python中list越界问题 python list切片越界

需求

很简单的知识点，做一下小结。

最近写代码需要做一下切片的操作，比如给定这样一个切片：

lst1 = [1,2,3,4,5,6,7,8,9,10,11,12,13]

将这个切片里面的元素按照每4个为一组，每一组组成单独的切片，然后再组合到外层的切片中，结果像这样：

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13]]

python的写法

# -*- coding:utf-8 -*-

lst1 = [1,2,3,4,5,6,7,8,9,10,11,12,13]
lst2 = ["w","wg","wa","jj","aa","ff","gg","hh","asd","ww","ee","gg","nn","mm"]

print(len(lst1)) # 13
print(len(lst2)) # 14

### 超限切片
ret_lst1 = list()
ret_lst2 = list()

# 根据lst1构建新的列表
for i in range(0,len(lst1),4):
    curr_lst = lst1[i:i+4]
    ret_lst1.append(curr_lst)

# 根据lst2构建新的列表
for i in range(0,len(lst2),4):
    curr_lst = lst2[i:i+4]
    ret_lst2.append(curr_lst)

### 注意结果最后一个列表不足的话不会报错
print(ret_lst1) # [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13]]
print(ret_lst2) # [['w', 'wg', 'wa', 'jj'], ['aa', 'ff', 'gg', 'hh'], ['asd', 'ww', 'ee', 'gg'], ['nn', 'mm']]

Go的写法

错误的写法

package t9

import (
    "fmt"
    "testing"
)


func TestOutRange(t *testing.T) {
    lst1 := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}
    lst2 := []string{"w", "wg", "wa", "jj", "aa", "ff", "gg", "hh", "asd", "ww", "ee", "gg", "nn", "mm"}

    fmt.Println(len(lst1)) // 13
    fmt.Println(len(lst2)) // 14

    // 结果
    var retLst1 [][]int
    var retLst2 [][]string

    // 超限切片
    for i := 0; i < len(lst1); i += 4 {
        currLst := lst1[i : i+4]
        retLst1 = append(retLst1, currLst)
    }

    for i := 0; i < len(lst2); i += 4 {
        currLst := lst2[i : i+4]
        retLst2 = append(retLst2, currLst)
    }

}

这样写会上报一个错误：

--- FAIL: TestOutRange (0.00s)
panic: runtime error: slice bounds out of range [:16] with capacity 13 [recovered]
    panic: runtime error: slice bounds out of range [:16] with capacity 13

错误的原因是，我们每次获取currLst的时候，是按照 lst[i i+4] 这样直接切片的，但是，如果i+4大于列表lst的长度的话，会上报越界错误！

正确的写法 ***

既然i+4可能会超出lst的长度越界，那我们在切片的时候判断一下二者的大小即可：

package t9

import (
    "fmt"
    "testing"
)

// 返回两个int最小的那个
func minInt(a, b int) int {
    if a < b {
        return a
    }
    return b
}

func TestOutRange(t *testing.T) {
    lst1 := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}
    lst2 := []string{"w", "wg", "wa", "jj", "aa", "ff", "gg", "hh", "asd", "ww", "ee", "gg", "nn", "mm"}

    fmt.Println(len(lst1)) // 13
    fmt.Println(len(lst2)) // 14

    // 结果
    var retLst1 [][]int
    var retLst2 [][]string

    // 切片
    for i := 0; i < len(lst1); i += 4 {
        // 保证不越界
        currLst := lst1[i:minInt(i+4, len(lst1))]
        retLst1 = append(retLst1, currLst)
    }

    for i := 0; i < len(lst2); i += 4 {
        // 保证不越界
        currLst := lst2[i:minInt(i+4, len(lst2))]
        retLst2 = append(retLst2, currLst)
    }

    // 注意结果有填充的默认值
    fmt.Println(retLst1) // [[1 2 3 4] [5 6 7 8] [9 10 11 12] [13]]
    fmt.Println(retLst2) // [[w wg wa jj] [aa ff gg hh] [asd ww ee gg] [nn mm]]
}

～～～