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• Name two ways to build a list containing five integer zeros.

• Name two ways to build a dictionary with two keys, 'a' and 'b', each having an associated value of 0.

• Name four operations that change a list object in place.

• Name four operations that change a dictionary object in place.

• Why might you use a dictionary instead of a list?

  
  

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关键词：knowledge python Edge test Your dictionary instead change having

• A literal expression like [0, 0, 0, 0, 0] and a repetition expression like [0] * 5 will each create a list of five zeros. In practice, you might also build one up with a loop that starts with an empty list and appends 0 to it in each iteration, with L.append(0). A list comprehension ([0 for i in range(5)]) could work here, too, but this is more work than you need to do for this answer.

• A literal expression such as {'a': 0, 'b': 0} or a series of assignments like D = {}, D['a'] = 0, and D['b'] = 0 would create the desired dictionary. You can also use the newer and simpler-to-code dict(a=0, b=0) keyword form, or the more flexible dict([('a', 0), ('b', 0)]) key/value sequences form. Or, because all the values are the same, you can use the special form dict.fromkeys('ab', 0). In 3.X and 2.7, you can also use a dictionary comprehension: {k:0 for k in 'ab'}, though again, this may be overkill here.

• The append and extend methods grow a list in place, the sort and reverse methods order and reverse lists, the insert method inserts an item at an offset, theremove and pop methods delete from a list by value and by position, the del statement deletes an item or slice, and index and slice assignment statements replace an item or entire section. Pick any four of these for the quiz.

• Dictionaries are primarily changed by assignment to a new or existing key, which creates or changes the key’s entry in the table. Also, the del statement deletes a key’s entry, the dictionary update method merges one dictionary into another in place, and D.pop(key) removes a key and returns the value it had. Dictionaries also have other, more exotic in-place change methods not presented in this chapter, such as setdefault; see reference sources for more details.

• Dictionaries are generally better when the data is labeled (a record with field names, for example); lists are best suited to collections of unlabeled items (such as all the files in a directory). Dictionary lookup is also usually quicker than searching a list, though this might vary per program.