Informatics Practices · Ch 4 — Working with Lists and Dictionaries
Summary
Summary
- Lists are mutable sequences in Python — the elements of a list can be changed after it is created.
- A list is written by placing its elements inside square brackets, separated by commas, e.g.
[10, 20, 30]. - List indexing starts at 0. Python allows two-way indexing: forward with indices 0, 1, 2, … and backward with negative indices, so a list can be traversed in either direction.
- The
+operator concatenates one list to the end of another; the*operator repeats the contents of a list the specified number of times. - The membership operator
intests whether an element is present in a list, andnot intests the opposite. - Slicing extracts a part of a list using the
list[start:stop:step]form. - Lists come with many manipulation methods and functions, including
len(),list(),append()(add one element at the end),extend()(add all elements of another list),insert(),count(),index(),remove(),pop(),reverse(),sort(),sorted(),min(),max()andsum(). append()adds its argument as a single element, whileextend()adds each element of its argument individually;sort()changes the list itself, whilesorted()returns a new sorted list leaving the original untouched.- A dictionary is a mapping (non-scalar) data type: an unordered collection of key-value pairs written inside curly braces.
- Each key is separated from its value by a colon, and consecutive pairs are separated by commas, e.g.
{'Mohan': 95, 'Ram': 89}. - Keys are unique and act as the index used to access values; keys must be of an immutable type, while values can be mutable.
- A dictionary is traversed with a
forloop — either looping over the dictionary itself (which yields keys, with values fetched asd[key]) or looping overd.items()(which yields each key-value pair directly). …