In Place Functions
This weeks homework had a lot of problems where we wanted you to solve them in-place (meaning without creating a new list). Instead, we wanted you to swap the items in the list and directly modify the input list. Here are a few more practice problems to make sure that you understand how in-place functions work. Write two functions (unrelated to one another) called map_in_place and sort_in_place that each take in a list. map_in_place will modify the original list by mapping the input function over each of the elements of the list. sort_in_place can use any sorting algorithm you want and will modify the original list so that it is sorted after calling this function on it.
def map_in_place(fn, lst):
"""
Takes in a function, fn, and maps it over
each item of the input list, lst.
Returns None and instead modifies the input list.
>>> r = [3, 2, 4, 5, 6]
>>> map_in_place(lambda x: x*x, r)
>>> r
[9, 4, 16, 25, 36]
"""
"***YOUR CODE HERE***"
def sort_in_place(lst):
"""
Takes in a list and sorts it in-place,
just like the builtin .sort() method does.
>>> r = [1, 5, 4, 6, 3, 12]
>>> sort_in_place(r)
>>> r
[1, 3, 4, 5, 6, 12]
"""
"***YOUR CODE HERE***"
def map_in_place(fn, lst):
"""
Takes in a function, fn, and maps it over
each item of the input list, lst.
Returns None and instead modifies the input list.
>>> r = [3, 2, 4, 5, 6]
>>> map_in_place(lambda x: x*x, r)
>>> r
[9, 4, 16, 25, 36]
"""
for i in range(len(lst)):
lst[i] = fn(lst[i])
def sort_in_place(lst):
"""
Takes in a list and sorts it in-place,
just like the builtin .sort() method does.
>>> r = [1, 5, 4, 6, 3, 12]
>>> sort_in_place(r)
>>> r
[1, 3, 4, 5, 6, 12]
"""
for i in range(len(lst)):
index_of_min = lst.index(min(lst[i:]))
lst[i], lst[index_of_min] = lst[index_of_min], lst[i]
For each of these, the important note is that we are iterating over the elements of the lists using the index values to directly mutate the list. If we were to do a for loop such as for i in lst then the value of i would change at each iteration but that is separate from the lst itself. The result is that you would not be mutating the list.
For the sort_in_place function, our general algorithm is to find the minimum of the rest of the list and set that to be the next item. The first time, we are finding the minimum of the whole list and setting that as our first item. Then the next time, we are finding the minimum of the rest of the list and setting that as our next item and so on and so forth. Let me know if the comments below if this method is confusing for you!
I don't claim to be perfect so if you find an error on this page, please send me an email preferably with a link to this page so that I know what I need to fix!