How to Fix expression must have a constant value in C++ Length

Error expression must have a constant value in C++ length occurs when you try to use a non-constant value in a context that requires a constant expression.

To fix the error in C++, use a constant size for arrays or replace the array with a dynamically sized container like std::vector.

Reproducing the error

int n;

cin >> n;

int array[n]; 

Output

expression must have a constant value in C++ length

How to fix it?

Here are three ways to fix this error programmatically:

  1. Use a constant expression for array size
  2. Use dynamic allocation
  3. Use a standard library container

Solution 1: Use a constant expression for array size

If the size can be determined at compile time, define it as a constant.

const int SIZE = 10;

int array[SIZE];

Solution 2: Use dynamic allocation

If the size is known only at runtime, use dynamic memory allocation:

int* array = new int[n];

// Remember to delete the array after use
delete[] array;

Solution 3: Use a standard library container

A more modern and safer approach is to use a standard library container like std::vector, which handles dynamic sizing.

#include <vector>

std::vector<int> vec(n);

That’s it!

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