Why C/C++ Variables Can't Start with Numbers

In C/C++, a variable name can have alphabets, numbers, and the underscore( _ ) character. There are some keywords in the C/C++ language. Apart from them, everything is treated as an identifier. Identifiers are the names of variables, constants, functions, etc.

Why Variables in C/C++ Can't Start with Numbers?

In C and C++, variable names (also, known as identifiers) cannot start with a digit due to how the compiler processes code during compilation.

First, we need to understand the phases of compilation or compiler. There are seven phases in a typical compiler:

• Lexical Analysis
• Syntax Analysis
• Semantic Analysis
• Intermediate Code Generation
• Code Optimization
• Code Generation
• Symbol Table Management

The Lexical Analysis phase is responsible for reading the source code and breaking it into tokens (like identifiers, keywords, literals, etc.).

Why Starting a Variable with a Number Is Invalid?

We can not specify the identifier that starts with a number in C/C++ because:

• None of the above supports that a variable starts with a number.
• Suppose that a variable starts with a number. First, it will scan the numbers, and then scan the alphabet later on.
• When it finds the alphabet after the numbers, it gets confused.
• So the compiler will have to backtrack to the lexical analysis phase, but backtracking is not supported in most compilers.
• The compiler should be able to identify a token as an identifier or a literal after looking at the first character.

Examples Demonstrating Why Variables Can't Start with Numbers

Practice these examples to understand variable (identifier) naming conventions, especially why variables cannot start with a number.

Example 1

In this example, we are initializing three variables. Out of three variables, one variable's name starts with a number. We will get an error when we run the code in C and C++.

#include <stdio.h>
int main()
{
    int 5s = 8;
    int _4a = 3;
    int b = 12;
    printf("The value of variable 5s : %d", 5s);
    printf("The value of variable _4a : %d", _4a);
    printf("\nThe value of variable b : %d", b);
    return 0;
}

This will throw an error:
error: invalid suffix "s" on integer constant

#include <iostream>
using namespace std;
int main()
{
    int 5s = 8;
    int _4a = 3;
    int b = 12;
    cout << "The value of variable 5s :" << 5s;
    cout << "The value of variable _4a :" << _4a;
    cout << "\nThe value of variable b :" << b;
    return 0;
}

This will throw an error:
error: expected unqualified-id before numeric constant

Example 2

In the example given below, we have used two variables where one variable starts with an underscore (_) and the other variable starts with an alphabet. This will not give any error as both are valid ways to define variables.

#include <stdio.h>
int main()
{
    int _4a = 3;
    int b = 12;
    printf("The value of variable _4a : %d", _4a);
    printf("\nThe value of variable b : %d", b);
    return 0;
}

The value of variable _4a : 3
The value of variable b : 12

#include <iostream>
using namespace std;
int main()
{
    int _4a = 3;
    int b = 12;
    cout << "The value of variable _4a :" << _4a;
    cout << "\nThe value of variable b :" << b;
    return 0;
}

The value of variable _4a :3
The value of variable b :12