Strings, Character Codes and Conversion

String length counts elements according to the language’s representation; indexing identifies a position. Python indexing starts at zero, so the last valid positive index is length minus one. A substring operation returns part of the text; a slice such as name[1:4] includes index 1 and excludes index 4.

Concatenation joins text without performing numerical addition. The strings "12" and "3" concatenate to "123". Searching for a substring requires a convention for failure: Python find returns -1 when absent, so treating that result as a successful index can accidentally access the last character.

A character coding system associates characters with numerical code points. In Python, ord converts a one-character string into its Unicode code point and chr performs the reverse operation. A character code is different from the numeric value described by a digit character.

Consecutive decimal digit characters have consecutive code points, allowing a deliberate conversion in simple contexts. General text needs an encoding when stored as bytes; a Unicode code point is not automatically a single byte. File decoding and in-memory string manipulation are different operations.

Parsing interprets text as a typed value. int("12") yields a whole-number value, whereas str(12) formats a value as text. Invalid syntax can cause an exception; silently replacing every invalid entry with zero changes the meaning of the input.

Date/time conversion requires an agreed format. A date such as 04/05/2026 is ambiguous without a day/month convention. Keep dates as date/time objects when comparing or calculating intervals, and format them for display separately. Document timezone assumptions when an instant rather than a local calendar date matters.

code = "AB-204"
print(len(code))
print(code[3:])
print(ord("5"), int("5"))

from datetime import datetime
value = datetime.strptime("31/10/2026", "%d/%m/%Y")
print(value.strftime("%Y-%m-%d"))

Attempt the task before opening its worked response. Record each change of state, explain the rule used, and check the result against the original requirements. Code examples labelled Python 3 use that language’s conventions; pseudocode states its own assumptions.