Question

def count_pairs(num_list): Given a list of numbers num_list, count how many ways unique pairs of values from this list have a sum equal to a value anywhere in the list (including equaling one of the values being added). Watch out for duplicates – two locations may sum up to a value equal to many places in the list, but they only count as one pairing overall. The two summed values must be at unique locations in the list.

• Parameters: num_list is a list (length > 2) of integers (no validation needed)

• Examples:

count_pairs([1,2,3,4,5]) → 4

# 1+2=3 # 1+3=4 # 1+4=5 # 2+3=5

count_pairs([1,2,3,1,0]) → 7

# 1+2=3

# 1+1=2 # 1+0=1 # 2+1=3 # 2+0=2 # 3+0=3 # 1+0=1

Answer 1

def count_pairs(num_list):
    count = 0
    for i in range(len(num_list)):
        for j in range(i + 1, len(num_list)):
            if num_list[i] + num_list[j] in num_list:
                count += 1
    return count


print(count_pairs([1, 2, 3, 4, 5]))
print(count_pairs([1, 2, 3, 1, 0]))