Special Occurrence

Source: Dec 2023, Task 3 (DTU 02002/02003). Study solution — not an official answer key.

Problem

A special occurrence is a 5 followed by two numbers where exactly one is 7.

Write special_occurrence(sequence) returning the index of that 5, or -1 if none.

>>> special_occurrence([2, 8, 11, 3, 12, 5, 7, 7, 11, 3, 12, 5, 2, 7, 5, 7, 2, 6])
11

Approach

Scan triples (sequence[i], sequence[i+1], sequence[i+2]). When the first is 5, check that exactly one of the next two equals 7.

Solution

def special_occurrence(sequence):
    for i in range(len(sequence) - 2):
        if sequence[i] == 5:
            a = sequence[i + 1]
            b = sequence[i + 2]
            if (a == 7) + (b == 7) == 1:
                return i
    return -1