Type Conversion

Go never converts between types automatically — not even from int to float64. Every conversion is written out.

Syntax

f := float64(n)   // Type(value) converts between numeric types
i := int(f)       // truncates toward zero

Converting between numbers and text is different: it uses the strconv ("string conversion") package.

• strconv.Itoa(n) — int to string ("integer to ASCII").
• strconv.Atoi(s) — string to int. Because the text might not be a number, it returns two values: the number and an error (which is nil when everything worked).

Example

package main

import (
    "fmt"
    "strconv"
)

func main() {
    count := 7
    total := 2
    average := float64(count) / float64(total)
    fmt.Println(average)

    price := 19.99
    fmt.Println(int(price))

    label := "Item " + strconv.Itoa(42)
    fmt.Println(label)

    n, err := strconv.Atoi("123")
    fmt.Println(n+1, err)

    bad, err := strconv.Atoi("abc")
    fmt.Println(bad, err)
}

Expected output

3.5
19
Item 42
124 <nil>
0 strconv.Atoi: parsing "abc": invalid syntax

<nil> means "no error". You'll learn to check errors properly with if err != nil in the Multiple Return Values & Errors lesson.

Common mistakes

• Writing count / total with two ints and expecting 3.5 — integer division gives 3. Convert before dividing.
• Using string(65) to turn a number into text. That produces "A" (the character with code 65), not "65"; use strconv.Itoa instead. go vet warns about this mistake.
• Ignoring the error from strconv.Atoi — on invalid input the number is silently 0.

Practice

Convert the string "25" to an int, add 5, and print the result along with the error value.

package main

import (
    "fmt"
    "strconv"
)

func main() {
    n, err := strconv.Atoi("25")
    fmt.Println(n+5, err) // 30 <nil>
}