Why can't variable names start with numbers?

Question

I was working with a new C++ developer a while back when he asked the question: "Why can't variable names start with numbers?"

I couldn't come up with an answer except that some numbers can have text in them (123456L, 123456U) and that wouldn't be possible if the compilers were thinking everything with some amount of alpha characters was a variable name.

Was that the right answer? Are there any more reasons?

string 2BeOrNot2Be = "that is the question"; // Why won't this compile?

Answer 1

I think the simple answer is that it can, the restriction is language based. In C++ and many others it can't because the language doesn't support it. It's not built into the rules to allow that.

The question is akin to asking why can't the King move four spaces at a time in Chess? It's because in Chess that is an illegal move. Can it in another game sure. It just depends on the rules being played by.

Answer 2

Well think about this:

int 2d = 42;
double a = 2d;

What is a? 2.0? or 42?

Hint, if you don't get it, d after a number means the number before it is a double literal

Answer 3

It's likely a decision that came for a few reasons, when you're parsing the token you only have to look at the first character to determine if it's an identifier or literal and then send it to the correct function for processing. So that's a performance optimization.

The other option would be to check if it's not a literal and leave the domain of identifiers to be the universe minus the literals. But to do this you would have to examine every character of every token to know how to classify it.

There is also the stylistic implications identifiers are supposed to be mnemonics so words are much easier to remember than numbers. When a lot of the original languages were being written setting the styles for the next few decades they weren't thinking about substituting "2" for "to".

Answer 4

The variable may be considered as a value also during compile time by the compiler so the value may call the value again and again recursively

Answer 5

Suppose you did allow symbol names to begin with numbers. Now suppose you want to name a variable 12345foobar. How would you differentiate this from 12345? It's actually not terribly difficult to do with a regular expression. The problem is actually one of performance. I can't really explain why this is in great detail, but it essentially boils down to the fact that differentiating 12345foobar from 12345 requires backtracking. This makes the regular expression non-deterministic.

There's a much better explanation of this here.

Answer 6

There could be nothing wrong with it when comes into declaring variable.but there is some ambiguity when it tries to use that variable somewhere else like this :

let 1 = "Hello world!" print(1) print(1)

print is a generic method that accepts all types of variable. so in that situation compiler does not know which (1) the programmer refers to : the 1 of integer value or the 1 that store a string value. maybe better for compiler in this situation to allows to define something like that but when trying to use this ambiguous stuff, bring an error with correction capability to how gonna fix that error and clear this ambiguity.