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data-science-ipython-notebooks/core/structs_utils.ipynb

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{
"metadata": {
"name": "",
"signature": "sha256:893799227c6632abab6f36b3299512e1f724d3a9e69a9cc0b81a3ff9c872f370"
},
"nbformat": 3,
"nbformat_minor": 0,
"worksheets": [
{
"cells": [
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Data Structure Utilities"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"* slice\n",
"* range and xrange\n",
"* bisect\n",
"* sort\n",
"* sorted\n",
"* reversed\n",
"* enumerate\n",
"* zip\n",
"* list comprehensions"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## slice"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"![alt text](http://www.nltk.org/images/string-slicing.png)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Select a section of list types (arrays, tuples, NumPy arrays) using [start:stop]. start is included, stop is not. The number of elements in the result is stop - start."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq = 'Monty Python'\n",
"seq[6:10]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 1,
"text": [
"'Pyth'"
]
}
],
"prompt_number": 1
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Omit start to default to start of the sequence"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq[:5]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 2,
"text": [
"'Monty'"
]
}
],
"prompt_number": 2
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Omit end to default to end of the sequence"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq[6:]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 3,
"text": [
"'Python'"
]
}
],
"prompt_number": 3
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Negative indices slice relative to the end"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq[-12:-7]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 4,
"text": [
"'Monty'"
]
}
],
"prompt_number": 4
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Slice can also take a step such as the one below, which takes every other element"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq[::2]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 5,
"text": [
"'MnyPto'"
]
}
],
"prompt_number": 5
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Passing -1 for the step reverses the list or tuple:"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq[::-1]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 6,
"text": [
"'nohtyP ytnoM'"
]
}
],
"prompt_number": 6
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Assign elements to a slice. Slice range does not have to equal number of elements to assign."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq = [1, 1, 2, 3, 5, 8, 13]\n",
"seq[5:] = ['H', 'a', 'l', 'l']\n",
"seq"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 7,
"text": [
"[1, 1, 2, 3, 5, 'H', 'a', 'l', 'l']"
]
}
],
"prompt_number": 7
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Compare assigning into a slice (above) versus assigning into an index (below)"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq = [1, 1, 2, 3, 5, 8, 13]\n",
"seq[5] = ['H', 'a', 'l', 'l']\n",
"seq"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 8,
"text": [
"[1, 1, 2, 3, 5, ['H', 'a', 'l', 'l'], 13]"
]
}
],
"prompt_number": 8
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## range and xrange"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Generate a list of evenly spaced integers. Note: range is no longer avalable in Python 3 "
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"range(10)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 9,
"text": [
"[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]"
]
}
],
"prompt_number": 9
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Range can take start, stop, and step arguments"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"range(0, 20, 3)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 10,
"text": [
"[0, 3, 6, 9, 12, 15, 18]"
]
}
],
"prompt_number": 10
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"It is very common to iterate through sequences by index with range"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq = [1, 2, 3]\n",
"for i in range(len(seq)):\n",
" val = seq[i]\n",
" print(val)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"1\n",
"2\n",
"3\n"
]
}
],
"prompt_number": 11
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"xrange is recommended for longer ranges and is available in Python 3 as range. It returns an iterator that generates integers one by one rather than all at once and storing them in a large list."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"sum = 0\n",
"for i in xrange(100000):\n",
" if i % 2 == 0:\n",
" sum += 1\n",
"print(sum)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"50000\n"
]
}
],
"prompt_number": 12
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## bisect"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"The bisect module does not check whether the list is sorted, as this check would be expensive O(n). Using bisect on an unsorted list will not result in an error but could lead to incorrect results."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"import bisect"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 13
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Find the location where an element should be inserted to keep the list sorted"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq = [1, 2, 2, 3, 5, 13]\n",
"bisect.bisect(seq, 8)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 14,
"text": [
"5"
]
}
],
"prompt_number": 14
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Inserts an element into a location to keep the list sorted"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"bisect.insort(seq, 8)\n",
"seq"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 15,
"text": [
"[1, 2, 2, 3, 5, 8, 13]"
]
}
],
"prompt_number": 15
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## sort"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Sort in-place O(n log n)"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq = [1, 5, 3, 9, 7, 6]\n",
"seq.sort()\n",
"seq"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 16,
"text": [
"[1, 3, 5, 6, 7, 9]"
]
}
],
"prompt_number": 16
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Sort by secondary key: str length"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq = ['the', 'quick', 'brown', 'fox', 'jumps', 'over']\n",
"seq.sort(key=len)\n",
"seq"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 17,
"text": [
"['the', 'fox', 'over', 'quick', 'brown', 'jumps']"
]
}
],
"prompt_number": 17
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## sorted"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Return a new sorted list from the elements of a sequence"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"sorted([2, 5, 1, 8, 7, 9])"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 18,
"text": [
"[1, 2, 5, 7, 8, 9]"
]
}
],
"prompt_number": 18
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"sorted('foo bar baz')"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 19,
"text": [
"[' ', ' ', 'a', 'a', 'b', 'b', 'f', 'o', 'o', 'r', 'z']"
]
}
],
"prompt_number": 19
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"It's common to get a sorted list of unique elements by combining sorted and set"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq = [2, 5, 1, 8, 7, 9, 9, 2, 5, 1, (4, 2), (1, 2), (1, 2)]\n",
"sorted(set(seq))"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 20,
"text": [
"[1, 2, 5, 7, 8, 9, (1, 2), (4, 2)]"
]
}
],
"prompt_number": 20
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## reversed"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Iterates over the sequence elements in reverse order"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"list(reversed(seq))"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 21,
"text": [
"[(1, 2), (1, 2), (4, 2), 1, 5, 2, 9, 9, 7, 8, 1, 5, 2]"
]
}
],
"prompt_number": 21
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## enumerate"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"enumerate gives you the index of a collection and the value"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"strings = ['foo', 'bar', 'baz']\n",
"for i, string in enumerate(strings):\n",
" print(i, string)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"(0, 'foo')\n",
"(1, 'bar')\n",
"(2, 'baz')\n"
]
}
],
"prompt_number": 22
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## zip"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Pairs up the elements of sequences to create a list of tuples"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq_1 = [1, 2, 3]\n",
"seq_2 = ['foo', 'bar', 'baz']\n",
"zip(seq_1, seq_2)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 23,
"text": [
"[(1, 'foo'), (2, 'bar'), (3, 'baz')]"
]
}
],
"prompt_number": 23
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Takes an arbitrary number of sequences. The number of elements it produces is determined by the shortest sequence."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"seq_3 = [True, False]\n",
"zip(seq_1, seq_2, seq_3)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 24,
"text": [
"[(1, 'foo', True), (2, 'bar', False)]"
]
}
],
"prompt_number": 24
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"It is common to use zip for simultaneously iterating over multiple sequences combined with enumerate"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"for i, (a, b) in enumerate(zip(seq_1, seq_2)):\n",
" print('%d: %s, %s' % (i, a, b))"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"0: 1, foo\n",
"1: 2, bar\n",
"2: 3, baz\n"
]
}
],
"prompt_number": 25
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"zip can unzip a zipped sequence, which you can think of as converting a list of rows into a list of columns"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"numbers = [(1, 'one'), (2, 'two'), (3, 'three')]\n",
"a, b = zip(*numbers)\n",
"a"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 26,
"text": [
"(1, 2, 3)"
]
}
],
"prompt_number": 26
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"b"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 27,
"text": [
"('one', 'two', 'three')"
]
}
],
"prompt_number": 27
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## List Comprehensions"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Concisely form a new list by filtering the elements of a sequence and transforming the elements passing the filter. List comprehensions take the form:\n",
"```python\n",
"[expr for val in collection if condition]\n",
"```\n",
"Which is equivalent to the following for loop:\n",
"```python\n",
"result = []\n",
"for val in collection:\n",
" if condition:\n",
" result.append(expr)\n",
"```"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Convert to upper case all strings that start with a 'b'"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"strings = ['foo', 'bar', 'baz', 'f', 'fo', 'b', 'ba']\n",
"[x.upper() for x in strings if x[0] == 'b']"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 28,
"text": [
"['BAR', 'BAZ', 'B', 'BA']"
]
}
],
"prompt_number": 28
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"List comprehensions can be nested"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"list_of_tuples = [(1, 2, 3), (4, 5, 6), (7, 8, 9)]\n",
"[x for tup in list_of_tuples for x in tup]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 29,
"text": [
"[1, 2, 3, 4, 5, 6, 7, 8, 9]"
]
}
],
"prompt_number": 29
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Dict Comprehension"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Create a lookup map of strings and their locations in the list for strings that start with a 'b'"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"{index : val for index, val in enumerate(strings) if val[0] == 'b'}"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 30,
"text": [
"{1: 'bar', 2: 'baz', 5: 'b', 6: 'ba'}"
]
}
],
"prompt_number": 30
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Set Comprehension"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Get the unique lengths of strings that start with a 'b'"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"{len(x) for x in strings if x[0] == 'b'}"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 31,
"text": [
"{1, 2, 3}"
]
}
],
"prompt_number": 31
}
],
"metadata": {}
}
]
}
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