## Returning string not list for vehicle count from tuples in a list python - python

I need my function to use only a while loop and return the date, from a list of tuples, where the total number of vehicles reaches or exceeds n.
def date_reached_n_vehicles(vehicle_records, n):
"""Returns number of days taken to reach or exceed a total of n vehicles"""
cumulative_total = 0
num_days = 0
index = 0
date_count = None
while index < len(vehicle_records):
cumulative_total += vehicle_records[index][1]
index += 1
num_days += 1
if cumulative_total >= n:
date_count = vehicle_records[index][0]
break
return date_count
Currently for the below test code it is returning
2010-01-03
and not the correct output of
2010-01-02
can someone please tell me where it needs to be change? I think it is iterating one count ahead but not sure how to fix it.
vehicle_records = [('2010-01-01',1),
('2010-01-02',2),
('2010-01-03',3)]
date = date_reached_n_vehicles(vehicle_records, 3)
print(date)

You increase index too soon:
while index < len(vehicle_records):
cumulative_total += vehicle_records[index][1]
if cumulative_total >= n:
date_count = vehicle_records[index][0]
break
index += 1
num_days += 1

Here's a tip: If you've looked at your code and you're still stuck on where the bug is, an easy way for finding bugs in your code is to insert a couple of print calls around the problematic area, printing out relevant variables1. In your case, you probably want to print the value of index before you use it to index vehicle_records:
print('index:', index)
if cumulative_total >= n:
date_count = vehicle_records[index][0]
break
Once you do this, your problem should start to become clear. Here's the output of your modified code:
index: 1
index: 2
2010-01-03
On the first iteration of your loop, index is 1. As you correctly noted, since python list indexing starts at zero, index is one count ahead. Why is this though? Well, right before you index vehicle_records, you're updating index:
while index < len(vehicle_records):
cumulative_total += vehicle_records[index][1]
index += 1
num_days += 1
This is where your problem lies. On the first iteration, index should be zero until after you've indexed vehicle_records, to be able to reach vehicle_records's first element. Thus, you shouldn't only update index until after you've used it:
while index < len(vehicle_records):
cumulative_total += vehicle_records[index][1]
if cumulative_total >= n:
date_count = vehicle_records[index][0]
break
index += 1
num_days += 1
1Although using print is a convenient way to find bugs, you really should take the time to learn to use a debugger. There are plenty of good ones out there. In fact, Python has a debugger that comes shipped with it: pdb

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