replacing and adding to a slice of array in python3

Question

I want to make a slice of array and then assign each element of a slice to a number and then update according to indices I provide.

For example:

[0, 0, 0, 0, 0] --> initial array

I want to assign 100 to first two elements i.e

a[0:2] = 100 for j in range(0,2)

so now the array becomes

[100, 100, 0, 0, 0]

now if i want to add 100 to the first three elements so the array should now become

[200, 200, 100, 0, 0]

what's the correct way to do this? I am getting a syntax error for the following code:

def arrayManipulation(n, queries):
n = 10
//queries is list of [[starting index, ending index, value to be updated]]
initialArray = [0]*n;
for i in queries:
    firstIndex = i[0]-1
    secondIndex = i[1]
    initialArray[firstIndex:secondIndex] = ((initialArray[j] += i[2]) for j in range(firstIndex, secondIndex))
    print(initialArray)

vash_the_stampede · Accepted Answer · 2018-09-27 01:24:34Z

1

Use enumerate here It will let you access the indexs you are interested in and assign values or manipulate them

l = [0, 0, 0, 0, 0]

for idx, item in enumerate(l[:2]):
    l[idx] = 100

for idx, item in enumerate(l[:3]):
    l[idx] += 100

print(l)

*Note : Initial assignment can be done via (@ShadowRanger)

l[:2] = [100] * 2

edited Sep 27, 2018 at 1:24

answered Sep 26, 2018 at 23:14

vash_the_stampede

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2 Comments

ShadowRanger Over a year ago

Note that for simple assignment (rather than mutation), you can do slice assignment more simply/efficiently, e.g. for setting the initial 100s, l[:2] = [100]*2. Bulk mutation (e.g. l[:3] += 100) would only work with third party types like numpy arrays.

vash_the_stampede Over a year ago

@ShadowRanger Yep that is correct I do know that, I'm still fresh to programming, I have brief use of some techniques as such that slip my mind when responding but yes, I agree and should have known

Olivier Melançon · Accepted Answer · 2018-09-27 01:24:26Z

0

The common way is to use range and a loop.

a = [0, 0, 0, 0]

for i in range(2):
    a[i] = 100

for i in range(3):
    a[i] += 100

print(a) # [200, 200, 100, 0]

Although, it is good to know that Python supports slice assignation.

a = [0, 0, 0, 0]

a[0:2] = [100, 100]

# To some extent, this even can be done
a[0:3] = map(lambda x: x + 100, a[0:3])

print(a) # [200, 200, 100, 0]

edited Sep 27, 2018 at 1:24

answered Sep 27, 2018 at 1:18

Olivier Melançon

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