Numbers & Math

Ruby is a dynamic, beginner-friendly programming language, and numbers power nearly every program — from totals and counters to prices and percentages.

Learn Numbers & Math in our free Ruby course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Ruby course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll do arithmetic, understand the Integer-vs-Float trap, and use the numeric methods Ruby gives every number.

What You'll Learn in This Lesson

1️⃣ Arithmetic and the Division Trap

Ruby's arithmetic operators look familiar: + - * / , plus ** for exponent and % for the remainder. The one surprise is division: when both sides are Integers, Ruby throws away the fraction. Make one side a Float to keep the decimals.

That 7 / 3 == 2 result trips up nearly every beginner. Remember: a Float anywhere in the division "infects" the result with decimals.

2️⃣ Numeric Methods

Because numbers are objects, they carry useful methods. You'll reach for round , floor , and ceil for rounding, even? / odd? for tests, and abs for absolute value. Note to_i truncates rather than rounds.

Your turn. Split a bill with a tip among friends. Replace the three TODO values — keep total and tip as Floats so cents survive — then run it.

Mini-Challenge: A Temperature Converter

Convert Celsius to Fahrenheit, remembering to use Floats in the formula. Run it with ruby temp.rb .

Common Errors (and the fix)

• Unexpected integer division — 1 / 2 is 0 . Use 1.0 / 2 for 0.5 .
• to_i rounds? No — it truncates — 9.9.to_i is 9 . Use round if you want 10.
• "divided by 0" (ZeroDivisionError) — Integer division by zero raises an error. Check the divisor first, or use Floats where 1.0/0 gives Infinity .
• Adding a string and a number — "5" + 1 errors. Convert with "5".to_i + 1 .
• Float surprises — 0.1 + 0.2 isn't exactly 0.3 . Round for display, or use cents/BigDecimal for money.

Pro Tips

• 💡 Underscores in big numbers: write 1_000_000 for readability — Ruby ignores the underscores.
• 💡 fdiv for safe division: 7.fdiv(3) always returns a Float, no Float conversion needed.
• 💡 Random numbers: rand(6) gives 0–5; rand(1..6) rolls a die.

📋 Quick Reference — Numbers & Math

Concept

Ruby Syntax

Exponent / modulo

2 ** 8 / 7 % 3

Float division

7.0 / 3

Round to n places

x.round(2)

Floor / ceil

x.floor / x.ceil

Even? / abs

n.even? / n.abs

Random

rand(1..6)

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ Ruby has Integers (whole) and Floats (decimal)
• ✅ Operators: + - * / ** %
• ✅ Integer ÷ Integer truncates — use a Float to keep decimals
• ✅ Numbers have methods: round , floor , ceil , even?
• ✅ Watch Float precision for money; round or use cents
• ✅ Next lesson: Control Flow — make decisions in code

Practice quiz

Continue this course

• Previous: Strings & Symbols
• Next: Control Flow