How can I create an external loop for going through elements of the array?

Question

I'm dealing with such script in python:

In the input I have such array:

a = [2,4,7,2]

On the output I need to return elements of the array, starting with the first one

[2]
[2, 4]
[2, 4, 7]
[2, 4, 7, 2]

How can I retrieve elements, starting with the second and further elements:

//start with a[1]
[4]
[4, 7]
[4, 7, 2]

//start with a[2]
[7]
[7,2]

//start with a[3]
[2]

So, by now my code is looking like this:

def ar(a):
    result = 0
    res_arr = []
    for i in a:
        res_arr.append(i)
        k = sum(res_arr)
        print (res_arr,k)

but I understand, that I need to add external loop, for going through all elements of the array. What is the most efficient way for writing it?

Do you want the lists starting with the second and further elements in the output, or is that your current strategy for producing the result in the second code block? — TigerhawkT3, Oct 30 '16 at 09:43

score 3 · Accepted Answer · edited May 23 '17 at 11:48

3

In Python you can use slicing (see this question for details).

For example,

for i in range(1, len(a) + 1):
    print(a[:i])

will output

[2]
[2, 4]
[2, 4, 7]
[2, 4, 7, 2]

To start with nth element you can use this code:

n = 1  # Start with second element (list indexes start with 0)
for i in range(n + 1, len(a) + 1):
    print(a[n:i])

which will output:

[4]
[4, 7]
[4, 7, 2]

edited May 23 '17 at 11:48

Community

1
1

answered Oct 30 '16 at 09:00

Franz Wexler

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TigerhawkT3 · Answer 2 · 2016-10-30T09:36:17.390

1

Just loop over a range of values to use in a slice:

def ar(a):
    for i in range(1, len(a)+1):
        print(a[:i])

Result:

>>> ar([1,2,3])
[1]
[1, 2]
[1, 2, 3]

You can start at an arbitrary index by simply adding it in a few key places:

def ar(a, start):
    for i in range(1+start, len(a)+1):
        print(a[start:i])

Result:

>>> ar([1,2,3], 1)
[2]
[2, 3]

edited Oct 30 '16 at 09:36

answered Oct 30 '16 at 08:58

TigerhawkT3

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This is not the question. Daniel is asking how to print the array from the second or further element. – Hugal31 Oct 30 '16 at 09:30
@Hugal31 - OP says "I need to return elements of the array, starting with the first one." I think the part about starting from the second element is part of his attempt at a solution. That attempt has a function that _only takes one argument_ - the list. There is no way for that function to start at an index of the user's choosing. Of course, it's trivial to add, and I'll do so. – TigerhawkT3 Oct 30 '16 at 09:35

score 0 · Answer 3 · answered Oct 30 '16 at 09:04

You can use the slices.

With you code:

def ar(a, start=0):
    result = 0
    res_arr = []
    for i in a[start:]:
        res_arr.append(i)
        k = sum(res_arr)
        print (res_arr,k)

With a more pythonic code:

def ar(a, start=0):
    for i in range(start + 1, len(a) + 1):
        print(a[start:i], sum(a[start:i]))

How can I create an external loop for going through elements of the array?

3 Answers3