# A Common-Sense Guide to Data Structures and Algorithms, Second Edition: Quickselect In Python (page 221)

Hi, I am trying to convert the ruby code of Quickselect into python and I noticed that I have to add return statements in the if else conditions like so

``````def partition(left_p, right_p, arr=[]):

pivot_index = right_p

pivot = arr[pivot_index]

right_p -= 1

while True:

while arr[left_p] < pivot:

left_p += 1

while arr[right_p] > pivot:

right_p -= 1

if left_p >= right_p:

break

else:

arr[left_p], arr[right_p] = arr[right_p], arr[left_p]

left_p += 1

arr[left_p], arr[pivot_index] = arr[pivot_index], arr[left_p]

return left_p

def quickselect(kth_lowest_value, left_index, right_index, arr=[]):

print(arr)

if right_index - left_index <= 0:

return arr[left_index]

pivot_index = partition(left_index, right_index, arr)

if kth_lowest_value < pivot_index:

return quickselect(kth_lowest_value, left_index, pivot_index-1, arr)

elif kth_lowest_value > pivot_index:

return quickselect(kth_lowest_value, pivot_index+1, right_index, arr)

else:

print(f"item = {arr[pivot_index]}")

return arr[pivot_index]

array = [200, 97, 100, 101, 211, 107, 63, 123, 11, 34]

index = quickselect(6, 0, len(array)-1, array)

print(index)
``````

In the book version written in Ruby, there “return” is only in “return @array[pivot_index]”, so I think we either remove the return or also put returns on the statements after the other conditionals. Unedited code below:

``````    attr_reader :array
def initialize(array)
@array = array
end
def quickselect!(kth_lowest_value, left_index, right_index)
# If we reach a base case - that is, that the subarray has one cell,
# we know we've found the value we're looking for:
if right_index - left_index <= 0
return @array[left_index]
end
# Partition the array and grab the index of the pivot:
pivot_index = partition!(left_index, right_index)
# If what we're looking for is to the left of the pivot:
if kth_lowest_value < pivot_index
# Recursively perform quickselect on the subarray to
# the left of the pivot:
return quickselect!(kth_lowest_value, left_index, pivot_index - 1)
# If what we're looking for is to the right of the pivot:
elsif kth_lowest_value > pivot_index
# Recursively perform quickselect on the subarray to
# the right of the pivot:
return quickselect!(kth_lowest_value, pivot_index + 1, right_index)
else # if kth_lowest_value == pivot_index
# if after the partition, the pivot position is in the same spot
# as the kth lowest value, we've found the value we're looking for
return @array[pivot_index]
end
end

def partition!(left_pointer, right_pointer)
# We always choose the right-most element as the pivot.
# We keep the index of the pivot for later use:
pivot_index = right_pointer
# We grab the pivot value itself:
pivot = @array[pivot_index]
# We start the right pointer immediately to the left of the pivot
right_pointer -= 1
while true
# Move the left pointer to the right as long as it
# points to value that is less than the pivot:
while @array[left_pointer] < pivot do
left_pointer += 1
end
# Move the right pointer to the left as long as it
# points to a value that is greater than the pivot:
while @array[right_pointer] > pivot do
right_pointer -= 1
end
# We've now reached the point where we've stopped
# moving both the left and right pointers.
# We check whether the left pointer has reached
# or gone beyond the right pointer. If it has,
# we break out of the loop so we can swap the pivot later
# on in our code:
if left_pointer >= right_pointer
break
# If the left pointer is still to the left of the right
# pointer, we swap the values of the left and right pointers:
else
@array[left_pointer], @array[right_pointer] = @array[right_pointer], @array[left_pointer]
# We move the left pointer over to the right, gearing up
# for the next round of left and right pointer movements:
left_pointer += 1
end
end
# As the final step of the partition, we swap the value
# of the left pointer with the pivot:
@array[left_pointer], @array[pivot_index] = @array[pivot_index], @array[left_pointer]
# We return the left_pointer for the sake of the quicksort method
# which will appear later in this chapter:
return left_pointer
end
end

array = [0, 50, 20, 10, 60, 30]
sortable_array = SortableArray.new(array)
p sortable_array.quickselect!(5, 0, array.length - 1)
``````