Python array logic

Question

I am trying to create a list of lists with the input of m and n, where m is the number of lists within the main list and n is the number of elements within each given list. The grid should contain the integers from start to start + rows * cols - 1 and be ascending. But, every odd numbered row should be descending instead.

The code I've written is returning the expected results, but my automated tester is saying it's incorrect. Maybe my logic is messed up somewhere?

inputs:

start = 1, m = 3, n = 5

expected:

[[1,2,3,4,5],[10,9,8,7,6],[11,12,13,14,15]]

result = []
mylist = []
start = 1

for x in range(0, rows):
    for x in range(0, cols):
        result.append(start)
        start += 1         
for y in range(0, rows):
    if y%2 != 0:
        mylist.append(result[cols - 1::-1])
        del result[cols - 1::-1]
    else:
        mylist.append(result[0:cols])
        del result[0:cols]
return mylist

Andrej Kesely · Accepted Answer · 2019-11-24 22:11:42Z

2

One possible solution, using itertools.count:

from itertools import count

def build(m, n, start=1):
    lst, c = [], count(start)
    for i in range(m):
        lst.append([next(c) for j in range(n)][::-1] if i % 2 else [next(c) for j in range(n)])
    return lst

print(build(3, 5, 1))

Prints:

[[1, 2, 3, 4, 5], [10, 9, 8, 7, 6], [11, 12, 13, 14, 15]]

print(build(3, 0, 1))

Prints:

[[], [], []]

answered Nov 24, 2019 at 22:11

Andrej Kesely

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5 Comments

Roozilla Over a year ago

This also came up incorrect using the automated tester. I'm wondering if it could be that the expected result does not contain spaces between the lists.

Andrej Kesely Over a year ago

@BeanieLeung What are allowed values for m, n and start?

Roozilla Over a year ago

It doesn't state any restrictions on the values

Andrej Kesely Over a year ago

@BeanieLeung Hmm, I don't know then. Could be anything...Maybe the output formatting is wrong...

Roozilla Over a year ago

I must have typed something in incorrectly. This code did work!

Chris Doyle · Accepted Answer · 2019-11-24 21:43:58Z

1

just generate the list of numbers you need which will be n * m, in your case that would generate 0 to 14 in the python range function. However as we want to start at ` then we need to add the start offset too the range end.

Now we can generate all the numbers we need we just need to think about how to create them. well we can add numbers to the list until the list reaches the size of n, then we need to start a new list, However if the list we just finished is an even numbered row then we need to reverse that list.

def build_lists(m, n, start=1):
    data =[[]]
    for i in range(start, n * m + start):
        if len(data[-1]) < n:
            data[-1].append(i)
        else:
            if len(data) % 2 == 0:
                data[-1] = data[-1][::-1]
            data.append([i])
    if len(data) % 2 == 0:
        data[-1] = data[-1][::-1]
    return data


print(build_lists(3, 5))
print(build_lists(6, 3))
print(build_lists(6, 2, 100))

OUTPUT

[[1, 2, 3, 4, 5], [10, 9, 8, 7, 6], [11, 12, 13, 14, 15]]
[[1, 2, 3], [6, 5, 4], [7, 8, 9], [12, 11, 10], [13, 14, 15], [18, 17, 16]]
[[100, 101], [103, 102], [104, 105], [107, 106], [108, 109], [111, 110]]

answered Nov 24, 2019 at 21:43

Chris Doyle

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1 Comment

Roozilla Over a year ago

The automated tester said this code is incorrect as well.

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