Algorithm

An algorithm is a finite sequence of well-defined steps that takes some input and produces an output, solving a particular problem. It is independent of any one programming language; the same algorithm can be written in Python, C or plain English.

Why it matters

The choice of algorithm often matters more than how fast the hardware is. Efficiency is described with Big O notation, which shows how running time grows with input size. Searching a sorted list of one million items by binary search takes about 20 comparisons, while a linear scan may need up to one million.

Example

Binary search on a sorted list:

def find(sorted_list, target):
    lo, hi = 0, len(sorted_list) - 1
    while lo <= hi:
        mid = (lo + hi) // 2
        if sorted_list[mid] == target:
            return mid
        if sorted_list[mid] < target:
            lo = mid + 1
        else:
            hi = mid - 1
    return -1

Each step halves the remaining range, so the work grows logarithmically.

Common mistakes

• Optimizing before measuring; a simple algorithm is fine for small inputs.
• Ignoring edge cases such as empty input or duplicates.
• Using binary search on data that is not sorted.

Related

• Data structure
• Recursion
• Function
• Unit test

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