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Declaring (global) array variables

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• Previously discussed: defining an array
  • Defining an array:

    DataType var[ size ]; // One dimensional array DataType var[ size1 ][ size2 ]; // Two dimensional array

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  • Example:

    double a[10]; // array of 10 double variables

    Result:

    Notice that:

    a[0] is located at the base address (in the example, we assume that the base address is 5000) a[1] is located at address base-address + 8 (a double variable occupies 8 bytes of memory) In general, a[i] is located at address base-address + i×8

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• Declaring (describing) a variable
  • Recall that:

    Declaring a variable = Provide sufficient information to the C compiler to access the variable in memory.

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• Declaring a one-dimensional array variable
  • Syntax to declare a one-dimensional array variable:

    extern DataTyp arrayVarName [ ] ;

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  • Example:

    File where the array is defined	Declaring the (global) array variable
    double a[10]; // Array definition void func( ); // Declare func( ) int main( int argc, char* argv[] ) { int i; for ( i = 0; i < 10; i++ ) a[i] = i*i; func( ); }	extern double a[ ]; // Array declaration void func( ) { int i; for ( i = 0; i < 10; i++ ) printf("a[%d] = %lf\n", i, a[i]); }

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  • Example Program: (Demo above code)                                                
    • The main Prog file: click here
    • The array declaration Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc declare-array1a.c declare-array1b.c To run:          ./a.out

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• How the C compiler uses the array declaration to access a[i]
  • Suppose the C compiler processed the following array declaration:

    extern double a[ ] ;

    The C compiler will handle accessing the array element a[i] as follows:

    It generate a dummy base address X (that the linker must update) The C compiler will use the type information (double is 8 bytes) to generate the address of X + i×8 for the array element a[i].

  • Summary:

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  • The linker is responsible to replace the dummy offset with the actual offset of the array variable.

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• Alternative syntax to declare a one-dimensional array
  • Fact:

    The C compiler will ignore any text between the brackets [...] in an array declaration: extern DataType arrayVarName [ /* ignored ! */ ] ;

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  • A common practice in C is to write the actual array size between the brackets in an array declaration (as a reminder to not exceed the array size)

  • Example:

    File where the array is defined	Declaring the (global) array variable
    double a[10]; // Array definition void func( ); // Declare func( ) int main( int argc, char* argv[] ) { int i; for ( i = 0; i < 10; i++ ) a[i] = i*i; func( ); }	extern double a[10]; // Array declaration void func( ) { int i; for ( i = 0; i < 10; i++ ) printf("a[%d] = %lf\n", i, a[i]); }

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• Declaring a two-dimensional array
  • Recall the syntax to define a two-dimensional array:

    DataType arrayVarName [ size1 ] [ size2 ] ;

    Example: (array definition)

    double a[3][5];

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  • Syntax to declare a two-dimensional array:

    extern DataType arrayVarName [ ] [ size2 ] ;

  • Example: (array declaration)

    File where the array is defined	Declaring the 2-dim. array variable
    double a[3][5]; // Array definition void func( ); // Declare func( ) int main( int argc, char* argv[] ) { int i, j; for ( i = 0; i < 3; i++ ) for ( j = 0; j < 5; j++ ) a[i][j] = i + j; func( ); }	extern double a[ ][5]; // Array declaration void func( ) { int i; for ( i = 0; i < 3; i++ ) { for ( j = 0; j < 5; j++ ) printf("a[%d][%d] = %lf ", i, j, a[i][j]); putchar('\n"); } }

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  • Example Program: (Demo above code)                                                
    • The main Prog file: click here
    • The array declaration Prog file: click here

    How to run the program:

    Right click on links and save in a scratch directory To compile:   gcc declare-array2a.c declare-array2b.c To run:          ./a.out

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• The reason for such a convoluted declaration
  • Fact:

    The C compiler stores a 2-dimensional array in a row-wise fashion inside the computer memory

    Example: double a[3][5] is stored in memory as follows:

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  • In order to enable the C compiler to compute the address of the variable a[i][j], we must tell the C compiler the length of the second dimension of the array:

    As you can see from the above figure:

    address of a[i][j] = base-address + ( i*SIZE-of-the-2nd-dimension + j ) * 8

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• Alternative syntax to declare a two-dimensional array
  • Fact:

    The C compiler will ignore any text between the brackets [...] in the leading (first) dimenison of an array declaration: extern DataType arrayVarName [ /* ignored ! */ ] [ SIZE ] ;

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  • A common practice in C is to write the actual array size of the leading (first) dimenison in an array declaration (as a reminder to not exceed the array size)

  • Example: (array declaration)

    File where the array is defined	Declaring the 2-dim. array variable
    double a[3][5]; // Array definition void func( ); // Declare func( ) int main( int argc, char* argv[] ) { int i, j; for ( i = 0; i < 3; i++ ) for ( j = 0; j < 5; j++ ) a[i][j] = i + j; func( ); }	extern double a[3][5]; // Array declaration void func( ) { int i; for ( i = 0; i < 3; i++ ) { for ( j = 0; j < 5; j++ ) printf("a[%d][%d] = %lf ", i, j, a[i][j]); putchar('\n"); } }