string Tcl：如果变量为空，则默认？
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Tcl: default if variable is empty?
提问by DevSolar
I've "inherited" some Tcl code, and while I worked through some tutorials and can make sense out of the language, my own Tcl constructs lack a certain... finesse.
我已经“继承”了一些 Tcl 代码，虽然我完成了一些教程并且可以理解该语言，但我自己的 Tcl 构造缺乏某种......技巧。
For example, I have this code:
例如，我有这个代码：
puts "Column 'name': [ $queryRs getString name ]"
$queryRsis the result set of a SQL query. The [ $queryRs getString name ]construct retrieves the content of the table column "name" from the current row in the result set. If the database field is NULL, the putsdoes not print anything.
$queryRs是 SQL 查询的结果集。该[ $queryRs getString name ]构造从结果集中的当前行检索表列“name”的内容。如果数据库字段为 NULL，puts则不打印任何内容。
I would like to print a "default" string instead, i.e. if [ $queryRs getString name ]results in nothing, I'd like to replace it with "--".
我想打印一个“默认”字符串，即如果[ $queryRs getString name ]没有结果，我想用"--".
Now, I could do something like this:
现在，我可以做这样的事情：
set nameVar "[ $queryRs getString name ]"
if { [ string length $nameVar ] == 0 } {
set nameVar "--"
}
puts "Column 'name': $nameVar"
But there hasto be a more compact solution, something that can be done inline instead of adding four lines and a temporary variable. Help, please?
但是必须有一个更紧凑的解决方案，可以内联完成而不是添加四行和一个临时变量。请帮忙？
回答by kostix
Two things.
两件事情。
First, Tcl doesn't have a notion of NULL(nil, undefinedor whatever) value,and when you want to simulate such a value you have to either use a variable and test for its existenceor use an entry in an arrayof dictionaryand test for its existence, too. If such a variable/entry exists, then the value is defined, otherwise it's not.
首先，TCL不具备的概念NULL（nil，undefined或其他）值，而当你想模仿你有这样的值或者使用一个变量，它的存在测试或在使用中的条目array的dictionary和测试其存在也是。如果存在这样的变量/条目，则定义该值，否则不定义。
Unfortunately, the creator of your inherited code apparently did not care about the case where a variable can be NULLso NULLsare indistinguishable from variables having default values (an empty string).
不幸的是，你的继承代码的创造者显然没有在意，其中一个变量可以是这样NULL那么NULLs是从具有默认值（一个空字符串）变量没有什么区别。
Next, you can use a helper procedure to do what you need:
接下来，您可以使用辅助程序来执行您需要的操作：
proc ValueOrDef {val {def --}} {
expr {$val ne "" ? $val : $def}
}
and then go like this:
然后像这样：
puts [ValueOrDef [$queryRs getString name]]
puts [ValueOrDef [$queryRs getString name] "some other default"]
回答by bmk
You could use the x?y:zconstruct of the exprcommand. xis the condition, yis the alternative if the condition is met and zis the alternative if xis not met.
您可以使用命令的x?y:z构造expr。x是条件，y如果满足条件，则为备选方案，如果不满足，z则为备选方案x。
E.g. (but still with a temporary variable):
例如（但仍然有一个临时变量）：
set nameVar [ $queryRs getString name ]
puts "Column 'name': [expr {[string length $nameVar]>0 ? $nameVar : "--"}]"
