c++ 2 dimensional array

Question

int main ()
{
  int line,column,i,j;
  int sortir = 1;
  float nbr;

  cout << "The number of column :";
  cin >> column;
  cout << "The number of lines: ";
  cin >> line;
  float matrice [column][line];   //creating my 2d array

  for(i=0;i <= line;i++) //asking the user to put the numbers he wants in the 2d array
    for(j=0;j <= column;j++){
      cout << "Enter a number";
      cin >> nbr;
      matrice[j][i] = nbr;
    }
  system("PAUSE");
  return 0;
}

Lets say I do an array of line = 1 and column = 1 which makes (memory zones) [0,0] [0,1] [1,0] [1,1]. Let's say the user inputs these following numbers:

[0,0]=1

[0,1]=2

[1,0]=3

[1,1]=4

When I want to show the user what he inputted at the end of the program: the zone [0][1] and [1][0] show the same number?

cout << matrice[0][0] = 1

cout << matrice[0][1] = 3 <-- why the f***

cout << matrice[1][0] = 3 <--His my for loop good?

cout << matrice[1][1] = 4

C++ doesn't have built-in dynamically sized two dimensional arrays. If the above code compiles, you are using a compiler extension. — Dietmar Kühl
– Dietmar Kühl, Commented Sep 14, 2013 at 22:47

juanchopanza · Accepted Answer · 2013-09-14 22:48:07Z

2

You are accessing your array out of bounds. You loop over too many elements. You need

for(i=0;i < line;i++) {
  for(j=0;j < column;j++){

Besides that, your code is not standards compliant C++, and is relying on an extension called variable length arrays (VLA). You cannot declare an automatic storage array with a size determined at runtime.

int i;
std::cin >> i;
int a[i];   // ERROR (non-standard VLA extension)
const int j = 42;
int b[j];   // OK
int c[42];  // OK

answered Sep 14, 2013 at 22:48

juanchopanza

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Comments

Cort Ammon · Accepted Answer · 2013-09-14 22:48:54Z

1

That is not valid syntax. float matrice[column][line] is illegal. You cannot have variable sized arrays in C++. Your compiler is providing such as an extension.

answered Sep 14, 2013 at 22:48

Cort Ammon

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2 Comments

opatut Over a year ago

You can of course create variable length arrays on the heap, using the new keyword. float* arr = new float[length];

Cort Ammon Over a year ago

Before you do, make sure you learn the delete command as well. new and delete always go hand in hand.

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