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<H2><A NAME="SECTION0071630000000000000000"></A><A NAME="itertools-recipes"></A>
<BR>
5.16.3 Recipes
</H2>

<P>
This section shows recipes for creating an extended toolset using the
existing itertools as building blocks.

<P>
The extended tools offer the same high performance as the underlying
toolset.  The superior memory performance is kept by processing elements one
at a time rather than bringing the whole iterable into memory all at once.
Code volume is kept small by linking the tools together in a functional style
which helps eliminate temporary variables.  High speed is retained by
preferring ``vectorized'' building blocks over the use of for-loops and
generators which incur interpreter overhead.

<P>
<div class="verbatim"><pre>
def take(n, seq):
    return list(islice(seq, n))

def enumerate(iterable):
    return izip(count(), iterable)

def tabulate(function):
    "Return function(0), function(1), ..."
    return imap(function, count())

def iteritems(mapping):
    return izip(mapping.iterkeys(), mapping.itervalues())

def nth(iterable, n):
    "Returns the nth item"
    return list(islice(iterable, n, n+1))

def all(seq, pred=bool):
    "Returns True if pred(x) is True for every element in the iterable"
    for elem in ifilterfalse(pred, seq):
        return False
    return True

def any(seq, pred=bool):
    "Returns True if pred(x) is True for at least one element in the iterable"
    for elem in ifilter(pred, seq):
        return True
    return False

def no(seq, pred=bool):
    "Returns True if pred(x) is False for every element in the iterable"
    for elem in ifilter(pred, seq):
        return False
    return True

def quantify(seq, pred=bool):
    "Count how many times the predicate is True in the sequence"
    return sum(imap(pred, seq))

def padnone(seq):
    """Returns the sequence elements and then returns None indefinitely.

    Useful for emulating the behavior of the built-in map() function.
    """
    return chain(seq, repeat(None))

def ncycles(seq, n):
    "Returns the sequence elements n times"
    return chain(*repeat(seq, n))

def dotproduct(vec1, vec2):
    return sum(imap(operator.mul, vec1, vec2))

def flatten(listOfLists):
    return list(chain(*listOfLists))

def repeatfunc(func, times=None, *args):
    """Repeat calls to func with specified arguments.

    Example:  repeatfunc(random.random)
    """
    if times is None:
        return starmap(func, repeat(args))
    else:
        return starmap(func, repeat(args, times))

def pairwise(iterable):
    "s -&gt; (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    try:
        b.next()
    except StopIteration:
        pass
    return izip(a, b)
</pre></div>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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