Python list sorting?

Question

An example of what I want to do:

What I want to do is have the first element equal itself, and every other equals itself minus the previous.

array = [12, 16, 31, 53, 70]
new = []
for x in range(5):
for value in array:
value -= array[x]
new.append(value)

#print new
#should be new = [12, 4, 15, 22, 17]

I'm running it in my interpreter and getting this:
[0, 4, 19, 41, 58, -4, 0, 15, 37, 54, -19, -15, 0, 22, 39, -41, -37, -22, 0, 17,
-58, -54, -39, -17, 0]

Is my code way off, any suggestions to get it working?

EDIT:

Ok getting really close here. Changed the code to this:

array = [12, 16, 31, 53, 70]
new = []
for x in range(0,5):
y = array[x] - array[x-1]
new.append(y)

now i'm getting
[-58, 4, 15, 22, 17]

I'll keep working on it, thanks for your response

array = [12, 16, 31, 53, 70]
new = []
new.append(array[0])
for x in range(1,5):
y = array[x] - array[x-1]
new.append(y)

Guess its worked out

Answer 1

I'm not a programming expert, but it looks like you've got a double loop. The code:

for x in range(5):
for value in array:

is causing two loops. Thats why you get so many entries in your list. You need two make only one loop. I would use a couple of variables to store the data that will be subtracted.
I would do it like this:

array = [12, 16, 31, 53, 70]
new = []
subtract = 0
for value in array:
{
value = array[x] - subtract
new.append(value)
subtract = subtract + array[x]
}

First we setup a variable to hold what we want to subtract, we start with zero because we don't want to subtract anything. Then we set value to the list item - 'subtract'. we then append the value to the new list and then add the list item to 'subtract' to have it subtract the new value the next round. The 'subtract' variable should keep its value and add the list items like you want.

Answer 2

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