如何在防范“切片边界超出范围”错误的同时轻松地在Go中获取子字符串？

Question

Using Go, I want to truncate long strings to an arbitrary length (e.g. for logging).

const maxLen = 100

func main() {
    myString := "This string might be longer, so we'll keep all except the first 100 bytes."

    fmt.Println(myString[:10])      // Prints the first 10 bytes
    fmt.Println(myString[:maxLen])  // panic: runtime error: slice bounds out of range
}

For now, I can solve it with an extra variable and if statement, but that seems very long-winded:

const maxLen = 100

func main() {
    myString := "This string might be longer, so we'll keep all except the first 100 bytes."

    limit := len(myString)
    if limit > maxLen {
        limit = maxLen
    }

    fmt.Println(myString[:limit]) // Prints the first 100 bytes, or the whole string if shorter
}

Is there a shorter/cleaner way?

Answer 1

Use a simple function to hide the implementation details. For example,

package main

import "fmt"

func maxString(s string, max int) string {
    if len(s) > max {
        r := 0
        for i := range s {
            r++
            if r > max {
                return s[:i]
            }
        }
    }
    return s
}

func main() {
    s := "日本語"
    fmt.Println(s)
    fmt.Println(maxString(s, 2))
}

Output:

日本語
日本