Lecture on String and List Concatenation in Python

Overview

• Concatenation involves combining two entities (strings or lists) into a new one.
• The + operator is used for concatenation.

String Concatenation

• Concatenating strings combines them into a new string.
  • Example: 'abc' + 'def' results in 'abcdef'.

List Concatenation

• Works similarly to string concatenation.
• Combines two lists into a new list.
  • Example: [a, b] + [c, d] results in [a, b, c, d].

Key Points

• Single Data Type: You can only concatenate lists with lists.
• Type Error: Concatenating a list with a non-list (e.g., integer) causes a Type Error.
  • Solution: Enclose the non-list in a literal list.
    • Example: alist + [5] (not alist + 5).
• Immutable Original Lists: Concatenation does not alter the original lists.
  • The original lists remain unchanged.
  • The result of concatenation is a new list.

Examples and Experimentation

• Creating Lists:

  • List A: [ ]
  • List B: [ ]
  • List C created by concatenating A and B: C = A + B
  • Observation: A and B remain unchanged, C contains elements of A followed by B.

• Appending Elements:

  • Direct concatenation of non-list element causes error.
  • Enclosing the element in a list is necessary.
    • C + ['d'] creates a new list with 'd' appended.
  • The variable must be reassigned to the new list.

References and Modifications

• Variable Reference: Multiple variables can reference the same list.

  • Example: D = C (C and D reference the same list).

• Effect of Modifications:

  • Changing an element in C affects D since both reference the same list.
  • Concatenating a new list changes C's reference, not D's.
  • Example: After C = C + ['7'], C changes but D still references the original list.

Conclusion

• Concatenation yields new lists without modifying the originals.
• Variable references determine whether changes affect multiple variables.
• Experimentation in Python interpreter is encouraged to understand list behavior.