Creating an alternating list of arrays with a loop in Python

Question

I'm trying to generate an alternating list of arrays with a loop, but I can't figure out the syntax. Currently I'm using the following code (as an example):

[ numpy.array([i,4,5]),numpy.array([31,4,i]) for i in range(5)) ]

It gives the following error:

"SyntaxError: invalid syntax"

I've tried , + and concatenate but it doesn't seem to work.

The desired output is the following list with alternating array entries:

[array([0, 4, 5]),
 array([31,  4,  0]),
 array([1, 4, 5]),
 array([31,  4,  1]),
 array([2, 4, 5]),
 array([31,  4,  2]),
 array([3, 4, 5]),
 array([31,  4,  3]),
 array([4, 4, 5]),
 array([31,  4,  4])]

Thanks everyone!

I think I have what you wanted. Did I guess the right output? — NightShadeQueen
– NightShadeQueen, Commented Jul 7, 2015 at 2:46
Yeah that's it, thanks a lot! Sorry for the confusion, I'll adjust the question — User01
– User01, Commented Jul 7, 2015 at 2:48

Sait · Accepted Answer · 2015-07-07 02:44:24Z

4

What do you want to produce?

In [3]: [ (numpy.array([i,4,5]),numpy.array([31,4,i])) for i in range(5) ]
Out[3]:
[(array([0, 4, 5]), array([31,  4,  0])),
 (array([1, 4, 5]), array([31,  4,  1])),
 (array([2, 4, 5]), array([31,  4,  2])),
 (array([3, 4, 5]), array([31,  4,  3])),
 (array([4, 4, 5]), array([31,  4,  4]))]

Using a for loop, the same thing:

myList = []
for i in range(5):
   item = ( numpy.array([i,4,5]),numpy.array([31,4,i]) )
   myList.append(item)
print(myList)

edited Jul 7, 2015 at 2:44

answered Jul 7, 2015 at 2:38

Sait

19.9k20 gold badges75 silver badges101 bronze badges

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

4 Comments

User01 Over a year ago

Yeah exactly! They should alternate in the list. When I use + instead of , btw, it returns this: [array([31, 8, 5]), array([32, 8, 6]), array([33, 8, 7]), array([34, 8, 8]), array([35, 8, 9])]

User01 Over a year ago

Thanks! I'm trying to get the result without the brackets, so just as list with the items being the arrays, is that also possible?

Sait Over a year ago

@fransheg Can you please edit the original question and write down the exact expected output? I am not sure what you are looking for.

User01 Over a year ago

Sorry for the confusion! I've now added the desired output. The question is now solved in the answer of NightShadeQueen, thanks again.

KlwntSingh · Accepted Answer · 2015-07-07 03:16:50Z

2

There's probably no easy way to get around making a list of list of arrays, so using sum to add all the internal lists together to get one list of arrays. (see documentation here)

In [6]: sum([ [numpy.array([i,4,5]), numpy.array([31,4,i])] for i in range(5) ],[])
Out[6]: 
[array([0, 4, 5]),
 array([31,  4,  0]),
 array([1, 4, 5]),
 array([31,  4,  1]),
 array([2, 4, 5]),
 array([31,  4,  2]),
 array([3, 4, 5]),
 array([31,  4,  3]),
 array([4, 4, 5]),
 array([31,  4,  4])]

edited Jul 7, 2015 at 3:16

KlwntSingh

1,0928 silver badges27 bronze badges

answered Jul 7, 2015 at 2:38

NightShadeQueen

3,3633 gold badges27 silver badges37 bronze badges

1 Comment

NightShadeQueen Over a year ago

Cool. Can you edit your question to provide an example of the output you wanted?

Collectives™ on Stack Overflow

Creating an alternating list of arrays with a loop in Python

2 Answers 2

4 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related