Python | Three-element sum in a list



Method # 1: Using Nested Loops

This is a naive method in which this particular problem can be solved and uses an outer loop to iterate over each element and the inner loop checks the remaining difference by adding pairs to the result.

# Python3 demo code
Sum of elements # 3
# using nested loops

 
# initializing list

test_list = [ 4 , 1 , 3 , 2 , 6 , 5 ]

 
# initial amount

sum = 9

 
# print original list

print ( "The original list:" + str (test_list))

 
# using nested loops
Sum of # 3 elements

res = []

for in range ( 0 , len (test_list) - 2 ):

for j in range (i + 1 , len (test_list) - 1 ):

for k in range (j + 1 , len (test_list)):

if test_list [i] + test_list [j] + test_list [k] = = sum :

temp = []

temp.append (test_list [i])

temp.append (test_list [j])

temp.append (test_list [k] )

  res.append ( tuple (temp))

 
# print result

print ( "The 3 sum element list is:" + str (res))

Output:

 The original list: [4, 1, 3, 2, 6, 5] The 3 sum element list is: [(4, 3, 2), (1, 3, 5), (1, 2, 6)] 

Method # 2: Using itertools .combination ()
This particular problem can also be accomplished in a succinct manner using a built-in function function. The combination function finds the entire combination that takes K arguments leading to a specific amount.

# Python3 demo code
Sum of # 3 items
# using itertools.combination ()

from itertools import combinations

 
# function to get the sum

def test (val):

return sum (val) = = 9

  
# initializing list

test_list = [ 4 , 1 , 3 , 2 , 6 , 5 ]

 
# initial amount

summation = 9

 
# prints the original list

print ( "The original list: " +   str (test_list))

 
# using itertools.combination ()
Sum of # 3 items

res = list ( filter (test, list (combinations (test_list, 3 ))))

  
# print result

print ( "The 3 sum element list is:" + str (res))

Exit:

 The o riginal list: [4, 1, 3, 2, 6, 5] The 3 sum element list is: [(4, 3, 2), (1, 3, 5), (1, 2, 6)]