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pythonarrays
Hellfish
Hellfish

Reputation: 1705

How can I add differents elements in the same array in Python?

I am a very new in Python so please forgive me the basic question.

I have an array with 400 float elements, and I need to add the first term with the second and divide by two.

I was trying something like:

x1=[0,...,399]

n = len(x1)

x2 = []

i = 0
for i in range(0,n): 
    x2[i]=(x1[i]+x1[i+1])/2

But it gives me the error: IndexError: list assignment index out of range

Thank you in advance.

Upvotes: 2

Views: 211

Answers (4)

Aleksei astynax Pirogov
Aleksei astynax Pirogov

Reputation: 2491

FP-pythonic way:

x1 = [1.0, 2.0, 3.0, 4.0, 5.0]
x2 = map(lambda x, y: (x + y) / 2, x1, [0] + x1[:-1])

Upvotes: 0

Greg E.
Greg E.

Reputation: 2742

The most succinct way I can think of expressing this:

[(i + j)/2 for i, j in zip(xrange(400), xrange(1,400))]

Or, equivalently:

xs = range(400)
[(i + j)/2 for i, j in zip(xs, xs[1:])]

Obviously, in Python3, xrange is obsolete, so there you could use range instead. Also, in Python3, the default behavior of / changes, so you'd have to use // instead if you want integers.

Upvotes: 1

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 250891

A shorter and faster one-line solution using list comprehensions:

x1=range(0,400)  #use xrange if on python 2.7
x2=[(x1[i]+x1[i+1])/2 for i in range(len(x1)) if i<len(x1)-1]

Upvotes: 1

Andrew Clark
Andrew Clark

Reputation: 208425

The problem here is that you cannot assign a value to an index in a list that is higher than the length of the list. Since you just want to keep adding items to the list, use the list.append() method instead:

n = len(x1)

x2 = []

i = 0
for i in range(n-1): 
    x2.append((x1[i]+x1[i+1])/2)

Note that I also decreased the range by one, otherwise x1[i+1] will cause an IndexError.

Upvotes: 3

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