Three-Way Partitioning of an Array Around a Given Range Using Python

Given an array and the range of the array [startval, endval]. Array is divided by three parts.

• All elements smaller than startval come first.

• All elements in range startval to endval come next.

• All elements greater than endval appear in the end.

Let's say we have the following input ?

A = [1, 14, 51, 12, 4, 2, 54, 20, 87, 98, 3, 1, 32]
startval = 14, endval = 54

The output should be ?

A = [1, 12, 4, 2, 3, 1, 14, 51, 20, 32,54, 87, 98]

Three-way partitioning of an array around a given range with List Comprehension

In this example, we will see how to three-way partition an array around a given range ?

Example

def partition_array(input, lowVal, highVal):
	# Separate input list in three parts
	my_first = [ num for num in input if num<lowVal ]
	my_second = [ num for num in input if (num>=lowVal and num<=highVal) ]
	my_third = [ num for num in input if num>highVal ]
	# Concatenate all the three parts
	print(my_first + my_second + my_third)

# Driver program
if __name__ == "__main__":
	my_input = [10, 140, 50, 200, 40, 20, 540, 200, 870, 980, 30, 10, 320]
	my_lowVal = 140
	my_highVal = 200
	partition_array(my_input, my_lowVal, my_highVal)

Output

[10, 50, 40, 20, 30, 10, 140, 200, 200, 540, 870, 980, 320]

Three-way partitioning of an array around a given range with while loop

In this example, we will see how to three-way partition an array around a given range with while loop ?

Example

def partitionFunc(my_input, n, lowVal, highVal):
	begn = 0
	end = n - 1
	i = 0

	# Looping
	while i <= end:
		if my_input[i] < lowVal:
			my_input[i], my_input[begn] = my_input[begn], my_input[i]
			i += 1
			begn += 1

		elif my_input[i] > highVal:
			my_input[i], my_input[end] = my_input[end], my_input[i]
			end -= 1
		else:
			i+=1

# Driver code
if __name__ == "__main__":
	my_input = [1, 14, 51, 12, 4, 2, 54, 20, 87, 98, 3, 1, 32]
	n = len(my_input)

	partitionFunc(my_input, n, 14, 54)

	for i in range(n):
		print(my_input[i], end = " ")

Output

1 12 4 2 1 3 54 20 32 51 14 98 87