Expression must have a constant value error when creating an array C++

Question

Good day. Faced an error "Expression must have a constant value" when creating an array. Find this question c++ array - expression must have a constant value, but these tips didn’t solve the problem. Please help and I'm sorry for such requests, I just started to learn C ++

Code:

#include <iostream>
#include <math.h>
#include <cstdlib>
#include <string>
#include <sstream>
#include <conio.h>
#include <random>

int main()
{
   int userInput = 0, sumEvenIndexElements = 0, sumElementsBeetwenZero = 0;
   bool checkZeroElements = false;
   std::cout << "Write array length:" << std::endl;
   std::cin >> userInput;
   const int lengthOfArray = userInput;
   int numbers [lengthOfArray];
   std::default_random_engine generator;
   std::uniform_int_distribution<int> distribution(-10, 10);

   for (int index = 0; index < lengthOfArray; index++)
   {
      numbers[index] = new int[distribution(generator)];
      std::cout << numbers << std::endl;

      if (index % 2 == 0)
      {
         sumEvenIndexElements += *numbers[index];
      }

   }

   std::cout << "Sum even index elements: " << sumEvenIndexElements << std::endl;

   for (int index = 0; index < lengthOfArray; index++)
   {
      numbers[index] = new int[distribution(generator)];

      if (numbers[index] == 0)
      {
         checkZeroElements = !checkZeroElements;
      }

      if (checkZeroElements)
      {
         sumElementsBeetwenZero += *numbers[index];
      }

   }

   if (checkZeroElements)
   {
      std::cout << "Sorry, array have less than two zero elements.";
   }
   else
   {
      std::cout << "Sum even index elements: " << sumEvenIndexElements << std::endl;
   }

}

The answer is in the link you posted. "The standard requires the array length to be a value that is computable at compile time so that the compiler is able to allocate enough space on the stack." — lars
– lars, Commented Jan 3, 2020 at 10:38
There are two types of arrays static and dynamic array. in the static array you must have a constant value because this will be generated at compile time. in the dynamic array you can put any number to initialize array. — Aqeel Ahmad
– Aqeel Ahmad, Commented Jan 3, 2020 at 13:55
one more thing, in the dynamic array you will have to delete it after using it. otherwise result will be in the leakage of memory. — Aqeel Ahmad
– Aqeel Ahmad, Commented Jan 3, 2020 at 13:57

Nikita Smirnov · Accepted Answer · 2020-01-03 11:09:59Z

1

lengthOfArray is a constant variable, it's value would not be chagned during runtime after initialization.

But it's value - userInput is not a constant, it dependes on runtime user input. As pointed here

A constant value is an explicit number or character such as 1 or 0.5 or ‘c’.

You should use std::vector instead of an array, as suggested in Paul Evans's answer, or assign a proper constant value to the lengthOfArray, which will be known at the time of compilation, e.g.:

const int lengthOfArray = 10;

edited Jan 3, 2020 at 11:09

answered Jan 3, 2020 at 10:43

Nikita Smirnov

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4 Comments

Sergey Holman Over a year ago

Yes, have already seen this. Don't quite understand why it works that way.

Nikita Smirnov Over a year ago

@SergeyHolman , Did you get the difference between constant variable and constant value?

Sergey Holman Over a year ago

But is not a constant variable means that its value cannot be changed after initialization and the ability to use it as a constant value, it was logical or no?

Nikita Smirnov Over a year ago

No, because it is initialized with non-const value. Updated my answer with a ref to explanation, what is a const value.

Paul Evans · Accepted Answer · 2020-01-03 10:38:28Z

0

Your code:

   std::cout << "Write array length:" << std::endl;
   std::cin >> userInput;
   const int lengthOfArray = userInput;
   int numbers [lengthOfArray];

won't work as per the compiler error you're getting.

std::vector plays a lot nicer:

   std::cout << "Write array length:" << std::endl;
   std::cin >> userInput;
   const int lengthOfArray = userInput;
   std::vector<int> numbers(lengthOfArray, 0);

will create a std::vector of int of length lengthOfArray all initialised to 0.

answered Jan 3, 2020 at 10:38

Paul Evans

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1 Comment

Sergey Holman Over a year ago

Thanks it's work. I think soon I will spend a lot of fun. Can you recommend a book on C++?

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