Conversion with strconv

Learn string-number conversions with strconv.

Conversion with strconv

The strconv package converts safely between numbers and text:

n, err := strconv.Atoi("42")         // string → int
s := strconv.Itoa(42)                 // int → string
f, err := strconv.ParseFloat("3.14", 64)
b, err := strconv.ParseBool("true")
t := strconv.FormatFloat(3.14159, 'f', 2, 64)  // "3.14"

Functions converting text to numbers always return an error; user input may be invalid.

The error type

A conversion error is of type *strconv.NumError and says why it failed:

_, err := strconv.Atoi("on iki")
fmt.Println(err)
// strconv.Atoi: parsing "on iki": invalid syntax

_, err = strconv.Atoi("99999999999999999999")
// ... value out of range

You can check the cause with errors.Is(err, strconv.ErrSyntax) and strconv.ErrRange.

Exercise Sum the inputs

Walk the inputs []string{"10", " 20 ", "otuz", "5"}; convert each with strconv.Atoi after strings.TrimSpace, skip the invalid ones and sum the valid ones. Print the sum and the number skipped on one line.

Show example solution

package main

import (
    "fmt"
    "strconv"
    "strings"
)

func main() {
    toplam, atlanan := 0, 0
    for _, s := range []string{"10", " 20 ", "otuz", "5"} {
        n, err := strconv.Atoi(strings.TrimSpace(s))
        if err != nil {
            atlanan++
            continue
        }
        toplam += n
    }
    fmt.Println(toplam, atlanan)
}

Test yourself

What does <code>strconv.Atoi(" 42")</code> (with a leading space) return?
- 0 and a syntax error
- 42 and nil
- A panic
- -42
Show answer

0 and a syntax error — Atoi doesn't accept spaces; user input should first be cleaned with strings.TrimSpace.

Convert to a number

n, err := strconv.____("17")

Show answer

n, err := strconv.Atoi("17")

Function and direction

strconv.Atoi

string → int
strconv.Itoa

int → string
strconv.ParseFloat

string → float64
strconv.ParseBool

string → bool

Next lesson: The strings package

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