Accessing rows of an array, inside an array of arrays?

Question

Say i have:

H = [array(a), array(b), array(c)...]

a = [[1,2,3,4,5,6],
     [11,22,33,44,55,66], #row 1 of H[0]
     [111,222,333,444,555,666]]
b = [[7,8,9,0,1,2],
     [77,88,99,00,11,22],
     [777,888,999,000,111,222]]
c = ...

I want to access row 1 of H[0], but then move onto accessing the rows within H[1] and so on.

My question(s):

1) How do i call on row 1 of H[0]?

2) How do i then, after going through all rows in H[0], make a loop run to H[1]?

I know i need something like

for i in range(len(H)):
    do stuff to rows until the last row in H[i]
    move onto H[i+1]
    do same stuff
    stop when (criteria here reached)

All help is appreciated. Thank you.

Edit: I now know I can access this by H[0][1] but i actually need the 0th column values within each row of the arrays. So, i need 11 from H[0][1]. How do i do this?

Row 1 of H[0] should simply be H[0][1], no?

Joachim Isaksson
– Joachim Isaksson

2016-03-16 17:24:13 +00:00
Commented Mar 16, 2016 at 17:24 — Joachim Isaksson
– Joachim Isaksson, Commented Mar 16, 2016 at 17:24
I definitely didn't realize that..

layces
– layces

2016-03-16 18:14:02 +00:00
Commented Mar 16, 2016 at 18:14 — layces
– layces, Commented Mar 16, 2016 at 18:14

Mikhail Gerasimov · Accepted Answer · 2016-03-16 19:25:36Z

4

H = [a, b, c]

Right? If so, then answers:

1)

H[0][1]  # 0 - "a" array; 1 - row with index 1 in "a"

2)

for arr in H:  # for array in H: a,b,c,...
    for row in arr:  # for row in current array
        # do stuff here

Upd:

One more example shows iterating through arrays, their's rows and elements:

a = [[1, 2],
     [3, 4]]
b = [[5, 6],
     [7, 8]]


H = [a, b]

for arr_i, arr in enumerate(H):
    print('array {}'.format(arr_i))

    for row_i, row in enumerate(arr):
        print('\trow {}'.format(row_i))

        for el in row:
            print('\t{}'.format(el))

Output:

array 0
    row 0
    1
    2
    row 1
    3
    4
array 1
    row 0
    5
    6
    row 1
    7
    8

edited Mar 16, 2016 at 19:25

answered Mar 16, 2016 at 17:33

Mikhail Gerasimov

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4 Comments

layces Over a year ago

For that second part, how would i separate my actions/what i'm doing between those two? So if i had H[i][j] and for arr in H: acted on i but for row in arr: acted on j , i'm confused on where exactly i would place my i+=1 or j+=1 @germn

Mikhail Gerasimov Over a year ago

@TC, in most cases you don't need i, j. Python is not C: in python you work with objects in list, not with it's indexes. In case you really need index of element, you can use enumerate(), see: stackoverflow.com/a/522578/1113207

layces Over a year ago

Okay so i had improperly named my own things, so at first i wasn't getting a proper recreation, but now i am! My very last question is still: how do i specifically only call on the very first column values for each row? Ex: 1,3,5,7 in your output example? I do not know what they are /called/, using el, row_i or row, and arr_i or arr ?

layces Over a year ago

I just ended up erasing all the other columns and backing it up into a separate list of lists. Thank you though, this as a whole answered my questions best.

ml-moron · Accepted Answer · 2016-03-16 19:06:29Z

2

What about:

for array in H:
    for row in array:
        # do stuff

This loop automatically moves on to the next array when the current array is finished.

If you need indices for the arrays then something like:

for array in H:
    for i, row in enumerate(array):
        for j, value in enumerate(row):
            # do stuff

edited Mar 16, 2016 at 19:06

answered Mar 16, 2016 at 17:30

ml-moron

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5 Comments

layces Over a year ago

Huh, so where would i put my break in this? Would it be at the end of the first for-loop? Or would it be better to put all of this in a while: ?

ml-moron Over a year ago

where do you want to put a break? you didn't mention it in your question.

layces Over a year ago

I need it to stop at the point i labeled stop when (criteria here reached). And thank you so much for your added part to the answer, can i use that to access only a certain column value inside of H[i][j]? So, in my example, if i only needed 11 from H[0][1]?

ml-moron Over a year ago

yeah, sure, if i == 0 and j == 1: # do stuff

ml-moron Over a year ago

Not sure what your break criteria is, but you can put an if...break statement in the inner loop if you're ok with it stoppint in the middle of a row.

Jules · Accepted Answer · 2016-03-16 17:35:13Z

It is difficult to understand what you are trying to do.

From what I understand though you are trying to loop through H and then thought the arrays that comprise it.

To do that you can do something like

for array in H:
    for sub_array in array:
        # do stuff

Otherwise if you want H[1] to access the information in a[1] and for H[3] to access the information in b[1] you can flatten the list. There are two ways to do this

flat_list = sum(H, [])

or

import itertools
for sub_array in itertools.chain(H):
    # do stuff

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