Bubble sort is the first sorting algorithm that you learn in many algorithm classes.
It's not a particularly fast technique, so it's never really used in practice. And generally if you're writing your own sorting algorithm instead of relying on one that your platform provides then you've made some poor decisions.
However bubble sort is simple enough that anyone can understand it, and most programmers can implement it or verify it.
Preparing for visualization
Before we get to the algorithm itself we need to create some useful functions for visualizing what it does.
We're going to be sorting a list of randomly selected numbers. So we'll need a way to visualize an array of numbers.
For this we'll make a simple bar graph. We'll need functions for drawing lines, for erasing lines, and (building on top of that) for drawing arrays of lines.
;export drawing to bar :side_a :side_b penwidth :side_b/2 fd :side_a back :side_a pu rt 90 fd :side_b lt 90 pd end to blank :side_b color [255 255 255] bar 500 :side_b pu lt 90 fd :side_b rt 90 pd end to min :a :b ifelse :a < :b output :a output :b end to showList :list :len :n setxy 0 500 make "index 0 make "b divide 500 :len make "b int :b + 1 make "prev 0 repeat min :len :n+1 [ blank :b color [0 250 :index*3] if (item :index :list) < :prev color [250 0 :index*3] bar item :index :list :b make "prev (item :index :list) make "index :index + 1 ] end
Initializing a random list
Now we need a function for generating a list of random numbers.
;export generateList to generateList :len make "list  make "index 0 repeat :len [ make "val rand 501 setitem :index :list :val make "index :index + 1 ] output :list end
And finally we're onto the bubble sort algorithm itself.
Structurally it's a loop, with an inner loop (i.e a nested loop). In the inner loop it compares two adjacent items from the list. If the first one is bigger than the second one, it's swaps them.
;export bubblesort ;import drawing to bubblesort :list :len make "n :len repeat :len - 1 [ make "i 0 repeat :n - 1 [ make "A1 item :i :list make "A2 item :i + 1 :list if :A1 > :A2 [ SETITEM :i :list :A2 SETITEM :i + 1 :list :A1 ] make "i :i + 1 ] showList :list :len :n make "n :n - 1 ] end
;import bubblesort ;import generateList make "len 20 + rand 20 make "list generateList :len showList :list :len :len bubblesort :list :len