List index out of range in 2D list

Question

I understand that there are similar questions to this here, here, and here. The first one addresses 1D lists, the second is great except it doesn't seem to work, and the third is close, but I still don't quite understand my issue.

Here is what I am trying to do. I need to create an 2D list (a 2D array in java and C++, which I am much more familiar with) filled with 0's. It needs to be size 20 across and 15 down.

Here is what I have tried:

self.grid = [[0 for x in range(GRID_COLUMN_SIZE)] for y in range(GRID_ROW_SIZE)] # where GRID_ROW_SIZE = 15, GRID_COLUMN_SIZE = 20

Note, I tried with the two constants switched (COLUMN first, then ROW), and it broke slightly later. In addition, I print the 2D list out and it's the wrong dimensions (15 across and 20 down).

Here is my later use of self.grid. Without getting too deep, I am iterating through all the values of the list (grid) and getting the surrounding points.

def populatePaths(self):
    for row in range(len(self.grid)):
        for column in range(len(self.grid[row])):
            if self.isPointAccessible(column, row):
                self.addPaths(column, row)

def addPaths(self, x, y):
    key = Point(x, y)
    print "Each: %s" % (key.toString())
    points = key.getSurroundingPoints()
    self.removeBarriersFromPath(points)
    self.paths[key] = points # a map from Points to lists of surrounding Points

Basically, I remove points along the path that can't be reached:

def removeBarriersFromPath(self, path):
    for point in list(path):
        print "Surrounding %s" % (point.toString())
        if not self.isPointAccessible(point.x, point.y):
            path.remove(point)
    return path

self.isPointAccessible() is trivial, but this is where it breaks. It checks to see if the value at the (x,y) location is 0: return self.grid[x][y] == 0

I added these print statements (point.toString() returns (x,y)) to show me the points as they happen, and I am able to iterate until x==14, but it breaks at x==15.

I suspect that I am getting the column/row order in the looping incorrect, but I'm not sure when/how.

Let me know if I didn't explain something clearly enough.

Edit Here is the traceback:

Traceback (most recent call last):
  File "/home/nu/catkin_ws/src/apriltags_intrude_detector/scripts/sphero_intrude_gui.py", line 70, in start
    self.populatePaths()
  File "/home/nu/catkin_ws/src/apriltags_intrude_detector/scripts/sphero_intrude_gui.py", line 156, in populatePaths
    self.addPaths(column, row)
  File "/home/nu/catkin_ws/src/apriltags_intrude_detector/scripts/sphero_intrude_gui.py", line 162, in addPaths
    self.removeBarriersFromPath(points)
  File "/home/nu/catkin_ws/src/apriltags_intrude_detector/scripts/sphero_intrude_gui.py", line 168, in removeBarriersFromPath
    if not self.isPointAccessible(point.x, point.y):
  File "/home/nu/catkin_ws/src/apriltags_intrude_detector/scripts/sphero_intrude_gui.py", line 173, in isPointAccessible
    return self.grid[x][y] == 0
IndexError: list index out of range

I neglected to put that part in. It now contains the traceback as well as the information about where it breaks (`self.isPointAccessible`) — Cache Staheli, May 10 '16 at 04:57

score 1 · Accepted Answer · answered May 10 '16 at 05:00

1

You did not post the whole source for isPointAccessible but from the error message it looks like your return line must be:

return self.grid[y][x] == 0

since y denotes the row number and x is the column.

answered May 10 '16 at 05:00

Selcuk

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Oh! that was it! Btw, `return self.grid[x][y] == 0` was the entire method. – Cache Staheli May 10 '16 at 05:02

List index out of range in 2D list

1 Answers1