CODEWITHSUNDEEP
pythonpandasrolling-sum
Data Science Beginner
Data Science Beginner

Reputation: 39

Rolling sum over an array in Python

I am given an array X and I am asked to return another array Y where y[t] = x[t] + x[t-1] and y[0] = 0 without using a for loop in Python.

What I can think of is using rolling sum but I am not sure if this is the fastest way since I need to convert x into a dataframe/ a series for rolling to work. Is there a faster way to do that? Thanks!

df = pd.DataFrame(X).rename(columns={0: 'X'})
df['Y'] = df['X'].rolling(2).sum().fillna(0)
Y = df['Y'].values

Upvotes: 0

Views: 1135

Answers (3)

Stuart
Stuart

Reputation: 9868

Well, you could use numpy, though that still turns the list into an array.

y = x + np.roll(x, 1)
y[0] = 0

This is fast, short, fairly transparent and doesn't (explicitly) use a for loop.

You could also use map, which is pretty much the same as the list comprehension and doesn't require any external libraries.

y = 0 + map(lambda (a, b): a+b, zip(x, x[1:]))

In Python3 this doesn't work, and instead you would need to write:

y = [0] + list(map(lambda a: a[0]+a[1], zip(x, x[1:])))

or 

y = [0] + list(map(sum, zip(x, x[1:])))

Upvotes: 0

mad_
mad_

Reputation: 8273

In case you are looking for one-liners might not give best performace

from toolz.itertoolz import sliding_window
[0]+map(sum,list(sliding_window(2, [1, 2, 3, 4]))) # [0, 3, 5, 7]

Upvotes: 1

Dani Mesejo
Dani Mesejo

Reputation: 61930

You could use a list comprehension and zip:

x = [1, 2, 3, 4]
y = [0] + [c + n for c, n in zip(x, x[1:])]
print(y)

Output

[0, 3, 5, 7]

This approach relies on built-in functions, so no need to import an external module such as pandas.

Upvotes: 1

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