Python sorting algorithms

Bubble sort swaps neighbours that are in the wrong order, pass after pass. Insertion sort takes each item in turn and slides it back into place among those before it. Merge sort splits the list, sorts each half and merges them.

These exercises ask for the steps, not just the result: write the algorithm yourself rather than calling sorted().

a = [5, 1, 4, 2]
for end in range(len(a) - 1, 0, -1):
    for i in range(end):
        if a[i] > a[i + 1]:
            a[i], a[i + 1] = a[i + 1], a[i]
print(a)

Watch for: Off-by-one: comparing a[i] with a[i + 1] means i must stop one before the end.