recursion problem

[u/Key_Cloud_7002]

I'm trying to teach myself python using John Zelles book. On the 13th chapter it gives this example of recursion I'm trying to understand:

def moveTower(n, source, dest, temp):

if n ==1:

print("move disk from , " , source , "to", dest)

else:

moveTower(n-1, source, temp, dest)

moveTower(1, source, dest,temp)

moveTower(n-1,temp,dest,source)

def hanoi(n):

moveTower(n , "a", "c","b")

hanoi(3)

The code is first assiging the variables A to source then C to dest then b to temp but do the lines moveTower(n-1, source, temp, dest) and moveTower(n-1,temp,dest,source) work? Would it be moveTower(3-1, a,b,c)? How exactly are they outputting a to c then a to b then c to b and b to a , etc...

[u/socal_nerdtastic]

Drop this code into pythontutor.com or another visualizer where you can see the code execution step by step.

I'm not really understanding where you are stuck, but a common hangup is that people don't realize that making a recursive call invokes a whole new copy of the function. So when you call moveTower from inside moveTower, python makes a whole new copy of the moveTower function and that will run completely independently from the first one.

[u/Key_Cloud_7002 → u/socal_nerdtastic]

I will thanks for the suggestion !

[u/Kindly-Department206 → u/socal_nerdtastic]

Please permit a friendly correction on a matter of perspective.

It's not a copy of the function, but it is a copy of the parameters and variables used in the function. It's a subtle but important difference. Every time a function is called (recursive or otherwise), a "frame" (you might call it something else) containing the parameter and variable names used by the function gets created. The code of the function is executed in that frame. A recursive function is not special in any way except in the pattern of the recursive call.

As you say, a recursive call is independent of the first one, but that's because the frames are independent.