list index out of range for while loop

Question

def main():
    infile = open('charge_accounts.txt', 'r')
    chargeAccounts = infile.readlines()

    index = 0
    while index < len(chargeAccounts):
        chargeAccounts[index] = chargeAccounts[index].rstrip('\n')
        index += 1

    userAccount = input("Please enter a charge account number: ")

    count = 0
    while userAccount != chargeAccounts[count] or chargeAccounts[count] == chargeAccounts[17]:
        count += 1


    if chargeAccounts[index] == userAccount:
        print("The account number", userAccount, "is in the list.")
    else:
        print("The account number", userAccount, "in not in the list.")


main()

I'm trying to make a code in python that has the user input a number and checks to see if that number is in a list. When I try to run this code, I get an error that says that the list index is out of range in the while loop. There are only 18 items in the list, and I have the while loop set to terminate at 17(which should be 18 in the list). Edit: It also doesn't work if the while loop is set to chargeAccounts[count] != chargeAccounts[17]:

Here's the exact error code:

Traceback (most recent call last):
  File "F:/CPT 168/Ch 7/AndrewBroughton_Chapter7_Excersizes/7-5/7-5.py", line 23, in <module>
    main()
  File "F:/CPT 168/Ch 7/AndrewBroughton_Chapter7_Excersizes/7-5/7-5.py", line 13, in main
    while userAccount != chargeAccounts[count] or chargeAccounts[count] != chargeAccounts[17]:
IndexError: list index out of range

Here's the content of the original text file:

5658845
4520125
7895122
8777541
8451277
1302850
8080152
4562555
5552012
5050552
7825877
1250255
1005231
6545231
3852085
7576651
7881200
4581002

Answer 1

You get the error at the string if chargeAccounts[index] == userAccount: because index is already bigger than the index of the last element of the list (because you left the loop with this index above).

I would recommend you to follow a few rules to work with lists that could save you from similar errors with indices.

1. Use for-loop instead of while-loop, if you lineary go through each element.
2. Use break if you want to leave the loop and keep the index

So you code may look like this:

with open('charge_accounts.txt', 'r') as infile:
    chargeAccounts = infile.readlines()

for index in range(len(chargeAccounts)):
    chargeAccounts[index] = chargeAccounts[index].strip()

userAccount = input("Please enter a charge account number: ")

found = False
for chargeAccount in chargeAccounts:
    if chargeAccount = userAccount:
        found = True
        break

if found:
    print("The account number", userAccount, "is in the list.")
else:
    print("The account number", userAccount, "in not in the list.")

Answer 2

A while loop will keep looping as long as its condition is True.

count = 0
while userAccount != chargeAccounts[count] or chargeAccounts[count] == chargeAccounts[17]:
    count += 1

If I enter an invalid userAccount to your program, the first part of the condition userAccount != chargeAccounts[count] is always going to be True. That makes the entire condition True, since you're using or logic.

Furthermore, if you want to check to see if you've reached the end of a list, you don't have to check the contents of the last list element (chargeAccounts[count] == chargeAccounts[17]). Check the length instead (count == len(chargeAccounts)).

To fix this, change that loop condition to something like

while count < len(chargeAccounts) and userAccount != chargeAccounts[count]:

(I'm not sure if this is exactly what you need, because I don't really follow the logic of your program. This should get you past the current error, though.)