Getting the next value within for loop

Question

I am writing a program in Python that reads in bank data from a file and stores it in data structures for output at a later time.

I have a list that stores the transactions like

D,520,W,20,D,100

Where the letter is the transaction type (Withdrawl or Deposit) and the numbers are the amount.

I have a for loop that will calculate the balance, but I am having trouble getting to the next element.

Essentially what I want to do is:

for item in theList:
    if item == 'D':
        balance = balance + int(NEXT_ITEM)
    if item == 'W':
        balance = balance - int(NEXT_ITEM)

Thanks for the help

Answer 1

data = 'D,520,W,20,D,100'.split(',')

def pairs(lst):
    it = iter(lst)
    return zip(it, it)

balance = 0
for trans,amt in pairs(data):
    if trans == 'D':
        balance += int(amt)
    else:
        balance -= int(amt)
print(balance)

Answer 2

An easy way, here.

for i, v in enumerate(l):
    if v == 'D':
        balance = balance + int(l[i+1])

Or just read two items at once:

for i in range(0, len(l), 2):
    sl = l[i:i+2]
    if sl[0] == 'W':
        balance = balance - int(sl[1])

Answer 3

data = 'D,520,W,20,D,100'.split(',')
it = iter(data)
balance = sum({'W': -1, 'D': +1}[item] * int(next(it)) for item in it)
print(balance)

Create an iterator and iterate over it. Then you can call next to get the next item.

---

Or without the need of next, by pairing the items of the list via zip:

data = 'D,520,W,20,D,100'.split(',')
balance = sum({'W': -1, 'D': +1}[a] * int(b) for a, b in zip(data[::2], data[1::2]))
print(balance)

---

Or following your example:

theList = 'D,520,W,20,D,100'.split(',')
theIterator = iter(theList)
balance = 0
for item in theIterator:
    if item == 'D':
        balance = balance + int(next(theIterator))
    if item == 'W':
        balance = balance - int(next(theIterator))
print(balance)

Answer 4

If your data is pairs of a transaction type code and a transaction amount, the natural data type is a list of dictionaries or a list of tuples. Or named tuples if you like. Other answers are showing how you can work around your choice of a flat list, but I think the best fix is to keep the bundling of the associated elements in the list you create from your file:

data = [('D', 520), ('W', 20), ...]

Or if your data is as simple as shown here, a list of signed numbers. Probably of the decimal.Decimal type unless you're dealing with whole dollars only.

I am assuming from your description that the creation of the list from your file is under your control. If not, I think Hugh Bothwell's answer is the cleanest way to adjust.