Most common way to evaluate truthiness of string in vim - vim

Question

In python to see if a string is non-empty, I'll do something like:
if STRING_VAR:
...
However, in vim a string doesn't really give a real truth-value when used directly. Is there a suggested way to evaluate the truthiness of a string? Usually I will use either:
if (STRING_VAR != "")
...
Or:
if (len(STRING_VAR) != 0)
...
What's the suggested/cleanest way to do this?

Answer 1

However, in vim a string doesn't really give a real truth-value when used directly.
If programming were a religion then the Pythonistas would become zealots ;-) Say, for a C programmer the real truth is a number, and for an assembly one it's totally unaddressable and the Strings do not exist at all. So how can you speak of a "real truth" then?
VimScript is partially modelled after C and AWK. In particular, :if accepts only numeric values (zero is false, non-zero is true) including those "convertible" to numbers, such as... Strings. However, String-to-Number conversion is done like with :h str2nr(). And so :if "hello world" is legal but false.
So you'll probably want :if empty(var) instead.
Note however that :h empty() accepts any Vim type. Usually this is quite good, but on some rare occasions you prefer :if len(var) or :if strlen(var).
The difference between :h len() and :h strlen() is that strlen() accepts Strings, while len() also accepts Lists and Dictionaries.
Still note that :echo strlen(0) is 1, as Numbers are auto-converted to Strings anyway. And so if you want to check simultaneously that 1) foo has type of String, and 2) foo is empty then you have to write :if foo is# ''

Answer 2

I would do !empty(STRING_VAR) *
I haven't looked how Vim compares strings. But remembering Vim is written in C, then if it is in similar manner to
int string_compare(const char* s1, const char* s2)
{
while(*s1 && (*s1 == *s2)) {
++s1;
++s2;
}
return *s1 == *s2;
}
then STRING_VAR != "" is definitely better than len(STRING_VAR) != 0
The time complexity of former is O(1) - the result is after comparing first characters. The latter requires O(n) - for counting the length.
On the other hand, if string in Vim is represented by structure, e.g.
struct string {
char str[1000];
int len;
};
then len(STRING_VAR) != 0 will be just simple integer comparison (var.len != 0).
Actually, len(STRING_VAR) will be enough.
* As we rather can trust Bram, he implemented empty() in most efficient way
In Vim, like in Python, if doesn't require brackets