multiplying in a list python

Question

I am doing some homework and I am stuck. I supposed to write a code in Python 3, that reads a text file that is like a bill with an amount of numbers. I am supposed to write the code so it will calculate the total amount.

I figured that a bill(simple example) will contain a number and the a prise. like:

2 10$
1 10 $
and so on

So I figured that I create a list with all numbers in it and then I want to multiply the first with the second element in the list and then jump in the list so the tird and the fourth element get multiply and so on, until there is no more numbers in my list. During this happens I want the sums of every multiplication in a new list called sums there they later will be summed.

my code looks like this so far:

file = open('bill.txt')
s = file.read()
file.close()



numbers = []
garbage = []

for x in s.split():
    try:
        numbers.append(float(x))
    except ValueEror:
        garbage.append()
print(numbers)


for n in numbers:
    sums = []
    start = 0
    nxt = start + 1
    t = numbers[start]*numbers[nxt]    
    if n <= len(numbers):
        start += 2
        nxt += 2
        summor.append(t)
    if n == len(numbers):
        print(sum(sums))

Answer 1

This may be a typo when you entered it but on your 2nd line of data there is a space between the amount and $ and no space on the line above. This will cause some problems when striping out the $.

Try this:

total = 0
with open('data.txt', 'r') as f:
    lines = f.readlines()
    amounts = []
    garbage = []
    for line in lines:
        try:
            multiples = line.split()
            multiplier = multiples[0]
            amount = multiples[1].strip('$\n')
            amounts.append(int(multiplier)*float(amount))
        except:
            garbage.append(line)
    total = sum(amounts)
print(total)

With some minor formatting on output or the text data you should be able to get the result you want. Here is the txt data I used:

2 10$
1 10$
3 20.00$

and the output is:

90.0

Also, you may want to get away from file = open('data.txt', 'r') which requires you to explicitly close your file. Use with open('data.txt', 'r') as f: and your file will implicitly be closed.

Answer 2

Firstly, I would say, quite a good attempt. If you don't want to be spoiled, you should not read the code I have written unless you try out the corrections/improvements yourself. (I am not a pro)

Well, like you said every line contains 2 numbers that need be multiplied, you should start by reading line by line & not individual numbers as that might get troublesome.

file = open('bill.txt')
s = file.readlines()
file.close()

numbers = []
garbage = []

for line in s:
    try:
        line.strip('$') # removes the trailing $ sign for easy number extraction
        numbers.append(map(int, line.split()))
    except ValueEror:
        garbage.append(line)
print(numbers)

Later, when summing up the multiplication of the number & the bills, you probably won't require a list for that, instead a sum integer variable should be enough.

sum = 0
for num, bill in numbers:
    sum += num * bill
print("The total amount is", sum)

Answer 3

The problem in your code is likely that you keep resetting sums for every number in the list. So you basically keep forgetting the sums you previously collected. The same applies to start and nxt. If you move these definition outside of the loop, and also fix the summor.append(t) to sums.append(t) it should almost work. Also note that the loop variable n does not iterate over indexes of numbers but over the actual items (i.e. the numbers). So the check at the end n == len(numbers) is unlikely to do what you expect. Instead, you can just put the print after the loop because you know that when you reached that point the loop is over and all numbers have been looked at.

---

A common idiom is to use zip to combine two elements with each other. Usually, you use zip on two different iterators or sequences, but you can also use zip on the same iterator for a single list:

>>> numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> it = iter(numbers)
>>> for a, b in zip(it, it):
        print(a, b)

1 2
3 4
5 6
7 8

As you can see, this will group the numbers automatically while iterating through the list. So now you just need to collect those sums:

>>> sums = []
>>> it = iter(numbers)
>>> for a, b in zip(it, it):
        sums.append(a * b)

>>> sum(sums)
100

And finally, you can shorten that a bit more using generator expressions:

>>> it = iter(numbers)
>>> sum(a * b for a, b in zip(it, it))
100

Answer 4

Several things:

• sums = [] is inside the loop for n in numbers. This means you're resetting the sums list every time. You should move this outside the loop.
• The same applies to start = 0 and nxt = start + 1. These values now keep being reset to 0 and 1 respectively for each number in the list.
• You are comparing if n <= len(numbers): but what is n? It's an actual value in your list of numbers, it doesn't represent the position in the list. You should replace this with while start < len(numbers): - as long as the starting position is still within the list we should keep going. This also means you can remove n from your code.

The code now becomes:

sums = []
start = 0
nxt = start + 1
while start < len(numbers):
    t = numbers[start]*numbers[nxt]    
    start += 2
    nxt += 2
    sums.append(t)

print(sum(sums))