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| 54 | <H2><A NAME="SECTION0071630000000000000000"></A><A NAME="itertools-recipes"></A> |
| 55 | <BR> |
| 56 | 5.16.3 Recipes |
| 57 | </H2> |
| 58 | |
| 59 | <P> |
| 60 | This section shows recipes for creating an extended toolset using the |
| 61 | existing itertools as building blocks. |
| 62 | |
| 63 | <P> |
| 64 | The extended tools offer the same high performance as the underlying |
| 65 | toolset. The superior memory performance is kept by processing elements one |
| 66 | at a time rather than bringing the whole iterable into memory all at once. |
| 67 | Code volume is kept small by linking the tools together in a functional style |
| 68 | which helps eliminate temporary variables. High speed is retained by |
| 69 | preferring ``vectorized'' building blocks over the use of for-loops and |
| 70 | generators which incur interpreter overhead. |
| 71 | |
| 72 | <P> |
| 73 | <div class="verbatim"><pre> |
| 74 | def take(n, seq): |
| 75 | return list(islice(seq, n)) |
| 76 | |
| 77 | def enumerate(iterable): |
| 78 | return izip(count(), iterable) |
| 79 | |
| 80 | def tabulate(function): |
| 81 | "Return function(0), function(1), ..." |
| 82 | return imap(function, count()) |
| 83 | |
| 84 | def iteritems(mapping): |
| 85 | return izip(mapping.iterkeys(), mapping.itervalues()) |
| 86 | |
| 87 | def nth(iterable, n): |
| 88 | "Returns the nth item" |
| 89 | return list(islice(iterable, n, n+1)) |
| 90 | |
| 91 | def all(seq, pred=bool): |
| 92 | "Returns True if pred(x) is True for every element in the iterable" |
| 93 | for elem in ifilterfalse(pred, seq): |
| 94 | return False |
| 95 | return True |
| 96 | |
| 97 | def any(seq, pred=bool): |
| 98 | "Returns True if pred(x) is True for at least one element in the iterable" |
| 99 | for elem in ifilter(pred, seq): |
| 100 | return True |
| 101 | return False |
| 102 | |
| 103 | def no(seq, pred=bool): |
| 104 | "Returns True if pred(x) is False for every element in the iterable" |
| 105 | for elem in ifilter(pred, seq): |
| 106 | return False |
| 107 | return True |
| 108 | |
| 109 | def quantify(seq, pred=bool): |
| 110 | "Count how many times the predicate is True in the sequence" |
| 111 | return sum(imap(pred, seq)) |
| 112 | |
| 113 | def padnone(seq): |
| 114 | """Returns the sequence elements and then returns None indefinitely. |
| 115 | |
| 116 | Useful for emulating the behavior of the built-in map() function. |
| 117 | """ |
| 118 | return chain(seq, repeat(None)) |
| 119 | |
| 120 | def ncycles(seq, n): |
| 121 | "Returns the sequence elements n times" |
| 122 | return chain(*repeat(seq, n)) |
| 123 | |
| 124 | def dotproduct(vec1, vec2): |
| 125 | return sum(imap(operator.mul, vec1, vec2)) |
| 126 | |
| 127 | def flatten(listOfLists): |
| 128 | return list(chain(*listOfLists)) |
| 129 | |
| 130 | def repeatfunc(func, times=None, *args): |
| 131 | """Repeat calls to func with specified arguments. |
| 132 | |
| 133 | Example: repeatfunc(random.random) |
| 134 | """ |
| 135 | if times is None: |
| 136 | return starmap(func, repeat(args)) |
| 137 | else: |
| 138 | return starmap(func, repeat(args, times)) |
| 139 | |
| 140 | def pairwise(iterable): |
| 141 | "s -> (s0,s1), (s1,s2), (s2, s3), ..." |
| 142 | a, b = tee(iterable) |
| 143 | try: |
| 144 | b.next() |
| 145 | except StopIteration: |
| 146 | pass |
| 147 | return izip(a, b) |
| 148 | </pre></div> |
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| 184 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> |
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