Q4. Define a function make_counter that returns a counter function, which takes an immutable key and a numerical value argument and returns the sum of all arguments ever passed to counter with that same key.

Below is the solution:

def make_counter():
    """Return a counter function.

    >>> c = make_counter()
    >>> c('a', 3)
    >>> c('a', 5)
    >>> c('b', 7)
    >>> c('a', 9)
    >>> c2 = make_counter()
    >>> c2(1, 2)
    >>> c2(3, 4)
    >>> c2(3, c('b', 6))
    dict = {}   
    def counter(key, value):
        if key in dict:
            dict[key] = [value]
        lst = dict[key]
        total = 0
        length = len(lst)
        for index in range(length):
            total = total + lst[index]
        return total
    return counter
    """-m doctest gives some weird failure despite all above test case pass"""

Can we improve this solution?

  • 1
    \$\begingroup\$ Rather than asking three questions in rapid succession, why not ask one, wait for feedback, apply it to the others then ask a follow-up? Also, there's no need to include all of the boiler-plate twice. \$\endgroup\$ – jonrsharpe May 16 '15 at 9:34
  • 3
    \$\begingroup\$ Note that the reason for your doctest failure is trailing whitespace, after the 17 for the final test. It is quite picky about these things! \$\endgroup\$ – jonrsharpe May 16 '15 at 9:38

Choose another name for the dictionary besides dict. Don't ever hide the name of a built-in type.


d = {}     
def counter(key,value):
    d[key] = d.get(key,0) + value
    return d[key]

You're done. I see no need to build lists of all the passed values.


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