Array Error

[luckylukebrooks]

So I'm writing code that will prompt a user for the size they desire for an array, then I have to create an array of that size

later i will have to loop asking them to input a value for each element, but I think i have that down.

i have a few error messages popping up, and i'm not sure why. what errors do you see?

  #include <iostream>
using namespace std;

int main() 
{
	int arraySize;
	const int userArray;

	cout << "\nThis program creates an array of various numbers and will sort them." << endl;

	cout << "\nHow large would you like the array?" << endl;
	cin >> arraySize;

	int userArray = arraySize;
	const int userArray[arraySize];

[Ganado]

You should tell us what the errors are (copy them). Learning how to interpret error messages that a compiler tells you is a vital skill to learn in C++ programming.

const int userArray;You can't have an uninitialized const variable. It has to be assigned some value.

int userArray = arraySize; const int userArray[arraySize];Now, you're trying to re-declare the variable called 'userArray' in two more places. You can't re-use a variable name if it's already in use within the same scope.

const int userArray[arraySize];Having array lengths that aren't known at compile-time is not standard. You should instead either use a vector, or allocate a large enough array to handle what the user needs.

For example...

#include <iostream>
#include <algorithm>
using namespace std;

int main() 
{
	cout << "\nThis program creates an array of various numbers and will sort them." << endl;

	cout << "\nHow large would you like the array? ";
	int arraySize;
	cin >> arraySize;
	
	int userArray[10000];

	for (int i = 0; i < arraySize; i++)
	{
		userArray[i] = (1 + i * i * 41) % 13; // just some numbers 
	}
    
	std::sort(userArray, userArray + arraySize);

	// print
	for (int i = 0; i < arraySize; i++)
	{
		std::cout << userArray[i] << '\n';   
	}
}

[seeplus]

As Ganado points out above, the size of an array has to be known to the compiler at compile time. It can't be set at run-time. If the size of the array is to be determined at run-time, then usually a std::vector would be used.

#include <iostream>
#include <vector>
#include <algorithm>

int main()
{
	size_t arraySize {};

	std::cout << "\nThis program creates an array of various numbers and will sort them.\n";
	std::cout << "\nHow large would you like the array? ";
	std::cin >> arraySize;

	std::vector<int> userArray(arraySize);

	std::cout << "Please enter " << arraySize << " numbers:\n";

	for (auto& a : userArray)
		std::cin >> a;

	std::sort(std::begin(userArray), std::end(userArray));

	std::cout << "\nThe sorted numbers are:\n";

	for (const auto& a : userArray)
		std::cout << a << ' ';

	std::cout << '\n';
}

[Handy Andy]

Hello luckylukebrooks,

The comments should help to explain:

#include <iostream>

using namespace std;  // <--- Best not to use.

int main()
{
    int arraySize{};  // <--- ALWAYS initialize your variables.
    const int userArray;  // <--- Being defined as a constant it needs a value. But defined as a constant it can not be changed in the program.

    cout << "\nThis program creates an array of various numbers and will sort them.\n";  // <--- Prefer the new line (\n) over the function "endl".

    cout << "\nHow large would you like the array? ";  // <--- Writing your prompt this way puts the "cin" on the same line.
    cin >> arraySize;

    int userArray = arraySize;
    const int userArray[arraySize];  // <--- Not standard C++. May not work with some compilers.
}

Based on you code I think this is what you you are trying to do:

#include <iostream>

using namespace std;  // <--- Best not to use.

int main()
{
    int arraySize{};
    int* userArray{};

    cout << "\nThis program creates an array of various numbers and will sort them.\n";

    cout << "\nHow large would you like the array? ";
    cin >> arraySize;

    userArray =  new int[arraySize];

    // <--- Other code here.

    delete[] userArray;

    return 0;  // <--- Not required, but makes a good break point.
}

Andy

[seeplus]

Unless there is a specific requirement to use new it's always better to use a container such as std::vector - or at a minimum std::unique_memory. A raw pointer should always be the last resort.

[luckylukebrooks]

@ Handy Andy

A couple questions:

1. what does the star mean in line 8?

2. what does new int mean on line 15?

3. What is that delete on line 19?

I apologize, my teacher hasn't gone over that yet

[luckylukebrooks]

@ seeplus

for this assignment my teacher doesn't want us to use a vector, not sure why

[seeplus]

Andy's code is using dynamic memory - which you say you haven't covered. You're left with defining an array as large as might be used as mentioned by Ganado above. Not the best way.

[Ganado]

I agree it's not the best way -- it's just, you start to become conditioned to just expect students to not be able to use vector, unfortunately (even though something like a C# List/Java ArrayList would be perfectly acceptable, in any other language; C++ gets special unfair treatment).

Actually, a lot of the time I assume that the instructor expects students to use VLAs, even though they aren't standard and don't even compile in msvc.

[seeplus]

#include <iostream>
#include <algorithm>

int main()
{
	constexpr size_t MAXARR {200};

	int userArray[MAXARR] {};
	size_t arraySize {};

	std::cout << "\nThis program creates an array of various numbers and will sort them.\n";
	std::cout << "\nHow large would you like the array (between 1 and " << MAXARR << ") ? ";
	std::cin >> arraySize;

	if (arraySize > MAXARR) {
		std::cout << "Maximum size is " << MAXARR << '\n';
		arraySize = MAXARR;
	}

	std::cout << "Please enter " << arraySize << " numbers:\n";

	for (size_t i = 0; i < arraySize; ++i)
		std::cin >> userArray[i];

	std::sort(userArray, userArray + arraySize);

	std::cout << "\nThe sorted numbers are:\n";

	for (size_t i = 0; i < arraySize; ++i)
		std::cout << userArray[i] << ' ';

	std::cout << '\n';
}

[luckylukebrooks]

here is the code i have currently, the only error it says it has is the i on line 19

"expression must have pointer to object type"

int main() 
{
	int arraySize{};
	int userArray{};

	cout << "\nThis program creates an array of various numbers and will sort them." << endl;

	cout << "\nHow large would you like the array? ";
	cin >> arraySize;

	userArray = arraySize;
	const int userArray{};


	//make user input values
	for (int i = 0; i < arraySize; ++i)
	{
		cout << "Please input an integer." << endl;
		cin >> userArray[i];
	}

}

[Ganado]

Please read my first post again. Perhaps I wasn't clear, so I will reiterate more succinctly.

int userArray{};

const int userArray{};
(1) You are re-declaring the userArray variable. This is not allowed.
(2) This is not an array, but an int.

An array would look something like:
int userArray[1000];

[mbozzi]

This is pretty good, you seem to understand the concepts. Just pay close attention to your (syntactic) spelling.

#include <iostream>
using namespace std;
int main() 
{
	int arraySize{};
        // wrong: can't make an array of size arraySize
        //   until you know what the size will be.  
        //   Don't declare it until you have the size.
	// int userArray{}; 

	cout << "\nThis program creates an array of various numbers and will sort them." << endl;

	cout << "\nHow large would you like the array? ";
	cin >> arraySize;
        // Now the size is known. Declare an array of 
        // arraySize ints:
        int userArray[arraySize] {}; 

        // wrong: see above
	// userArray = arraySize;
        // const int userArray{};
	
	//make user input values
	for (int i = 0; i < arraySize; ++i)
	{
		cout << "Please input an integer." << endl;
		cin >> userArray[i];
	}
}

[Ganado]

Yeah that's a better way of putting it. Let us know if the above code compiles (assuming you're using GCC/clang/MinGW).

[luckylukebrooks]

will this still work if the user is choosing the size of the array?

[Ganado]

If you can compile it, and run it without it crashing, that is good evidence that it works*. Of course, if you enter a huge number, or a negative number, then bad things might happen.

*"working" is not the same thing as "correct", but it's a good enough approximation for this particular thread.

[luckylukebrooks]

When I compile this code, it says there's an error in the array. says needs constant

int main() 
{

	int arraySize;

	cout << "\nThis program creates an array of various numbers and will sort them." << endl;

	cout << "\nHow large would you like the array? ";
	cin >> arraySize;
	
	int userArray[arraySize]{};

	//make user input values
	for (int i = 0; i < arraySize; ++i)
	{
		cout << "Please input an integer." << endl;
		cin >> userArray[i];
	}

}

[seeplus]

This is effectively the same code as post #1 for which Ganado stated "Having array lengths that aren't known at compile-time is not standard".Don't you read/understand the replies provided?

Let me repeat -

YOU CANNOT SPECIFY AN ARRAY SIZE AT RUN TIME. THE SIZE MUST BE KNOWN AT COMPILE TIME.

Some compilers allow this, but THIS IS NOT STANDARD

[lastchance]

#include <iostream>
using namespace std;

int main() 
{
	int arraySize;
	cout << "\nHow large would you like the array? ";
	cin >> arraySize;
	int * userArray = new int[arraySize]{};     // <***** not constant, so you need dynamic memory; i.e. "new"

	for ( int i = 0; i < arraySize; ++i )
	{
		cout << "Please input element " << i << ": ";
		cin >> userArray[i];
	}
	
	// ... Do something with your array
	
	delete [] userArray;                        // <***** nobody else can use it 'til you give it back
}

[seeplus]

If you really want a run-time sized array and don't/wouldn't use std::vector, then prefer std::unique_ptr (via std::make_unique) rather than new/delete.

#include <iostream>
#include <memory>

int main()
{
	size_t arraySize {};

	std::cout << "How large would you like the array? ";
	std::cin >> arraySize;

	const auto userArray {std::make_unique<int[]>(arraySize)};

	for (size_t i = 0; i < arraySize; ++i) {
		std::cout << "Please input element " << i << ": ";
		std::cin >> userArray[i];
	}

	std::cout << '\n';

	for (size_t i = 0; i < arraySize; ++i)
		std::cout << userArray[i] << ' ';

	std::cout << '\n';

	// ... Do something with array
}