Ch 4Working with Lists and Dictionaries — Class 11 Informatics Practices, concept-first.

A list is Python's way of keeping several values together in a single, ordered container. Formally, the list data type is an ordered sequence which is mutable and is made up of one or more elements.

Once data sits inside a list, you need a way to reach any single element directly — without walking through the others.

The definition of a list calls it an ordered sequence which is mutable — and mutability is the property this section pins down.

A list is not just a container to store values — the list data type lets you manipulate its contents through a set of built-in operations.

Concatenation is the merging of two or more values into one — the way two strings can be joined end to end.

Where concatenation joins different lists, repetition replicates one list. Python allows us to replicate the contents of a list using the repetition operator, depicted by the symbol .

Often the question is not where an element sits in a list but simply whether it is there at all. Python answers this with the membership operators, which test a list's contents and reply with a Boolea…

Indexing pulls out one element; slicing pulls out a whole run of them. A slicing operation creates a new list by taking out elements from an existing list — you name the range of indexes you want, and…

Creating a list is only half the story — most programs need to visit every element of a list one after another, whether to display them, add them up, or test each one against a condition.

Lists come with a rich set of ready-made tools: methods that belong to the list type itself (called as list1.append(...), list1.sort(), and so on) and built-in functions that take a list as an argumen…

Everything learnt so far — creating lists, indexing them, and the methods append(), insert(), extend(), pop(), remove(), sort() — comes together in complete programs.

A list answers the question "what is the element at position 3?" — but many real situations ask a different kind of question: "what are Mohan's marks?" or "what is the capital of Kerala?".

A dictionary is written between curly braces , with the items separated by commas, and each item written as a key-value pair with a colon (:) between the key and its value.

Sequences — strings, lists and tuples — hand out their elements through indexing: you ask for position 0, 1, 2 and so on. A dictionary works on a different principle.

Accessing a missing key with square brackets ends in a KeyError, so a program often needs to check first whether a key exists.

Like lists — and unlike strings and tuples — dictionaries are mutable: the contents of a dictionary can be changed after it has been created.

Traversing a dictionary means visiting every item it holds — each key together with its value — one at a time, usually to display them or to process them in some way.

Python supplies a set of ready-made functions and methods for working on dictionaries, so that everyday jobs — counting the pairs, pulling out all the keys or all the values, fetching a value safely,…

Creating a dictionary is only the starting point; the real skill is applying the methods learnt in this chapter to inspect it, query it and change it.

- 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.