DSA Introduction

Data structures store data in useful ways, and algorithms are the steps that work on that data.

What is a Data Structure?

A data structure is a way of organising data so it can be used well. You already use them in daily life:

What is an Algorithm?

An algorithm is a clear set of steps to solve a problem. A recipe for making chai is an algorithm: boil water, add tea, add milk, add sugar, boil again, pour.

In programming, algorithms do jobs like searching for an item, sorting a list, or finding the shortest path between two places.

A good algorithm is:

• Correct: it always gives the right answer
• Clear: each step is exact, with no guessing
• Finite: it finishes after some steps
• Efficient: it does not waste time or memory

Why Does It Matter?

Imagine a shop in Karachi with 1,000,000 products. Two ways to find a product by its code:

1. Check every product one by one. In the worst case that is 1,000,000 checks.
2. Keep the codes sorted and use binary search. That needs only about 20 checks.

Both are correct, but the second is enormously faster. Choosing the right method is what DSA teaches you.

A Simple Algorithm in Python

This algorithm counts how many students passed:

marks = [72, 45, 88, 39, 91, 60]

passed = 0
for m in marks:
    if m >= 50:
        passed += 1

print("Passed:", passed, "out of", len(marks))

Passed: 4 out of 6

The steps are simple: start a counter at zero, look at each mark, and add one when the mark is 50 or more.

Choosing a Data Structure

The same data can be stored in different ways. Here a list and a dictionary both hold phone numbers, but finding a number is easier with the dictionary:

contacts_list = [("Ali", "0300-1111111"), ("Sara", "0321-2222222")]
for name, phone in contacts_list:
    if name == "Sara":
        print("From list:", phone)

contacts_dict = {"Ali": "0300-1111111", "Sara": "0321-2222222"}
print("From dict:", contacts_dict["Sara"])

From list: 0321-2222222
From dict: 0321-2222222

The list has to be checked item by item. The dictionary jumps straight to the answer. You will see why in the hash tables lesson.

Linear and Non-linear Structures

• Linear: items are in a sequence, one after another. Arrays, linked lists, stacks and queues.
• Non-linear: items connect in branches or networks. Trees and graphs.

Exercise