1. What does computer science deal with?

Computer science is the study of problem-solving methods together with the computer systems that execute them

1. Problem definition
2. Algorithm design
3. Program implementation
4. Execution on a computer system

The whole flow in one line

Problem → algorithm → program (C code) → executable → process → executed by the CPU

2. Problems and algorithms

• Algorithmic thinking: turning a problem into clear steps and data forms

Algorithm

• 5 basic conditions: input, output, definiteness, finiteness, effectiveness
• 2 evaluation criteria: correctness, efficiency

Example (Euclidean algorithm)

• gcd(m, n) = gcd(n, m mod n); when n becomes 0, m is the answer.
• gcd(48,18) → (18,12) → (12,6) → (6,0) → 6

3. Connecting programs and computer systems

• Until C code runs: preprocessing → compiling → assembling → linking → executable → OS loads it into memory

• Program execution path: user → application program → standard library → operating system → hardware

Abstraction

Hide the complex internals and expose only what the user needs

Address of arr[i] = start address + i × element size