Programming Forums > Application Development > C++

Irhaha · Jan 11th, 2005, 9:50 AM

Dizzutch,
Hi! Thanks for replying me in "Read and print a square matrix". But I have tried your suggestion and errors came out.
I inputted
#include <iostream.h>
#include <stdlib.h>

void main()
{
int i,j,N;
cout<<"Enter N:";
cin>>N;

for(i=0;i<N;i++)
for(j=0;j<N;j++)
int array[i][j]=(int)malloc(sizeof(int));
}

Dizzutch · Jan 11th, 2005, 10:20 AM

was the error during compile time or during run time? and what is the error? I'm not that good with C++, but the syntax looks correct for ANSII C.

Tama · Jan 14th, 2005, 9:27 PM

>void main()
int main(). A void return value has never been correct C or C++.

>int array[i][j]=(int)malloc(sizeof(int));
This will fail to work for quite a few reasons. First, arrays must have a constant size in C++, variables won't work. Second, malloc returns a pointer yet you cast it to int. Third, array isn't a valid lvalue and thus cannot be assigned the return value of malloc, even if you do cast it to int.

Here is the correct C++ code to do what you want (using malloc):

(Toggle Plain Text)

#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
  int N;
  int **array;

  cout<<"Enter N: ";
  cin>> N;

  // Create
  array = (int **)malloc(N * sizeof *array);
  for (int i = 0; i < N; i++)
    array[i] = (int *)malloc(N * sizeof *array[i]);

  // Initialize
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++)
      array[i][j] = i * j;
  }

  // Display
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++)
      cout<< array[i][j] <<'\t';
    cout<<endl;
  }

  // Destroy
  for (int i = 0; i < N; i++)
    free(array[i]);
  free(array);
}

However, with C++ you should be using new and delete instead of malloc and free because they understand C++ better:

(Toggle Plain Text)

#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
  int N;
  int **array;

  cout<<"Enter N: ";
  cin>> N;

  // Create
  array = new int*[N];
  for (int i = 0; i < N; i++)
    array[i] = new int[N];

  // Initialize
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++)
      array[i][j] = i * j;
  }

  // Display
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++)
      cout<< array[i][j] <<'\t';
    cout<<endl;
  }

  // Destroy
  for (int i = 0; i < N; i++)
    delete [] array[i];
  delete [] array;
}

Or, even better, use the vector class that comes with the C++ standard library:

(Toggle Plain Text)

#include <iostream>
#include <cstdlib>
#include <vector>

using namespace std;

int main()
{
  int N;

  cout<<"Enter N: ";
  cin>> N;

  vector< vector<int> > array(N, vector<int>(N));

  // Initialize
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++)
      array[i][j] = i * j;
  }

  // Display
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++)
      cout<< array[i][j] <<'\t';
    cout<<endl;
  }
}

Dizzutch · Jan 14th, 2005, 9:31 PM

sorry, i wrote that on a limb, didn't really test it.

Jan 11th, 2005, 9:50 AM	#1
Irhaha Newbie Join Date: Jan 2005 Posts: 3 Rep Power: 0	To Dizzutch Dizzutch, Hi! Thanks for replying me in "Read and print a square matrix". But I have tried your suggestion and errors came out. I inputted #include <iostream.h> #include <stdlib.h> void main() { int i,j,N; cout<<"Enter N:"; cin>>N; for(i=0;i<N;i++) for(j=0;j<N;j++) int array[i][j]=(int)malloc(sizeof(int)); }

Jan 11th, 2005, 10:20 AM	#2
Dizzutch Professional Programmer Join Date: Dec 2004 Location: Worcester, MA Posts: 441 Rep Power: 4	was the error during compile time or during run time? and what is the error? I'm not that good with C++, but the syntax looks correct for ANSII C. __________________ naked pictures of you \| PFO F@H stats

Jan 14th, 2005, 9:27 PM	#3
Tama Programmer Join Date: Dec 2004 Posts: 35 Rep Power: 0	>void main() int main(). A void return value has never been correct C or C++. >int array[i][j]=(int)malloc(sizeof(int)); This will fail to work for quite a few reasons. First, arrays must have a constant size in C++, variables won't work. Second, malloc returns a pointer yet you cast it to int. Third, array isn't a valid lvalue and thus cannot be assigned the return value of malloc, even if you do cast it to int. Here is the correct C++ code to do what you want (using malloc): (Toggle Plain Text) #include <iostream> #include <cstdlib> using namespace std; int main() { int N; int array; cout<<"Enter N: "; cin>> N; // Create array = (int )malloc(N * sizeof array); for (int i = 0; i < N; i++) array[i] = (int )malloc(N * sizeof array[i]); // Initialize for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) array[i][j] = i j; } // Display for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) cout<< array[i][j] <<'\t'; cout<<endl; } // Destroy for (int i = 0; i < N; i++) free(array[i]); free(array); } #include <iostream> #include <cstdlib> using namespace std; int main() { int N; int array; cout<<"Enter N: "; cin>> N; // Create array = (int )malloc(N * sizeof array); for (int i = 0; i < N; i++) array[i] = (int )malloc(N * sizeof array[i]); // Initialize for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) array[i][j] = i j; } // Display for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) cout<< array[i][j] <<'\t'; cout<<endl; } // Destroy for (int i = 0; i < N; i++) free(array[i]); free(array); } However, with C++ you should be using new and delete instead of malloc and free because they understand C++ better: (Toggle Plain Text) #include <iostream> #include <cstdlib> using namespace std; int main() { int N; int *array; cout<<"Enter N: "; cin>> N; // Create array = new int[N]; for (int i = 0; i < N; i++) array[i] = new int[N]; // Initialize for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) array[i][j] = i * j; } // Display for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) cout<< array[i][j] <<'\t'; cout<<endl; } // Destroy for (int i = 0; i < N; i++) delete [] array[i]; delete [] array; } #include <iostream> #include <cstdlib> using namespace std; int main() { int N; int *array; cout<<"Enter N: "; cin>> N; // Create array = new int[N]; for (int i = 0; i < N; i++) array[i] = new int[N]; // Initialize for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) array[i][j] = i * j; } // Display for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) cout<< array[i][j] <<'\t'; cout<<endl; } // Destroy for (int i = 0; i < N; i++) delete [] array[i]; delete [] array; } Or, even better, use the vector class that comes with the C++ standard library: (Toggle Plain Text) #include <iostream> #include <cstdlib> #include <vector> using namespace std; int main() { int N; cout<<"Enter N: "; cin>> N; vector< vector<int> > array(N, vector<int>(N)); // Initialize for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) array[i][j] = i * j; } // Display for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) cout<< array[i][j] <<'\t'; cout<<endl; } } #include <iostream> #include <cstdlib> #include <vector> using namespace std; int main() { int N; cout<<"Enter N: "; cin>> N; vector< vector<int> > array(N, vector<int>(N)); // Initialize for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) array[i][j] = i * j; } // Display for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) cout<< array[i][j] <<'\t'; cout<<endl; } }

Jan 14th, 2005, 9:31 PM	#4
Dizzutch Professional Programmer Join Date: Dec 2004 Location: Worcester, MA Posts: 441 Rep Power: 4	sorry, i wrote that on a limb, didn't really test it. __________________ naked pictures of you \| PFO F@H stats

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Programming Forums > Application Development > C++

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