TCL 中带有括号和大括号的双美元是什么意思? [英] What does a double dollar with parentheses and braces following mean in TCL?

问题描述

下面这行有什么作用?

eval "set abc \$${SID}(abc)"

我的尝试

我知道 $someArray(index) 是在 TCL 中访问数组的方式.所以

My try

I know that $someArray(index) is how arrays are accessed in TCL. So

set abc $SID(abc)

希望 abc 成为数组 SID 的键(据我所知，abc 可以是 TCL 中的所有数组是关联的.这是正确的吗?).然后将值赋给 abc.

would expect abc to be a key of the array SID (as far as I know abc could be anything as all arrays in TCL are associative. Is this correct?). Then it assigns the value to abc.

所以在伪代码中该行是

So in Pseudocode that line does

Ensure: SID is array, abc exists

abc <- SID[abc]

但是为什么 SID 周围有大括号?为什么是 eval?

But why are there braces around SID? Why the eval?

推荐答案

大括号用于防止替换超过必要字符数的变量.

Braces are used to prevent substitution of variables of more characters than necessary.

$variableonetwo

引用名为variableonetwo的变量并返回其值:

Refers to a variable named variableonetwo and returns its value:

% set variableonetwo 1
% puts $variableonetwo
1

但是……

${variableone}two

指名为variableone的变量和字符串two:

Refers to the variable named variableone and the string two:

% set variableone 1
% puts ${variableone}two
1two
% puts $variableonetwo
can't read "variableonetwo": no such variable

<小时>

eval 用于...很好地评估/执行命令.

---

eval is used to... well evaluate/execute a command.

eval "set abc \$${SID}(abc)"

如果 $SID 的值为 foo 是一个数组，则将尝试评估以下内容.

Will try to evaluate the following if $SID has the value foo which is an array.

set abc $foo(abc)

在此处使用 eval 的一个优点是，您可以在同一行中使用 两个 替换，第一个替换 $SID 到数组名称，第二个来自数组值(来自提供的键).

An advantage of using eval here is that you are able to have not one but two substitutions in the same line, the first being substituting $SID to an array name and the second being from the array value (from the provided key).

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