In Python, how do I restart a for loop within a while loop? The furthest out loop is a while loop that runs while r < y(length of integer list)

Question

The integer list is essentially a random integer list of mostly integers in an excel file but there are some blocks of zeroes I want to identify. Ex. [3,3,3,0,0,4,0,6,0,0,7]. I just want to run through the list so that I can print: first block of zeroes start at index 3 and end at index 4, second block etc.

The list is a single column in excel, and spans thousands of rows.

The way I am doing that is with the following code:

r = 1
while r+1000 < y:
   for q in range(r,y-1):
      if sheet.cell_value(q,3) == '0':
          start = q
          print(str(start))
          for e in range(q,y-1):
              if sheet.cell_value(e,3) != 0:
                  r = e
                  print(str(end))
                  break
          break

The prints are there so I can check if it is going right. But what's happening now in the output is it never stops and only prints the start value.

Don't use nested loops. Instead, just run through all the rows once, but use a variable to remember between rows whether or not you're within a block of zeroes. — Karl Knechtel, Apr 05 '20 at 21:43
could you please give me a quick example of what you mean? how would you make a variable to remember blocks of zeroes? — Isaac Naim, Apr 05 '20 at 21:45
All the variable's value needs to be is a boolean, "are we currently in a block of zeroes or not?". Come up with a rule that looks at the current value of the variable, and whether or not the current row is a zero, and decides whether we are still in the block, and if we should print a start or an end value. — Karl Knechtel, Apr 05 '20 at 21:47
while troubleshooting this, I would add something like `iterations = 0 while(r+1000 < y) and (iterations < 10000):` and have `iterations += 1` in the code. that way, if your intended end, based on `r` and `y` doesn't come to pass, you will break out at some point. — JL Peyret, Apr 05 '20 at 21:48
Haha I hate to keep bugging you, you've given me helpful information so far, but IM not there yet. Maybe you could be more specific to my example? — Isaac Naim, Apr 05 '20 at 21:50
To simplify a lot your problem you should also read your excel file and convert it into a "python" format. For this case, you could use a list or a numpy array and look for consecurive zeros, e.g. https://stackoverflow.com/questions/24885092/finding-the-consecutive-zeros-in-a-numpy-array — Mathieu, Apr 05 '20 at 21:50
Thank you so much @Mathieu! That question solves my problem completely! How do I give you credit? — Isaac Naim, Apr 05 '20 at 21:56
@IsaacNaim As it is not an answer but a comment, and as your question is actually already answered elsewhere, you can't. Glad I could help, have a good evening :) — Mathieu, Apr 05 '20 at 22:30

Todd · Accepted Answer · 2020-04-05T22:50:30.267

You could do something similar to this to get a listing of where every group of 0's start and end.

>>> li = [3,3,3,0,0,4,0,6,0,0,7]
>>> tracking_zeros = False
>>>
>>> for i, n in enumerate(li):
...     if n == 0:
...         if not tracking_zeros:
...             print(f"start zeros: {i}")
...             tracking_zeros = True
...     else:
...         if tracking_zeros:
...             print(f"  end zeros: {i}")
...             tracking_zeros = False
...             
start zeros: 3
  end zeros: 5
start zeros: 6
  end zeros: 7
start zeros: 8
  end zeros: 10

Not counting single zeroes:

>>> tracking_zeros = False
>>>
>>> for i, n in enumerate(li):
...     if n == 0:
...         if not tracking_zeros and li[i+1] == 0:
...             print(f"start zeros: {i}")
...             tracking_zeros = True
...     else:
...         if tracking_zeros:
...             print(f"  end zeros: {i}")
...             tracking_zeros = False
...             
start zeros: 3
  end zeros: 5
start zeros: 8
  end zeros: 10

If the last item in the list is a 0, it'll throw an index error so you might need to account for that.

Could just append a non-zero value at the end before you do the loop to prevent that: li.append(1)

In Python, how do I restart a for loop within a while loop? The furthest out loop is a while loop that runs while r < y(length of integer list)

1 Answers1