Algorithm Expression (College Board AP® Computer Science Principles): Study Guide

Algorithm fundamentals

What is an algorithm?

• An algorithm is a set of finite instructions that, when followed in order, accomplish a specific task

• Finite means the algorithm must eventually reach an end; it cannot run forever

• Every algorithm is constructed from three fundamental building blocks:

  • Sequencing: executing steps one after another in a specific order

  • Selection: making a decision based on a condition (e.g. IF a score is above 50, display "Pass")

  • Iteration: repeating a set of steps until a condition is met (e.g. REPEAT UNTIL all items have been checked)

• These three components can be combined to structure any algorithm, regardless of complexity

• Algorithm implementation is the process of translating an algorithm into working code in a programming language

• Algorithms can be expressed in several ways before being turned into code:

Natural language example

1. Ask the user to enter a number
2. Divide the number by 2 and check the remainder
3. If the remainder is 0, display "Even"
4. Otherwise, display "Odd"
 

Flowchart diagram example

Pseudocode example

number ← INPUT()
IF (number MOD 2 = 0)
{
   DISPLAY("Even")
}
ELSE
{
   DISPLAY("Odd")
}
 

• All three representations describe the same algorithm; only the format changes

• Natural language is the easiest to understand but the least precise

• Pseudocode is the closest to actual code and the most useful for algorithm implementation