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Declaring Array Variable

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• Declaring a one-dimensional array
  • Recall this is how you define a one-dimensional array:

    TYPE arrayName[ SIZE ];

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  • You declare an array as follows:

    extern TYPE arrayName[ ];

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  • Example;

    int a[10]; // Defines the variable a (array of 10 integers) extern int a[]; // Declaration

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  • Example: Program file that defines an array

    int a[10]; // Define array int f(int i) { return a[i]; }

    Program file that declares an array

    extern int a[]; // Declare array int main(int argc, char *argv[]) { int i; int f(int); // Declare function for ( i = 0; i < 10; i++ ) a[i] = i*i; for ( i = 0; i < 10; i++ ) cout << f(i) << endl; }

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  • Example Program: (Demo above code)                                                
    • Prog file with array definition: click here
    • Prog file with array declaration: click here

    Compile and run:

    CC array01a.C array01b.C a.out

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• C++ allowance for convenience
  • For convenience (and clarity), C++ will allow the user to specify the SIZE in an array declaration

    Example:

    extern int a[ 10 ]; // Declare array ^^^^ (ignored by C++) int main(int argc, char *argv[]) { int i; int f(int); // Declare function for ( i = 0; i < 10; i++ ) a[i] = i*i; for ( i = 0; i < 10; i++ ) cout << f(i) << endl; }

    (In fact, you can lie and put in any value you want. C++ could care less...)

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• Declaring a two-dimensional array
  • Recall this is how you define a two-dimensional array:

    TYPE arrayName [ SIZE1 ] [ SIZE2 ];

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  • You declare an array as follows:

    extern TYPE arrayName [ ] [ SIZE2 ];

    In general:

    In an array declaration, you must tell C++ all dimensions, except the first dimension

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  • Example;

    int a[3][4]; // Defines the variable a (array of 3x4 integers) extern int a[][4]; // Declaration

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  • Example: Program file that defines an array

    int a[3][4]; // Define array int f(int i) { return a[i]; }

    Program file that declares an array

    extern int a[][4]; // Declare array int main(int argc, char *argv[]) { int i; int f(int); // Declare function for ( i = 0; i < 10; i++ ) a[i] = i*i; for ( i = 0; i < 10; i++ ) cout << f(i) << endl; }

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  • Example Program: (Demo above code)                                                
    • Prog file with array definition: click here
    • Prog file with array declaration: click here

    Compile and run:

    CC array03a.C array03b.C a.out

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• Why does C++ need all but the first dimension...
  • C++ stores the elements of a multi-dimensional array row-wise in memory

    Example: a 3x4 array

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  • The figure shows the placement of the elements of a 2-dimensional array in a linear fashion

    Example:

    a[0][0] is placed at position 0 a[0][1] is placed at position 1 a[0][2] is placed at position 2 a[0][3] is placed at position 3 a[1][0] is placed at position 4 a[1][1] is placed at position 5 ... a[i][j] is placed at position i*4 + j

  • So, in order to compute the position that the element a[i][j] occupied, C++ need to know the number of elements in a row

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  • The above situation can be generalized for an N-dimensional array

    (It's easier to explain using a 2-dimensional array)