Python memory - The memory limit exceeded for Python code

Question

I got this problem while I was solving a problem of self crossing. It's like the snake game.

Given x = [2, 1, 1, 2], we start from 0, and then we go up for 2, go left for 1, go right for 1 and go down for 2. The goal is to calculate if it will self cross or not. As the given steps are always integers, I thought about using a matrix and every time when it get through a point, I set the point to 1. Like this I could check if the point has been visited or not.

I think in this way, all the memory that Python will make is just the assignment of the matrix. That's why I don't understand when the input x is huge, and why it says "memory out of limit" . For me, I think the memory is the same as the matrix has been initiated before.

How can I fix this?

class Solution(object):

    def isSelfCrossing(self, x):
        """
        :type x: List[int]
        :rtype: bool
        """
        m = [[0 for j in xrange(1000)] for j in xrange(1000)]
        m[0][0] = 1
        a, b = 0, 0
        flag = 0
        for i in x:
            flag = (flag + 1) % 4
            if flag == 1:
                for temp in xrange(1, i+1):
                    b += 1
                    if m[a][b] != 0:
                        return True
                    else: m[a][b] = 1
            elif flag == 2:
                for temp in xrange(1, i+1):
                    a -= 1
                    if m[a][b] != 0:
                        return True
                    else: m[a][b] = 1
            elif flag == 3:
                for temp in xrange(1, i+1):
                    b -= 1
                    if m[a][b] != 0:
                        return True
                    else: m[a][b] = 1
            else:
                for temp in xrange(1, i+1):
                    a += 1
                    if m[a][b] != 0:
                        return True
                    else: m[a][b] = 1
        return False

Answer 1

How huge could "x" be? If "x" is not so big, does your code run successfully?

Assuming "x" is really really huge, it might be related to your "m" initialization which is the straw that broke the camel's back . In other words, your "x" (and some other memory allocations) might consume almost all of the allowed memory allocation of the process, and "m" initialization passes over the limit.

Answer 2

Using lists for a boolean matrix in Python rarely makes sense. Try storing pairs of integers in a set and replace

m = ...       # with m = set()
m[a][b] != 0  # with (a, b) in m
m[a][b] = 1   # with m.add((a, b))

Alternatively, you can create a flat bytearray with the same number of elements as m and index it as a * 1000 + b . This, however, might lead to subtle indexing bugs, so I strongly suggest to give set a try first.