Class 11 Working with Lists and Dictionaries

Lists: indexing, slicing, operators and methods; dictionaries: keys and values, adding, updating, deleting and traversing them.

Key ideas

List indexes start at 0, and a negative index counts from the end: a[-1] is the last item.

a[i:j] runs from index i up to, not including, j; a[::2] takes every second item and a[::-1] reverses.

+ joins two lists and * repeats one; on lists they never do arithmetic on the items.

append(x) adds one item at the end, extend(L) adds each item of L, insert(i, x) puts x at index i; pop() removes and returns the last item, pop(i) the item at i, remove(x) the first x.

count(x) counts x and index(x) finds its first position; sort() sorts the list itself (reverse=True: largest first).

A dictionary maps unique, immutable keys to values; d[k] = v replaces the value of an existing key or adds a new pair at the end, and del d[k] removes it.

x in d checks the keys only; keys(), values() and items() follow the order the keys were added, and d1.update(d2) adds the pairs of d2, replacing the values of shared keys.

Remember

a = [10, 20, 30, 40, 50]

a[1] → 20; a[-1] → 50; a[1:4] → [20, 30, 40]; a[::2] → [10, 30, 50]

+ and *; built-ins

[1, 2] + [3] → [1, 2, 3]; [1, 2] * 2 → [1, 2, 1, 2]; for [4, 9, 2]: len 3, min 2, max 9, sum 15

pop and remove

pop() removes the LAST item and returns it; remove(x) removes the first x, by value

Third largest

a.sort() then a[-3]; third smallest: a[2]

Dictionary

d = {'Asha': 78}; d['Asha'] = 81 updates; d['Ravi'] = 64 adds; del d['Ravi'] removes

len(d) after changes

updating a key: no change; a new key: one more; del: one fewer

Traversing

for k in d:
if d[k] >= 75:
print(k) # keys in the order added

Answer (a) to (c) for this program.
a = [45, 47, 6, 37, 27]
a.extend([63, 73])
a.insert(1, 6)
p = a.pop()
print(p)
print(len(a))
print(a.index(6))

Working: after insert: [45, 6, 47, 6, 37, 27, 63, 73]; pop() removes the last item

Answer: (a) 73; (b) 7; (c) 1

Working with Lists and Dictionaries

Key ideas

Remember

Watch out

Worked example