英文:
In Bash, what is the difference between a scalar and single-element array?
问题
除了通过 declare -p 报告的差异外，标量是否与单元素数组不可区分？ 将它们视为等同是否安全？
X="cat" # 标量
Y=("cat") # 单元素数组
一般来说，如果在表达式中将数组视为标量，则会得到第一个数组元素。此外，如果将标量用作数组，则它会表现得像一个单元素数组。
declare -p 显示它们是不同的。（我在注释中显示命令输出。）
declare -p X Y # declare -- X="cat"
# declare -a Y=([0]="cat")
如果将数组视为标量，它会表现得像一个标量。
echo 字符串长度: ${#X} ${#Y} # 字符串长度: 3 3
echo 值: $X $Y # 值: cat cat
[ "$X" == "$Y" ] && echo "相等测试为真" # 相等测试为真
如果将标量视为数组，它会表现得像一个单元素数组。
echo "数组长度: ${#X[@]} ${#Y[@]}" # 数组长度: 1 1
echo "数组索引: ${X[0]} ${Y[0]}" # 数组索引: cat cat
echo "${X[1]}" "${Y[1]}" # ""
"M. Nejat Aydin" 和 "Ed Morton" 的回复显示它们并不总是等同。谢谢。
英文:
Except for the difference in how the are reported by declare -p, is a scalar indistinguishable from a single-element array? Is it safe to treat them as equivalent?
X="cat" # scalar
Y=("cat") # single-element array
In general, if you if you use an array in an expression as if it were a scalar, you get the first array element. Furthermore, if you use a scalar as an array, it acts like a single-element array.
declare -p shows that they are distinct. (I show command output in comments.)
declare -p X Y # declare -- X="cat"
# declare -a Y=([0]="cat")
If you treat the array as a scalar, it acts like a scalar.
echo string length: ${#X} ${#Y} # string length: 3 3
echo values: $X $Y # values: cat cat
[ "$X" == "$Y" ] && echo "equality test is true" # equality test is true
If you treat the scalar like an array, it acts a single-element array.
echo "array length: ${#X[@]} ${#Y[@]}" # array length: 1 1
echo "array indexing: ${X[0]} ${Y[0]}" # array indexing: cat cat
echo \"${X[1]}\" \"${Y[1]}\" # "" ""
Responses from "M. Nejat Aydin" and "Ed Morton" show that they are not always equivalent. Thanks.
答案1
得分: 3
根据 man bash：
如果使用语法
name[subscript]=value赋值给任何变量，将自动创建一个 索引数组。
...引用一个数组变量（无论是 索引数组 还是 关联数组），如果没有下标，等同于使用下标
0引用该数组。
英文:
According to man bash:
> An indexed array is created automatically if any variable is assigned to using the syntax name[subscript]=value.
> ...
>
> Referencing an array <sup>(either indexed array or associative array)</sup> variable without a subscript is equivalent to referencing the array with a subscript of 0.
答案2
得分: 2
根据bash源代码：
/* 执行数组赋值 name[ind]=value。如果NAME已经存在且不是数组，IND为0，则执行name=value。如果NAME存在且不是数组，IND不为0，则将其转换为数组，以现有值作为name[0]。
如果NAME不存在，无论IND的值如何，都只需创建一个数组变量。 */
SHELL_VAR *
bind_array_variable (name, ind, value, flags)
英文:
According to the bash sourcecode:
/* Perform an array assignment name[ind]=value. If NAME already exists and
is not an array, and IND is 0, perform name=value instead. If NAME exists
and is not an array, and IND is not 0, convert it into an array with the
existing value as name[0].
If NAME does not exist, just create an array variable, no matter what
IND's value may be. */
SHELL_VAR *
bind_array_variable (name, ind, value, flags)
答案3
得分: 2
$ X="cat" # scalar
$ Y=("cat") # single-element array
$ echo "${X[@]:1}"
at
$ echo "${Y[@]:1}"
$
英文:
$ X="cat" # scalar
$ Y=("cat") # single-element array
<p>
$ echo "${X[@]:1}"
at
$ echo "${Y[@]:1}"
$
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论