copy or view

view_or_copy.ipynb

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    "import numpy as np"
   ]
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   "source": [
    "A modified exercise from [Advanced Numpy](https://paris-swc.github.io/advanced-numpy-lesson/02-indexing.html#view-or-copy) lecture:\n",
    "\n",
    "> **View or copy?**\n",
    ">\n",
    "> Create an array of elements from 0 to 4. Increment the second element by 1 using `x[1] += 1`. Compare the new array with the original. Now define `y = x[:2]` and increment its second element? Does `x` also change? What happens with `x` when you select a single element from `x` (`y = x[1]`) and increment it? \n",
    "\n",
    "\n"
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   "source": [
    "The rationale of this exercise is to explain what a \"view\" is and the difference between selecting a single element (copy) and a slice (view). "
   ]
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   "source": [
    "## NumPy: indexing by an integer \n",
    "\n",
    "We can change an element of NumPy array in place:"
   ]
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     "text": [
      " Before: x = [0 1 2 3 4]\n",
      " After:  x = [0 2 2 3 4]\n"
     ]
    }
   ],
   "source": [
    "x = np.arange(5)\n",
    "\n",
    "print \" Before: x =\", x\n",
    "x[1] += 1\n",
    "print \" After:  x =\", x"
   ]
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   "source": [
    "**Result**: assignment incremented the second element.\n",
    "\n",
    "**Conclusion**: Assigning/incrementing/etc. an array element in place modifies the array."
   ]
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   "source": [
    "## A  catch\n",
    "\n",
    "The previous example does not work if we first assign the element to a new variable"
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      " Before: x = [0 1 2 3 4]\n",
      " After:  x = [0 1 2 3 4]\n"
     ]
    }
   ],
   "source": [
    "# indexing by a slice creates a view\n",
    "\n",
    "x = np.arange(5)\n",
    "\n",
    "# take first two elements\n",
    "y = x[1]\n",
    "\n",
    "print \" Before: x =\", x\n",
    "y += 1\n",
    "print \" After:  x =\", x"
   ]
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   "source": [
    "**Result**: Incrementing the value of `y` didn't change `x`!\n",
    "\n",
    "**Conclusion**: Selecting a single element from an array creates a copy of the element."
   ]
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   "source": [
    "## NumPy: indexing by a slice\n",
    "\n",
    "However indexing NumPy array by a slice gives a different result:"
   ]
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     "text": [
      " Before: x = [0 1 2 3 4]\n",
      " After:  x = [0 2 2 3 4]\n"
     ]
    }
   ],
   "source": [
    "# indexing by a slice creates a view\n",
    "\n",
    "x = np.arange(5)\n",
    "\n",
    "# take first two elements\n",
    "y = x[:2]\n",
    "\n",
    "print \" Before: x =\", x\n",
    "y[1] += 1\n",
    "print \" After:  x =\", x"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Result**: incrementing the second element of `y` (slice of `x`) changed also `x`.\n",
    "\n",
    "**Conclusion**: Indexing a NumPy array by a slice creates a *view* of the sub-array."
   ]
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