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Answer to Question #75361 in Python for Andrew

Question #75361
Write a function flipSwitches that accepts one argument, a sequence of upper or lower case letters (the sequence can either be a str or a list, if you write your code correctly, it shouldn’t matter). This is a sequence of switches, uppercase means to turn a switch on, and lowercase to turn a switch off. For example, ‘A’ means to turn the switch ‘A’ on, and ‘a’ means to turn the switch ‘A’ off. The function should then return the set of those switches that are still on at the end of the sequence of switches. To receive full credit, your code must use a set that keeps track of the switches that are on.You can then iterate over the sequence and modify the set of switches that are on as appropriate.
>>> flipSwitches( ['A','A','a','B'] )
{'B'}
Expert's answer
def flipSwitches(arr):
tracking_set = set() # a set that keeps track of the switches that are on

for c in arr: # loop over all elements in set
if c.isupper(): # if an element is upper letter add to the tracking set (turn on)
tracking_set.add(c)
if c.islower() and (c.upper() in tracking_set): # if an element is lower letter and the upper letter is in the tracking set than remove it (turn off)
tracking_set.remove(c.upper())

return tracking_set

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