Pointer and double converting errors and the code has none

Question

I'm writing a code that's supposed to get input for the the size of a 2D array, location of an area inside the array, and the contents of the array. It doesn't do anything with those at this point, but when I tried to compile this code, it gives me errors about pointers. I've never used pointers before, and this code's no exception.

I've actually solved the problem by making the variable names numberless, but I'm really curious why this error occurs.

#include <bits/stdc++.h>
using namespace std;

int n, m, a, b;
int x0, x1, y0, y1;
int main()
{
//  freopen("seri3.gir","r",stdin);
//  freopen("seri3.cik","w",stdout);
    cin >> n >> m;
    cin >> x0 >> x1 >> y0 >> y1;
    x0--, x1--, y0--, y1--;
    int f[n][m],g[n][m];
    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++)
        {
            cin >> f[i][j];
            if ((j > y0 || j < y1) && (i > x0 || i < x1)) {
                g[j][i] = f[j][i];
                f[j][i] = 0;
                cout << g[j][i] << endl;
            }
        }


}

This is one of the longest error messages I've ever gotten. It's a compiler error, basically telling that I can't compare pointers and integers and that it couldn't convert doubles over and over again. Note that I haven't used any pointers or doubles in the code. Here's the full error (The compiler is g++, and the IDE is Geany)

Answer 1

There are two errors that I am able to reproduce.

Firstly, POSIX standard specifies functions:

double y0(double x);
double y1(double x);

These functions are in conflict with your variable declarations, and cause the following diagnostic:

‘int y0’ redeclared as different kind of symbol
note: previous declaration ‘double y0(double)’
‘int y1’ redeclared as different kind of symbol
note: previous declaration ‘double y1(double)’

There will also be other errors because you are using these functions as if they were integers. To fix this: Don't declare names that are already in use.

A general solution to avoid name clashes to declare everything inside a namespace of your own. Then you'll just need to come up with one global name that isn't in use. A simple solution in this case is to simply not declare the variables globally at all, but use local variables instead.

---

Secondly, the program is ill-formed because the array size that you use is not a compile time constant. The resulting diagnostic:

ISO C++ forbids variable length array 'f' [-Wvla]
ISO C++ forbids variable length array 'g' [-Wvla]

To fix this, either use constant size arrays, or allocate them dynamically.

Answer 2

I've fixed it by using #include <stdio.h> and #include <iostream> instead of #include <bits/stdc++>. Thanks to eerorika for mentioning that y0 and y1 are functions.

Answer 3

Your code written in c++ rather than in g++ …

#include <iostream>

int main()
{
    using std::cin;
    using std::cout;
    using std::endl;
    int n, m;// , a, b; a and b are never used
    int x0, x1, y0, y1; // Declare them as local variables to avoid clash with ::y0() function
    //  freopen("seri3.gir","r",stdin);
    //  freopen("seri3.cik","w",stdout);
    cin >> n >> m;
    cin >> x0 >> x1 >> y0 >> y1;
    x0--, x1--, y0--, y1--;
    //  int f[n][m], g[n][m]; // Run-time sized arrays are g++ - not c++ !!!
    int f[10][10], g[10][10];
    if (n > 10 || m > 10)
        cout << "Array to big!" << endl;
    else for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cin >> f[i][j];
            if ((j > y0 || j < y1) && (i > x0 || i < x1)) {
                g[j][i] = f[j][i];
                f[j][i] = 0;
                cout << g[j][i] << endl;
            }
        }
    }
}