Why Can\'t I Use a Variable to Define an Array Size in C ?

Const Int as Array Size: Conditional Allowance

Despite both examples involving the use of a const int to specify the size of an array, only the first expression is permitted in C . The reason for this discrepancy lies in the semantics of the two expressions.

The first expression, const int size = 2, is a constant expression, meaning that its value can be fully evaluated during compilation. This allows the compiler to determine the size of the array at compile time, enabling it to reserve the appropriate amount of memory.

In contrast, the second expression, const int size = a, represents a non-constant expression because a is a mutable variable. Therefore, the compiler cannot guarantee the value of size until runtime, making it impossible to allocate the array during compilation.

This distinction highlights the significance of the "kind of expression" in C . The use of a non-constant expression as an array size precludes compile-time memory reservation because the compiler cannot determine the required size without executing the program. This limitation is a consequence of the strict rules and complexities of flow analysis, which is the process of determining the values of variables at runtime.

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