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DSAPath18 mincore

Python String Patterns

Python string patterns cover splitting, joining, searching, prefix checks, character codes, slicing costs, and frequency windows.

DifficultyCore
TierTier 2
ModulePython Practice
LanguagesPython
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Why This Matters

String problems are usually sequence problems with extra costs. The characters are easy to scan, but copying substrings, rebuilding strings repeatedly, or using the wrong search primitive can change the complexity.

Use this page before KMP string matching and Rolling hash.

Core Idea

Treat strings as immutable sequences. Scan with indices when performance matters, and build output with lists plus join when repeated concatenation would copy too much.

NeedPython moveWatch for
Tokenize textsplitseparators and empty fields
Build outputjoinjoin strings, not arbitrary objects
Find substringfindreturns -1 when absent
Prefix checkstartswithclearer than slicing for intent
Suffix checkendswithavoids manual index math
Character codeordmaps char to integer
Integer to charchrmaps code point to char
Substringslicingcreates a new string

Non-Example or Failure Mode

Repeatedly doing answer += ch inside a long loop can allocate many intermediate strings. Prefer parts.append(ch) and "".join(parts) when building a result incrementally.

Worked Example

Normalize a comma-separated tag string:

raw = " graph, heap , search "
tags = [part.strip() for part in raw.split(",")]
result = "-".join(tags)

The useful point is not the syntax. It is the model: split into parts, clean each part, then join once.

Common Mistakes

MistakeCorrection
Assuming string slicing is free.Slices allocate new strings. Use indices in hot loops.
Rebuilding strings one character at a time.Collect parts in a list and call join once.
Forgetting find returns -1.Check index != -1 before using it.
Using slicing for every prefix check.startswith makes the intent clear.
Confusing characters with bytes.Use encode and decode when crossing the byte boundary.

Diagnostic Questions

Question typeQuestionAnswer signal
DefinitionAre Python strings mutable?No. Operations create new strings.
Example / non-exampleIs text[:i] free?No. It creates a substring copy.
ComputationWhat does "abc".find("z") return?-1.
TransferWhy use ord in rolling hash?It converts characters to integer values for arithmetic.

Runnable Drill

Python string pattern drill

Checks split, join, find, prefix checks, ord, chr, slicing, and a small frequency window.

Output will appear here.

Exercises

Beginner:

  • Use split and join to normalize a list of comma-separated words.
  • Explain the difference between find and index.
  • Convert characters to integer codes with ord.

Intermediate:

  • Detect whether a string has any repeated character.
  • Replace repeated slicing in a palindrome check with two indices.

Challenge:

  • Implement fixed-window anagram search with Counter, then explain the update invariant.

Diagram Recommendation

Type: sliding-window character count.

Caption: "A fixed-width window moves across a string while the frequency table is updated by one outgoing and one incoming character."

Purpose: Connect string methods to the same invariants used in array sliding-window problems.

Next Topics

References