Write a program to input your friends’ names and their Phone Numbers and store them in the dictionary as the k

Programming Problems · Q5

Q. Write a program to input your friends’ names and their Phone Numbers and store them in the dictionary as the key-value pair. Perform the following operations on the dictionary:

a) Display the name and phone number of all your friends

b) Add a new key-value pair in this dictionary and display the modified dictionary

c) Delete a particular friend from the dictionary

d) Modify the phone number of an existing friend

e) Check if a friend is present in the dictionary or not

f) Display the dictionary in sorted order of names

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Concept understanding — Dictionary Key Access

Dictionary Key Access

Imagine you have a physical address book. Each entry has a name (like "Ravi") and a phone number (like "[PII:phone]"). To find Ravi's number, you don't flip through every page — you look up "Ravi" directly. That name is the key, and the number is the value.

A dictionary in programming is exactly that: a collection of key-value pairs. The key is what you know; the value is what you want to retrieve. Key access is the operation of giving the dictionary a key and getting back the value tied to it.

The Intuition

Think of a real dictionary (the book). You look up a word (the key) to find its definition (the value). You never look up a definition to find the word — that would be backwards. Similarly, in a Python dictionary, you always access values by their keys, never the other way around.

Note

Keys must be unique and immutable (strings, numbers, tuples). Values can be anything — numbers, strings, lists, even other dictionaries.

The Precise Statement

In Python, dictionary key access is done using square brackets [] with the key inside, or the .get() method.

Syntax:

dictionary_name[key]

dictionary_name.get(key, default_value)

Example:

student = {"name": "Anjali", "age": 17, "grade": "A"}
print(student["name"])   # Output: Anjali
print(student.get("age")) # Output: 17

What Happens Under the Hood

When you write student["name"], Python:

Takes the key "name".
Computes its hash (a fixed-size number derived from the key's content).
Uses that hash to jump directly to the memory location where the value "Anjali" is stored.

This is why dictionary access is fast — it doesn't scan through all entries. It's a direct lookup, like using an index in a book's index rather than reading every page.

Watch out

If you use a key that does not exist with square brackets, Python raises a KeyError and your program crashes. The .get() method is safer — it returns None (or a default you provide) instead of crashing.

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A phone book is the textbook dictionary: name = key, phone number = value. Each requested operation is one dictionary idiom — loop to display, d[key] = value to add or modify, del d[key] to delete, key in d to check presence, and sorted(d) to visit names alphabetically.

friends = {'Tanvi': '9876512340', 'Rahul': '9123456780', 'Zoya': '9988776655'}
for name in friends: print(name, friends[name])      # a) display
friends['Kabir'] = '9012345678'                      # b) add
del friends['Rahul']                                 # c) delete
friends['Zoya'] = '9000011111'                       # d) modify …

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Store friends as name -> phone-number pairs; then a) loop and print, b) assign a new key, c) del a key, d) reassign an existing key, e) test with in, f) iterate over sorted(friends).

The idea. Key-value data with unique, meaningful keys (names) and one value each (phone number) is exactly what a dictionary models. Every operation the question asks for maps to a single dictionary idiom — no searching or index bookkeeping needed. (In a real program you would first fill the dictionary with an input() loop, friends[input("Name: ")] = input("Phone: "); here the data is pre-filled so each operation's effect is visible.)

Program:

friends = {'Tanvi': '9876512340', 'Rahul': '9123456780', 'Zoya': '9988776655'}

# a) display the name and phone number of all friends
print("a) All friends:")
for name in friends:
    print("  ", name, ":", friends[name])

# b) add a new key-value pair
friends['Kabir'] = '9012345678'
print("b) After adding Kabir :", friends)

# c) delete a particular friend
del friends['Rahul']
print("c) After deleting Rahul :", friends)

# d) modify the phone number of an existing friend
friends['Zoya'] = '9000011111'
print("d) After changing Zoya's number :", friends)

# e) check if a friend is present or not
name = 'Tanvi'
if name in friends:
    print("e)", name, "is present in the dictionary")
else:
    print("e)", name, "is not present in the dictionary")

# f) display the dictionary in sorted order of names
print("f) Dictionary in sorted order of names:")
for name in sorted(friends):
    print("  ", name, ":", friends[name])

Verified output:

a) All friends:
   Tanvi : 9876512340
   Rahul : 9123456780
   Zoya : 9988776655
b) After adding Kabir : {'Tanvi': '9876512340', 'Rahul': '9123456780', 'Zoya': '9988776655', 'Kabir': '9012345678'}
c) After deleting Rahul : {'Tanvi': '9876512340', 'Zoya': '9988776655', 'Kabir': '9012345678'}
d) After changing Zoya's number : {'Tanvi': '9876512340', 'Zoya': '9000011111', 'Kabir': '9012345678'}
e) Tanvi is present in the dictionary
f) Dictionary in sorted order of names:
   Kabir : 9012345678
   Tanvi : 9876512340
   Zoya : 9000011111

Operation-by-operation: …

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Q.Write a program to input your friends’ names and their Phone Numbers and store them in the dictionary as the key-value pair. Perform the following operations on the dictionary:

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