Numeric Types, Strings, Runes & Byte Slices

Go's built-in types separate fixed-width numbers, UTF-8 text, Unicode code points (runes), and raw bytes.

Mixing them without conversion is a common source of off-by-one and encoding bugs.

Summary

Integers and floats have explicit sizes; untyped constants adapt to context.

Strings are immutable UTF-8 byte sequences.

A rune is an alias for int32 representing a Unicode code point.

Use []byte for binary data and []rune when you need character-indexed text operations.

Recipe

Quick-reference recipe card - copy-paste ready.

package main
 
import (
    "fmt"
    "strconv"
    "unicode/utf8"
)
 
func main() {
    s := "café"
    fmt.Println(utf8.RuneCountInString(s)) // rune count: 4
 
    n, err := strconv.Atoi("42")
    if err != nil {
        panic(err)
    }
    fmt.Println(n + 1)
}

When to reach for this:

Count characters with utf8.RuneCountInString, not len(string).
Parse user input with strconv, not fmt.Sscanf, for clear errors.
Convert numbers explicitly with int64(x) when widening or narrowing.
Treat string and []byte as distinct unless copying for APIs.

Working Example

package main
 
import (
    "fmt"
    "strconv"
    "unicode/utf8"
)
 
func main() {
    price := "19.99"
    amount, err := strconv.ParseFloat(price, 64)
    if err != nil {
        fmt.Println("parse error:", err)
        return
    }
 
    title := "Hello, 世界"
    fmt.Println("bytes:", len(title))
    fmt.Println("runes:", utf8.RuneCountInString(title))
 
    for i, r := range title {
        fmt.Printf("byte index %d: %U\n", i, r)
    }
 
    b := []byte(title)
    b[0] = 'h' // mutates copy only if you reassign string from b
    fmt.Println(string(b))
}

What this demonstrates:

strconv.ParseFloat for string-to-number conversion with errors.
len on strings measures UTF-8 bytes, not runes.
range over strings decodes runes and yields byte indices.
[]byte(string) allocates a mutable copy of bytes.

Deep Dive

How It Works

Numeric types are signed/unsigned integers (int, int8..int64, uint, uint8..uint64), floats (float32, float64), and complex64/complex128.
int and uint size is platform-dependent (32 or 64 bit); prefer sized types for serialization.
Strings are read-only; converting to []byte copies unless using unsafe (avoid).
UTF-8 encodes runes as 1-4 bytes; invalid UTF-8 is replaced with U+FFFD when ranging.

Numeric Types at a Glance

Type	Typical use	Overflow
`int`	indexes, lengths	wraps in unchecked arithmetic
`int64`	IDs, timestamps	same
`uint8`	raw bytes	same
`float64`	measurements	IEEE rounding errors
`big.Int` (package)	arbitrary precision	not built-in

String, Rune, Byte

Operation	Code	Result
Byte length	`len(s)`	UTF-8 byte count
Rune count	`utf8.RuneCountInString(s)`	Unicode code points
Iterate runes	`for i, r := range s`	byte index + rune
Byte slice	`[]byte(s)`	mutable copy
Rune slice	`[]rune(s)`	one rune per code point

Go Notes

// strconv is the idiomatic string/number bridge
i, err := strconv.ParseInt("ff", 16, 64)
f := strconv.FormatFloat(3.5, 'f', 2, 64)
 
// rune literal vs byte
var letter rune = 'A'   // int32 code point
var raw byte = 'A'      // uint8 ASCII

Gotchas

Using len for character count - "é" has len 2 in UTF-8. Fix: utf8.RuneCountInString or range.
Truncating with int32(x) silently - Large int64 to int32 wraps. Fix: check bounds before conversion.
Mutating []byte derived from string - Safe for the slice, but reassigning string(b) creates new string; original unchanged. Fix: treat conversion as copy.
fmt.Sscanf for parsing - Weak error reporting vs strconv. Fix: prefer strconv.Parse* functions.
Comparing floats with == - 0.1+0.2 issues. Fix: epsilon compare or integers in cents.
Indexing string by byte - s[i] is a byte, not a rune; may split multibyte sequences. Fix: range or convert to []rune for indexing.

Alternatives

Alternative	Use When	Don't Use When
`[]byte`	IO, crypto, wire formats	You need character operations
`[]rune`	Reverse, insert by character index	Memory matters for huge ASCII text
`strings.Builder`	Building strings in loops	Few concatenations (`+` is fine)
`math/big`	Decimal money, crypto	Simple counters

FAQs

What is a rune in Go?

type rune = int32 - a Unicode code point.

Rune literals like '世' are typed as untyped rune constants.

Why are strings immutable?

Immutability enables safe sharing and efficient substring slices that reference the same backing array.

Mutations go through []byte or strings.Builder.

How do I convert int to string?

strconv.Itoa(42)
strconv.FormatInt(42, 10)
fmt.Sprintf("%d", 42) // allocates more; fine for logs

What is the difference between byte and uint8?

They are identical types; byte is an alias for uint8 used for ASCII/binary data.

How does range over string handle invalid UTF-8?

Invalid bytes are replaced with rune utf8.RuneError (U+FFFD) and width 1.

Should I use float64 for money?

No - use integer minor units (cents) or shopspring/decimal / big.Rat for exact arithmetic.

What is an untyped constant?

Constants without explicit type (like 42 or 3.14) take the type demanded by the context with default int/float64/complex128.

How do I trim UTF-8 safely?

Use strings.TrimSpace and utf8.ValidString when validating user text.

Avoid trimming by raw byte counts for display widths.

Can I range over []byte as runes?

for _, b := range []byte(s) yields bytes, not runes.

Use for _, r := range string(b) or utf8.DecodeRune.

What does strconv.Atoi do?

Shortcut for ParseInt(s, 10, 0) returning int.

It errors on overflow for the platform int size.

How do I concatenate strings efficiently?

Use strings.Builder in loops; the compiler optimizes simple + chains of few parts.

Are string and []byte interchangeable in JSON?

json.Marshal encodes both as strings or base64 depending on field type.

Pick one type in your API structs deliberately.

Arrays, Slices & Maps at a Glance - Slice mechanics behind []byte and []rune
Zero Values and Initialization - Empty string and numeric defaults
Variables, Constants & Scope - Typed vs untyped constants
Go Fundamentals Basics - String iteration example

Stack versions: This page was written for Go 1.26.x (Green Tea GC default, go fix modernizers - verify patch at build), chi (latest - verify at build), gin (latest - verify at build), echo (latest - verify at build), google.golang.org/grpc (latest - verify at build), sigs.k8s.io/controller-runtime (latest - verify at build), kubebuilder (latest - verify at build), tinygo (latest - verify board targets at build), wazero (latest - verify at build), and golangci-lint (latest - verify linter set at build).

Numeric Types, Strings, Runes & Byte Slices

Go's built-in types separate fixed-width numbers, UTF-8 text, Unicode code points (runes), and raw bytes.

Mixing them without conversion is a common source of off-by-one and encoding bugs.

Summary

Integers and floats have explicit sizes; untyped constants adapt to context.

Strings are immutable UTF-8 byte sequences.

A rune is an alias for int32 representing a Unicode code point.

Use []byte for binary data and []rune when you need character-indexed text operations.

Recipe

Quick-reference recipe card - copy-paste ready.

package main
 
import (
    "fmt"
    "strconv"
    "unicode/utf8"
)
 
func main() {
    s := "café"
    fmt.Println(utf8.RuneCountInString(s)) // rune count: 4
 
    n, err := strconv.Atoi("42")
    if err != nil {
        panic(err)
    }
    fmt.Println(n + 1)
}

When to reach for this:

Count characters with utf8.RuneCountInString, not len(string).
Parse user input with strconv, not fmt.Sscanf, for clear errors.
Convert numbers explicitly with int64(x) when widening or narrowing.
Treat string and []byte as distinct unless copying for APIs.

Working Example

package main
 
import (
    "fmt"
    "strconv"
    "unicode/utf8"
)
 
func main() {
    price := "19.99"
    amount, err := strconv.ParseFloat(price, 64)
    if err != nil {
        fmt.Println("parse error:", err)
        return
    }
 
    title := "Hello, 世界"
    fmt.Println("bytes:", len(title))
    fmt.Println("runes:", utf8.RuneCountInString(title))
 
    for i, r := range title {
        fmt.Printf("byte index %d: %U\n", i, r)
    }
 
    b := []byte(title)
    b[0] = 'h' // mutates copy only if you reassign string from b
    fmt.Println(string(b))
}

What this demonstrates:

strconv.ParseFloat for string-to-number conversion with errors.
len on strings measures UTF-8 bytes, not runes.
range over strings decodes runes and yields byte indices.
[]byte(string) allocates a mutable copy of bytes.

Deep Dive

How It Works

Numeric types are signed/unsigned integers (int, int8..int64, uint, uint8..uint64), floats (float32, float64), and complex64/complex128.
int and uint size is platform-dependent (32 or 64 bit); prefer sized types for serialization.
Strings are read-only; converting to []byte copies unless using unsafe (avoid).
UTF-8 encodes runes as 1-4 bytes; invalid UTF-8 is replaced with U+FFFD when ranging.

Numeric Types at a Glance

Type	Typical use	Overflow
`int`	indexes, lengths	wraps in unchecked arithmetic
`int64`	IDs, timestamps	same
`uint8`	raw bytes	same
`float64`	measurements	IEEE rounding errors
`big.Int` (package)	arbitrary precision	not built-in

String, Rune, Byte

Operation	Code	Result
Byte length	`len(s)`	UTF-8 byte count
Rune count	`utf8.RuneCountInString(s)`	Unicode code points
Iterate runes	`for i, r := range s`	byte index + rune
Byte slice	`[]byte(s)`	mutable copy
Rune slice	`[]rune(s)`	one rune per code point

Go Notes

// strconv is the idiomatic string/number bridge
i, err := strconv.ParseInt("ff", 16, 64)
f := strconv.FormatFloat(3.5, 'f', 2, 64)
 
// rune literal vs byte
var letter rune = 'A'   // int32 code point
var raw byte = 'A'      // uint8 ASCII

Gotchas

Using len for character count - "é" has len 2 in UTF-8. Fix: utf8.RuneCountInString or range.
Truncating with int32(x) silently - Large int64 to int32 wraps. Fix: check bounds before conversion.
Mutating []byte derived from string - Safe for the slice, but reassigning string(b) creates new string; original unchanged. Fix: treat conversion as copy.
fmt.Sscanf for parsing - Weak error reporting vs strconv. Fix: prefer strconv.Parse* functions.
Comparing floats with == - 0.1+0.2 issues. Fix: epsilon compare or integers in cents.
Indexing string by byte - s[i] is a byte, not a rune; may split multibyte sequences. Fix: range or convert to []rune for indexing.

Alternatives

Alternative	Use When	Don't Use When
`[]byte`	IO, crypto, wire formats	You need character operations
`[]rune`	Reverse, insert by character index	Memory matters for huge ASCII text
`strings.Builder`	Building strings in loops	Few concatenations (`+` is fine)
`math/big`	Decimal money, crypto	Simple counters

FAQs

What is a rune in Go?

type rune = int32 - a Unicode code point.

Rune literals like '世' are typed as untyped rune constants.

Why are strings immutable?

Immutability enables safe sharing and efficient substring slices that reference the same backing array.

Mutations go through []byte or strings.Builder.

How do I convert int to string?

strconv.Itoa(42)
strconv.FormatInt(42, 10)
fmt.Sprintf("%d", 42) // allocates more; fine for logs

What is the difference between byte and uint8?

They are identical types; byte is an alias for uint8 used for ASCII/binary data.

How does range over string handle invalid UTF-8?

Invalid bytes are replaced with rune utf8.RuneError (U+FFFD) and width 1.

Should I use float64 for money?

No - use integer minor units (cents) or shopspring/decimal / big.Rat for exact arithmetic.

What is an untyped constant?

Constants without explicit type (like 42 or 3.14) take the type demanded by the context with default int/float64/complex128.

How do I trim UTF-8 safely?

Use strings.TrimSpace and utf8.ValidString when validating user text.

Avoid trimming by raw byte counts for display widths.

Can I range over []byte as runes?

for _, b := range []byte(s) yields bytes, not runes.

Use for _, r := range string(b) or utf8.DecodeRune.

What does strconv.Atoi do?

Shortcut for ParseInt(s, 10, 0) returning int.

It errors on overflow for the platform int size.

How do I concatenate strings efficiently?

Use strings.Builder in loops; the compiler optimizes simple + chains of few parts.

Are string and []byte interchangeable in JSON?

json.Marshal encodes both as strings or base64 depending on field type.

Pick one type in your API structs deliberately.

Arrays, Slices & Maps at a Glance - Slice mechanics behind []byte and []rune
Zero Values and Initialization - Empty string and numeric defaults
Variables, Constants & Scope - Typed vs untyped constants
Go Fundamentals Basics - String iteration example

Stack versions: This page was written for Go 1.26.x (Green Tea GC default, go fix modernizers - verify patch at build), chi (latest - verify at build), gin (latest - verify at build), echo (latest - verify at build), google.golang.org/grpc (latest - verify at build), sigs.k8s.io/controller-runtime (latest - verify at build), kubebuilder (latest - verify at build), tinygo (latest - verify board targets at build), wazero (latest - verify at build), and golangci-lint (latest - verify linter set at build).

Numeric Types, Strings, Runes & Byte Slices

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Numeric Types, Strings, Runes & Byte Slices