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

C++ can't have it because the language designers made it a rule. If you were to create your own language, you could certainly allow it, but you would probably run into the same problems they did and decide not to allow it. Examples of variable names that would cause problems:

0x, 2d, 5555

Answer 2

Because if you allowed keyword and identifier to begin with numberic characters, the lexer (part of the compiler) couldn't readily differentiate between the start of a numeric literal and a keyword without getting a whole lot more complicated (and slower).

Answer 3

Because then a string of digits would be a valid identifier as well as a valid number.

int 17 = 497;
int 42 = 6 * 9;
String 1111 = "Totally text";

Answer 4

Variable names cannot start with a digit, because it can cause some problems like below:

int a = 2;
int 2 = 5;
int c = 2 * a; 

what is the value of c? is 4, or is 10!

another example:

float 5 = 25;
float b = 5.5;

is first 5 a number, or is an object (. operator) There is a similar problem with second 5.

Maybe, there are some other reasons. So, we shouldn't use any digit in the beginnig of a variable name.

Answer 5

Backtracking is avoided in lexical analysis phase while compiling the piece of code. The variable like Apple; , the compiler will know its a identifier right away when it meets letter ‘A’ character in the lexical Analysis phase. However, a variable like 123apple; , compiler won’t be able to decide if its a number or identifier until it hits ‘a’ and it needs backtracking to go in the lexical analysis phase to identify that it is a variable. But it is not supported in compiler.

Reference

Answer 6

It's a convention now, but it started out as a technical requirement.

In the old days, parsers of languages such as FORTRAN or BASIC did not require the uses of spaces. So, basically, the following are identical:

10 V1=100
20 PRINT V1

and

10V1=100
20PRINTV1

Now suppose that numeral prefixes were allowed. How would you interpret this?

101V=100

as

10 1V = 100

or as

101 V = 100

or as

1 01V = 100

So, this was made illegal.