Can this loop be done in a list comprehension?

Question

Comming from this question Adding the last position of an array to the same array

I was curious if the mentioned loop can be done in a list comprehension?

array = [3,4,2,5,4,5,8,7,8,9]
value = 10

for i in range(1,10):
   array[i] = array[i-1] + value

I thought maybe with the walrus operator.

My attempt gave me an error which lead to cannot-use-assignment-expressions-with-subscript

[array[count] := val if count == 0 else array[count] := array[count-1] + value for count,val in enumerate(array)]

Any ideas?

A list comprehension creates a new list, so it's not clear why you'd want to use one. — jonrsharpe
– jonrsharpe, Commented Jan 2, 2022 at 22:27
Is [3, 13, 23, 33, 43, 53, 63, 73, 83, 93] output of code? only depend on first item? — Amirreza Noori
– Amirreza Noori, Commented Jan 2, 2022 at 22:40
list(range(array[0], array[0] + len(array) * value, value)) — kaya3
– kaya3, Commented Jan 2, 2022 at 23:58
Amirreza Noori, it is as far as I understood from the 1. linked question. — RSale
– RSale, Commented Jan 3, 2022 at 0:00

user7864386 · Accepted Answer · 2022-01-03 00:15:42Z

1

The for-loop only uses the first element and overwrites array with brand new items. The very same outcome can be achieved using the below list comprehension:

array = [array[0] + i*value for i in range(len(array))]

Output:

[3, 13, 23, 33, 43, 53, 63, 73, 83, 93]

answered Jan 3, 2022 at 0:15

user7864386

Sign up to request clarification or add additional context in comments.

Comments

BrokenBenchmark · Accepted Answer · 2022-01-03 00:20:31Z

0

We can make the observation that, if our resulting array is result, then result[i] = array[0] + i * value.

For example:

result[0] = array[0]
result[1] = result[0] + value = array[0] + 1 * value
result[2] = result[1] + value = array[0] + 2 * value
etc.

It follows from the code that this can generally be expressed as:

result[0] = array[0]
result[i] = result[i - 1] + value for i > 0.

Then, the list comprehension becomes:

result = [array[0] + i * value for i in range(len(array))]

edited Jan 3, 2022 at 0:20

answered Jan 2, 2022 at 22:49

BrokenBenchmark

19.4k7 gold badges26 silver badges40 bronze badges

1 Comment

RSale Over a year ago

BrokenBenchmark, sorry but this is not the same as the result from the above loop.

MK14 · Accepted Answer · 2022-01-03 23:01:11Z

0

The error occured because array[count] is not a variable identifier, it is instead a position in the list array and thus you can't assign anything to it.

Instead you can use the following...

array = [3,4,2,5,4,5,8,7,8,9]
value = 10

res = [(first_val := array[count]) if count==0 else array[count-1]+(count*value) for count,val in enumerate(array)]

print(res)
print(first_val)

Output:-

[3, 13, 14, 12, 15, 14, 15, 18, 17, 18]
3

edited Jan 3, 2022 at 23:01

answered Jan 2, 2022 at 23:16

MK14

5163 silver badges9 bronze badges

3 Comments

RSale Over a year ago

MK14, I got your result with another lc.However, this is not the same as the result from the above loop.

RSale Over a year ago

I just ran that code and got [3, 13, 23, 33, 43, 53, 63, 73, 83, 93]

MK14 Over a year ago

@RSale Found the bug, it was in the line array[count-1] + value, here the elements from the main array are directly taken, but in your code, the elements are constantly being updated after adding the value, thus that's why it is was giving a different answer.

Collectives™ on Stack Overflow

Can this loop be done in a list comprehension?

3 Answers 3

Comments

1 Comment

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related