Calculate the Sum of Pairs in Python

The challenge

Given a list of integers and a single sum value, return the first two values (parse from the left please) in order of appearance that add up to form the sum.

sum_pairs([11, 3, 7, 5], 10) # ^--^ 3 + 7 = 10 == [3, 7] sum_pairs([4, 3, 2, 3, 4], 6) # ^-----^ 4 + 2 = 6, indices: 0, 2 * # ^-----^ 3 + 3 = 6, indices: 1, 3 # ^-----^ 2 + 4 = 6, indices: 2, 4 # * entire pair is earlier, and therefore is the correct answer == [4, 2] sum_pairs([0, 0, -2, 3], 2) # there are no pairs of values that can be added to produce 2. == None/nil/undefined (Based on the language) sum_pairs([10, 5, 2, 3, 7, 5], 10) # ^-----------^ 5 + 5 = 10, indices: 1, 5 # ^--^ 3 + 7 = 10, indices: 3, 4 * # * entire pair is earlier, and therefore is the correct answer == [3, 7]
Code language: Python (python)

Negative numbers and duplicate numbers can and will appear.

NOTE: There will also be lists tested of lengths upwards of 10,000,000 elements. Be sure your code doesn’t time out.

The solution in Python code

Option 1:

def sum_pairs(lst, s): cache = set() for i in lst: if s - i in cache: return [s - i, i] cache.add(i)
Code language: Python (python)

Option 2:

def sum_pairs(ints, s): num_dic = {} for num in ints: if num in num_dic: return [num_dic[num], num] else: num_dic[s - num] = num
Code language: Python (python)

Option 3:

def sum_pairs(ints, s): seen=[] for item in ints: if s-item in seen: return [s-item, item] if item not in seen: seen+=[item] return None
Code language: Python (python)

Test cases to validate our solution

l1= [1, 4, 8, 7, 3, 15] l2= [1, -2, 3, 0, -6, 1] l3= [20, -13, 40] l4= [1, 2, 3, 4, 1, 0] l5= [10, 5, 2, 3, 7, 5] l6= [4, -2, 3, 3, 4] l7= [0, 2, 0] l8= [5, 9, 13, -3] test.describe("Testing For Sum of Pairs") test.expect(sum_pairs(l1, 8) == [1, 7], "Basic: %s should return [1, 7] for sum = 8" % l1) test.expect(sum_pairs(l2, -6) == [0, -6], "Negatives: %s should return [0, -6] for sum = -6" % l2) test.expect(sum_pairs(l3, -7) == None, "No Match: %s should return None for sum = -7" % l3) test.expect(sum_pairs(l4, 2) == [1, 1], "First Match From Left: %s should return [1, 1] for sum = 2 " % l4) test.expect(sum_pairs(l5, 10) == [3, 7], "First Match From Left REDUX!: %s should return [3, 7] for sum = 10 " % l5) test.expect(sum_pairs(l6, 8) == [4, 4], "Duplicates: %s should return [4, 4] for sum = 8" % l6) test.expect(sum_pairs(l7, 0) == [0, 0], "Zeroes: %s should return [0, 0] for sum = 0" % l7) test.expect(sum_pairs(l8, 10) == [13, -3], "Subtraction: %s should return [13, -3] for sum = 10" % l8)
Code language: Python (python)
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