1.1 Introduction to Algorithms, Programming, and Compilers
AP Computer Science A

Overview

In AP Computer Science A, Unit 1 introduces how computers solve problems using algorithms, programs, and compilers. This section helps students understand how ideas become working computer programs.

1. Algorithms

An algorithm is a step-by-step process or set of rules designed to accomplish a specific task or solve a problem.

• In everyday life: A recipe is an algorithm for cooking a meal.
• In computer science: An algorithm is a logical sequence of instructions that a computer can follow.

Key Points:

• Algorithms must be clear, precise, and finite (they must eventually stop).
• They can be expressed in pseudocode, flowcharts, or programming languages like Java.
• Efficiency matters — good algorithms use fewer steps or resources like time and memory.

Example: Algorithm to find the largest number in a list:

1. Start with the first number as the “largest.”
2. Compare it to the next number.
3. If the next number is larger, update your “largest” value.
4. Continue until the end of the list.
5. Output the largest number.

2. Programming

Programming is the process of translating an algorithm into code that a computer can execute. In APCSA, students write programs in Java, a high-level, object-oriented language.

Key Concepts in Programming:

• Syntax: The correct structure of code (like grammar in English).
• Logic: The reasoning behind code behavior (loops, conditionals, methods).
• Debugging: Finding and fixing errors in code.
• Problem solving: Designing programs to achieve a goal.

Example:

int largest = numbers[0];
for (int i = 1; i < numbers.length; i++) {
    if (numbers[i] > largest) {
        largest = numbers[i];
    }
}
System.out.println("Largest: " + largest);

3. Compilers

A compiler is a special software tool that translates code written by humans (like Java) into machine language that the computer can understand.

How It Works:

1. You write code in Java (.java file).
2. The Java compiler (javac) checks for syntax errors and converts it into bytecode (.class file).
3. The Java Virtual Machine (JVM) executes that bytecode on any computer.

Why It Matters:

• The compiler ensures your code is error-free and efficient.
• It provides error messages to help identify mistakes.
• Understanding compilation helps students interpret errors and debug faster.

Summary

Concept	Description	Example
Algorithm	Step-by-step instructions to solve a problem	Sorting a list of numbers
Programming	Writing those steps in a programming language	Java code implementing the sort
Compiler	Converts Java code to machine-executable form	javac MyProgram.java

In AP Computer Science A

Students apply this understanding by:

• Designing algorithms before coding.
• Writing and debugging Java programs.
• Compiling and executing their programs to test for correctness.
• Learning the relationship between logic (algorithm) → syntax (program) → execution (compiler and computer).