String Formatting (format!, println!)

Rust's formatting macros — println! , format! , and eprintln! — substitute values into a template using {} placeholders that also control width, precision, alignment, and number base.

Learn String Formatting (format!, println!) in our free Rust course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a…

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

In this lesson you'll learn Display vs Debug formatting, positional and named arguments, the width/precision/fill mini-language, printing in binary and hex, and how to escape braces.

What You'll Learn in This Lesson

1️⃣ The Core Macros & Display vs Debug

println! writes to standard output, eprintln! to standard error, and format! returns a String instead of printing. The placeholder {} uses Display (user-facing); {:?} uses Debug (developer-facing, works for tuples and collections).

The (this went to stderr) line came from eprintln! . In your terminal both streams interleave, but they're separate channels — redirecting output ( cargo run > out.txt ) keeps stderr on screen.

2️⃣ Positional, Named & Inline Arguments

Empty {} consume arguments in order, but you can also reference them by index ( {0} , {1} ) to reuse or reorder, give them names, or — most concise — capture a variable directly inside the braces.

3️⃣ Width, Precision, Alignment & Radix

After a colon comes the format spec: an optional fill char and alignment ( < left, > right, ^ center), a minimum width, then a precision after a dot. You can also choose a number base with :b , :o , :x , and add # for the prefix.

These specs make aligned tables trivial: pick a width per column and an alignment, and rows line up perfectly. Note the last line — doubling braces ( {{}} ) prints a literal {} with no substitution.

Your turn. Fill in the blanks marked ___ , then run it.

Mini-Challenge: A Tidy Receipt

Use widths, alignment, and precision to print a perfectly columned receipt. Run it with cargo run and check the output.

Common Errors (and the fix)

• ❌ "X doesn't implement std::fmt::Display" — You used {} on a type without Display. ✅ Use {:?} and add #[derive(Debug)] .
• ❌ "invalid reference to positional argument" — A {0} index has no matching argument. ✅ Make sure indices line up with the arguments you passed.
• ❌ "argument never used" — You passed more arguments than placeholders. ✅ Add a placeholder or remove the extra argument.
• ❌ A literal { breaks the format — It's read as a placeholder. ✅ Double it: write {{ and }} for literal braces.

Pro Tips

• 💡 Prefer inline capture ( {name} ) for short, readable format strings.
• 💡 Use {:#?} when debugging nested data — the indented multi-line form is far easier to read.
• 💡 Build tables with fixed widths per column and alignment flags; everything lines up with zero manual spacing.

📋 Quick Reference — Format Specs

Spec

Effect

{} / {:?}

Display / Debug

{:#?}

Pretty (multi-line) Debug

{:>8} {:^8}

Right / center in width 8

{:.2}

2 decimal places

{:b} {:x} {:#x}

Binary / hex / hex w/ prefix

{{ }}

Literal braces

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ println! prints, format! returns a String, eprintln! writes to stderr
• ✅ {} = Display, {:?} = Debug, {:#?} = pretty Debug
• ✅ Arguments can be positional, named, or inline-captured
• ✅ Specs control width, precision, fill, alignment, and radix; {{}} escapes braces
• ✅ Next lesson: Pattern Matching — match , if let , and destructuring

Practice quiz

Continue this course

• Previous: Tuples & Arrays
• Next: Pattern Matching