Selection sort

Simple sorts · Article 2 of 8 · 3 min read

The easiest sort to explain: find the smallest item, put it first, repeat.

The short version

Selection sort scans the unsorted part for the smallest item and swaps it to the front. Then it repeats on what is left.

Details

It always does about n squared comparisons, even on a list that is already sorted. It makes few swaps, which matters only when writing is expensive.

Try it yourself

def selection_sort(items):
    for i in range(len(items)):
        smallest = i
        for j in range(i + 1, len(items)):
            if items[j] < items[smallest]:
                smallest = j
        items[i], items[smallest] = items[smallest], items[i]

data = [5, 2, 9, 1]
selection_sort(data)
print(data)

You should see [1, 2, 5, 9].

Explain it to the duck

Why does selection sort take the same time on a sorted list as on a shuffled one?

Tested with: Python 3.13.