swift - The Position of SpriteNode in SpriteKit -

i getting start spritekit, , in project. wanna create grids falling sky , form grids map follow:

i created physics world gravity -9.8

and grid, each of them created following constructor , add scene:

class numbertile:skspritenode{      convenience init(xpos:cgfloat, ypos:cgfloat){         var tilecolorarray:nsarray = ["#996666","#a65959","#b34d4d","#bf4040","#cc3333","#d92626","#e61919"]          //get random color array         var fillcolor:uicolor = uicolor(rgba:  tilecolorarray.objectatindex(int(arc4random_uniform(uint32(tilecolorarray.count)))) as! string)          var size:cgsize = cgsize(width: 38, height: 38);         self.init(color:fillcolor, size:size)          self.position = cgpointmake(xpos, ypos)         self.anchorpoint = cgpointmake(0,0)          self.physicsbody = skphysicsbody(             rectangleofsize: cgsize(width: 38,height: 40),             center: cgpointmake((cgfloat)(38/2),(cgfloat)(40/2)))          self.physicsbody?.mass = 1000         self.physicsbody?.friction = 1.0         self.physicsbody?.allowsrotation = false;     } } 

after running function drop grids sky. in case 6th column's height different others. happens randomly, not must happen in 6th column, idea?

//update adding tile creation code

func addtileinrow(row: nsinteger){         var xpos:cgfloat = (cgfloat)(self.counter - 1) * (38 + (2)) + (2)/2;         var ypos:cgfloat = (self.headersprite.frame.origin.y - self.headersprite.frame.size.height)          var tile:numbertile = numbertile(xpos: xpos, ypos: ypos)         addchild(tile)     } 

the counter start 1, when counter larger 7, reset 1 until 8 rows created.