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amy989
amy989

Reputation: 313

Python - change multiple elements in list based on indices

I'm new to python.. My question: I have a list rt, and a list with indices rtInd, which varies in size and content.

For all the indices in rtInd, I want to change the corresponding elements in rt.

Example: rt = [10,20,30,40]; rtInd = [2,3]

What I want:

rt[2] = 30 + x

rt[3] = 40 + x

Can someone help me?

Thanks in advance!

Amy

Upvotes: 0

Views: 1244

Answers (2)

ShadowRanger
ShadowRanger

Reputation: 155333

If rt is a numpy array, you could index with the list (or numpy.array) of indices itself (must not be a tuple, to be clear, due to tuples being used for multidimensional access, not a set of indices to modify):

# Setup
rt = numpy.array([10, 20, 30, 40], dtype=np.int64)
rtInd = [2, 3]  # Or numpy.array([2, 3])
x = 123  # Arbitrary value picked

# Actual work
rt[rtInd] += x

and that will push the work to a single operation inside numpy (potentially improving performance).

That said, numpy is a pretty heavyweight dependency; if you're not already relying on numpy, I'd stick to the simple loop described in M Z's answer.

Upvotes: 0

M Z
M Z

Reputation: 4799

If you have a list of indices, and you have x, you can loop through and update the values at the corresponding indices:

rt = [10,20,30,40]
rtInd = [2,3]
x = 10

for i in rtInd:
    rt[i] += x

# result:
# rt = [10, 20, 40, 50]

Upvotes: 1

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