# Data Structures — Looping Techniques

> When looping through dictionaries, the key and corresponding value can be retrieved at the same time using the ~dict.items method.

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## Knowledge

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When looping through dictionaries, the key and corresponding value can be retrieved at the same time using the ~dict.items method.

&gt;&gt;&gt; knights = {'gallahad': 'the pure', 'robin': 'the brave'} &gt;&gt;&gt; for k, v in knights.items(): ... print(k, v) ... gallahad the pure robin the brave

When looping through a sequence, the position index and corresponding value can be retrieved at the same time using the enumerate function.

&gt;&gt;&gt; for i, v in enumerate(['tic', 'tac', 'toe']): ... print(i, v) ... 0 tic 1 tac 2 toe

To loop over two or more sequences at the same time, the entries can be paired with the zip function.

&gt;&gt;&gt; questions = ['name', 'quest', 'favorite color'] &gt;&gt;&gt; answers = ['lancelot', 'the holy grail', 'blue'] &gt;&gt;&gt; for q, a in zip(questions, answers): ... print('What is your {0}? It is {1}.'.format(q, a)) ... What is your name? It is lancelot. What is your quest? It is the holy grail. What is your favorite color? It is blue.

To loop over a sequence in reverse, first specify the sequence in a forward direction and then call the reversed function.

&gt;&gt;&gt; for i in reversed(range(1, 10, 2)): ... print(i) ... 9 7 5 3 1

To loop over a sequence in sorted order, use the sorted function which returns a new sorted list while leaving the source unaltered.

&gt;&gt;&gt; basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana'] &gt;&gt;&gt; for i in sorted(basket): ... print(i) ... apple apple banana orange orange pear

Using set on a sequence eliminates duplicate elements. The use of sorted in combination with set over a sequence is an idiomatic way to loop over unique elements of the sequence in sorted order.

&gt;&gt;&gt; basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana'] &gt;&gt;&gt; for f in sorted(set(basket)): ... print(f) ... apple banana orange pear

It is sometimes tempting to change a list while you are looping over it; however, it is often simpler and safer to create a new list instead.

&gt;&gt;&gt; import math &gt;&gt;&gt; raw_data = [56.2, float('NaN'), 51.7, 55.3, 52.5, float('NaN'), 47.8] &gt;&gt;&gt; filtered_data = [] &gt;&gt;&gt; for value in raw_data: ... if not math.isnan(value): ... filtered_data.append(value) ... &gt;&gt;&gt; filtered_data [56.2, 51.7, 55.3, 52.5, 47.8]

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