*w):
        pix = getPixel(pict,min(width-1,x),y)
        nPix = getPixel(newPict,min(width-w,x-w),y)
        nc = getColor(nPix)
        c = getColor(pix)
        setColor(nPix,c)