Question

Can someone please explain the following line of code? Is it some kind of nested for loop? If so can someone rewrite it as an equivalent nested for loop. allPositions parameter is a list and synapsesPerSegment is an int variable.

for rx,ry in random.sample(allPositions, synapsesPerSegment):

Answer 1

It’s a normal loop. No nesting. random.sample returns a list of elements from allPositions, taking synapsesPerSegment many items. As the variables being assigned to in the for loop are a tuple in the form (rx, ry), this suggests that allPositions is a list (or collection) of tuples in the form (rx, ry), which are assigned to rx and ry each iteration. If you have a list of tuples, the for loop ‘unpacks’ them each iteration to those variables. For example if you have

(a, b) = (99, 100)

then this assigment will unpack:

(c, d) = (a, b)

so that c == 99 and d == 100.

To get back to the question, here is an walk through with some example data:

if we say:
allPositions = [(1,100), (2, 200), (3, 300), (4, 400)]

and, for example:
synapsesPerSegment = 3

then
random.sample(allPositions, synapsesPerSegment) might produce [(3, 300), (1,100), (2, 200)] because it takes 3 items from allPositions at random.

then iterating over that:

• Iteration 1: rx = 4, ry = 400
• Iteration 2: rx = 1, ry = 100
• Iteration 3: rx = 2, ry = 200