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 

Answer 1

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:

[[], [], []]

• Roozilla: 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: @BeanieLeung What are allowed values for m, n and start?
• Roozilla: It doesn't state any restrictions on the values
• Andrej Kesely: @BeanieLeung Hmm, I don't know then. Could be anything...Maybe the output formatting is wrong...
• Roozilla: I must have typed something in incorrectly. This code did work!

Answer 2

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]]

• Roozilla: The automated tester said this code is incorrect as well.