Substring Comparison (windows, slice equality)

Substring comparison checks if a slice equals another string—s[i:j] == pat or startswith/endswith for anchored checks. Rolling compare avoids rebuilding slices when you slide a window of fixed length.

Why this shows up in the real world

DNA k-mer scans compare each window to a motif. Search snippets bold the matching substring after compare.

Core idea (explained for students)

For every i, compare s[i : i + m] to pat—early break on first mismatch. Python slices copy—use character-wise loop if profiling shows allocation pressure (rare).

Try this in Python

def window_equals(s: str, i: int, pat: str) -> bool:
    return s[i : i + len(pat)] == pat


print(window_equals("abcdef", 2, "cde"))

Common mistakes

• Off-by-one window end i+m > n—loop i in range(n - m + 1).
• Case sensitivity—normalize once if needed.

Key takeaways

• Reuse m = len(pat) outside loops.
• For many patterns against one text, consider Aho–Corasick when complexity demands it.