Using constants in C++

Question

I'm working on a program that is supposed to load a 2D array from a file.

My file looks like this, with the first number indicating the size of the array:

10
7 6 9 4 5 4 3 2 1 0
6 5 5 5 6 5 4 3 2 1
6 5 6 6 7 6 8 4 3 2
1 5 6 7 7 7 6 5 4 3
5 5 6 7 6 7 7 6 5 9
5 6 7 6 5 6 6 5 4 3
5 6 7 9 5 5 6 5 4 3
5 5 6 7 6 6 7 6 5 4
5 9 5 6 7 6 5 0 3 2
5 5 5 5 6 5 4 3 2 7

And here is my code so far:

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

int main() {
ifstream prog;
prog.open("../prog1.dat");

    //If file can't be opened, exit
    if (!prog) {
        cerr << "File could not be opened" << endl;
        exit(EXIT_FAILURE);
    }
    else {
        while (!prog.eof()) {
            int size = 100, i, j;
            prog >> size; 
            int **numArray = new int* [size];
                for(i = 0; i < size; i++) {
                    numArray[i] = new int[size];
                    for(j = 0; j < size; j++) {
                        cout <<numArray[i][j] << " ";
                    }
                    cout << endl;
                }

        prog.close(); 
        return 0; 
        }
    }

}

But one of the errors I'm getting says that "expression must have a constant value" on the

int numArray[size][size];

portion of my code.

My problem is that I don't know how to go about making this a constant since I'm getting the size from the file as if I don't already know the size of the array.

This is my first C++ program and I'm pretty much teaching it to myself as I go since my professor seems to think that because we should know how to do these things in Java that we don't have to go over it in class. The examples I've found dealing with using constants say it should be something like this:

const int size = *value*;

but since my program is supposed to look in the file for the size, I'm not sure how to do this. Any suggestions? Also, like I said, I'm really new at this so if you happen to spot anything else in my code that needs to be fixed, that'd be greatly appreciated as well.

Answer 1

When you define an array in C++ the size must be known at compile time. That's why the compiler produces an error for the line:

int numArray[size][size];

You can use std::vector to create dynamic arrays.

std::vector<std::vector<int>> numArray(size, std::vecot<int>(size));

Now, numArray can be used just like a statically defined array.

Answer 2

It is impossible. As you don't know size of array at compile time, you should allocate memory dynamically.

Use std::vector:

std::vector<std::vector<int> > numbers;

Answer 3

The answers referring to vectors are correct and the preferred method for creating dynamic arrays in C++, however; your code can work. The problem you have is that you are trying to define a dynamic array on the stack and you can't do that. You need to create the array on the heap.

Here is a good example on how to do that: How do I declare a 2d array in C++ using new?

Answer 4

You could also do this by declaring numArray as a pointer to int pointer, then allocate numArray and its contents on loading the size.

Be careful of freeing the memory correctly after use, tho.

Answer 5

This should work. See, you just use the for loop.

for(i = 0; i < size; i++) {
    for(j = 0; j < size; j++) {
        cout <<numArray[i][j] << " ";
    }
    cout << endl;
 }

Answer 6

Constants can mean either a variable which doesn't change or a literal, which is any value not a variable, such as 1, 4, 5. These are integer literals. In this context, the compiler is saying that the value for the delcaraing the size of the array takes a literal. Replace 'Size' with an integer.

Better yet, use

#define SIZE 100

Macro in C/C++ are just values that are expanded during compilation.

Or if you want to change the array during runtime, you can use pointers.

int *array[10000]; // make the array large enough for most cases
// note the '*' that is a what makes a pointer

// read the size from file


for (int i = 0, i < size; ++i) 
{
    array[i] = new int [size];      // use 'new' to dynamically create an array
}

Pointers allows creating arrays with variables and literals.

Here a version of your program using pointers. Study it to learn how it works.

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

int main() {
ifstream prog;
prog.open("prog1.dat");

    //If file can't be opened, exit
    if (!prog) {
        cerr << "File could not be opened" << endl;
        exit(EXIT_FAILURE);
    }

    int size = 100, i, j;
    //prog >> size; 
    int **numArray = new int*;   // pointers used here

        for(i = 0; i < size; i++) {
            numArray[i] = new int[size];     // pointer used here
            for(j = 0; j < size; j++) {
                prog >> numArray[i][j];
            }
        }
        return 0; 
        cout << numArray[i][j];
}