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

Understanding the Restrictions on Using Const Int as Array Size

In C , the use of a const int as an array size is subject to certain restrictions. Let's delve into why these restrictions exist.

Consider the following scenarios:

1. Allowed:

   <code class="cpp">const int size = 2;
   int array[size] = {0}; // Compilation succeeds</code>

2. Compile Error:

   <code class="cpp">int a = 2;
   const int size = a;
   int array[size] = {0}; // Compilation fails</code>

Reason:

The underlying reason for these restrictions stems from the concept of constant expressions. In C , a constant expression is a compile-time constant that can be evaluated during compilation. In the first scenario, size is initialized with a compile-time constant (i.e., 2), allowing the compiler to determine the array's size at compile time. This information is crucial for memory allocation.

However, in the second scenario, size is initialized with the value of a, which is mutable and therefore non-constant. This means the compiler cannot determine the array's size until runtime when the value of a is known. Consequently, it cannot perform compile-time memory allocation for the array, leading to a compile error.

The rationale behind these limitations is to maintain consistency and predictability in the programming language. Allowing non-constant expressions in array size declarations would introduce uncertainties during compilation and potentially lead to runtime errors or unpredictable behavior.

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