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Tamil Nadu HSE First Year Computer Science study materials
Tamil Nadu HSE First Year Computer Science study resources

Tamil Nadu HSE First Year Computer Science: Strings, Lists, Tuples and Dictionaries

Tamil Nadu HSE First Year Computer Science · Work through your exam, one topic at a time.

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Strings, Lists, Tuples and Dictionaries

Strings, Lists, Tuples and Dictionaries tests the built-in collection types: indexing and slicing, mutability, common methods, conversion between types, and the effect of aliasing.

Core rules

  • Strings are ordered and immutable. Indexing starts at 0; negative indices count from the end. Slicing uses [start:stop:step] and excludes stop.
  • String methods such as upper(), lower(), strip(), split(), replace() and find() return new values; they do not change the original string.
  • Lists are ordered and mutable. Methods include append(), extend(), insert(), remove(), pop(), sort() and reverse().
  • Tuples are ordered and immutable, written with parentheses. A one-element tuple needs a trailing comma: (5,).
  • Dictionaries store key–value pairs. Keys must be unique and immutable; values can be anything. Use get() to avoid a KeyError.
  • list(), tuple() and dict() convert between types where the data fits.

Worked example

a = [10, 20, 30]
b = a
b.append(40)
print(a)

This prints [10, 20, 30, 40]. The assignment b = a copies the reference, not the list, so both names point to the same object. To make an independent copy, use b = a[:] or b = list(a). Similarly, 'COMPUTER'[2:5] gives 'MPU' because indices 2, 3 and 4 are included and 5 is excluded.

Traps that cost marks

  • Trying s[0] = 'X' on a string; strings are immutable.
  • Writing (5) and expecting a tuple; it is just the integer 5.
  • Using d['key'] for a missing key instead of d.get('key').
  • Confusing remove(value) with pop(index) in lists.
  • Assuming slicing includes the stop index.