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   "metadata": {},
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    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[[0, 0], [0, 0]]\n",
      "[[1, -3], [1, -3]]\n"
     ]
    }
   ],
   "source": [
    "after = [[1,2],[3,4]]\n",
    "rows, cols = (len(after), len(after[0])) \n",
    "\n",
    "before = [[0]*cols]*rows\n",
    "\n",
    "# before = [[0 for i in range(cols)] for j in range(rows)]  #uncomment this array definition and comment above one... see difference in output.\n",
    "\n",
    "print(before)\n",
    "\n",
    "def calculation(a,b):\n",
    "    s = after[0][0]\n",
    "    for x in range(a+1):\n",
    "        for y in range(b+1):\n",
    "                s += before[x][y]\n",
    "    before[a][b] = after[a][b] - s\n",
    "    \n",
    "def cal2():\n",
    "    for x in range(len(after)):\n",
    "        for y in range(len(after[0])):\n",
    "            calculation(x,y)\n",
    "    before[0][0] = after[0][0]\n",
    "    print(before)\n",
    "cal2()\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
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