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The equivalence of pointers and arrays in C

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• An alternative way to express array parameter
  • Recall: function with an array parameter

    void print( int N, double x[ ] ) { int i; for ( i = 0; i < N; i++ ) printf("%lf\n", x[i] ); } int main( int argc, char *argv[ ] ) { double a[10]; print( a ) ; }

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  • Fact:

    The data type of a in the example is: (double *) // address of the double variable &a[0]

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  • We can re-write the definition of the function as follows:

    void print( int N, double *x ) { int i; for ( i = 0; i < N; i++ ) printf("%lf\n", x[i] ); // x[i] <==> *(x + i) !!! } int main( int argc, char *argv[ ] ) { double a[10]; print( a ) ; }

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• Postscript: multi-dimensional arrays
  • Fact:

    The above syntax equivalence does not generalize to more than one-level of [ ] In other words: p[i][j] is invalid Multi-dimensional arrays: If you want dynamic multi-dimensional arrays in C, you have to maintain the array yourself !!!

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  • Implementing a 2-dimensional (dynamic) array:

    You must map the elements in a two-dimensional array onto a one-dimensional array. Example: The mapping is as follows: mapped to a[i][j] ===> a[ i + j*columnSize ]

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  • Example: a 3x4 (dynamically allocated) array

    #define NRows 3 #define NCols 4 int main(int argc, char *argv[]) { int i, j; double* p; // We uses this reference variable to access // dynamically created array elements p = calloc(NRows*NCols, sizeof(double) ); // 3x4 = 12 elements for ( i = 0; i < NRows; i++ ) for ( j = 0; j < NCols; j++ ) p[i*NCols+j] = i+j; // put value i+j in array element p[i][j] for ( i = 0; i < NRows; i++ ) { for ( j = 0; j < NCols; j++ ) printf("p[%d][%d] = %lf ", i, j, p[i*NCols+j] ); putchar('\n'); } free(p); // Un-reserve the first array }

    Result:

    p[0][0] = 0.000000 p[0][1] = 1.000000 p[0][2] = 2.000000 p[0][3] = 3.000000 p[1][0] = 1.000000 p[1][1] = 2.000000 p[1][2] = 3.000000 p[1][3] = 4.000000 p[2][0] = 2.000000 p[2][1] = 3.000000 p[2][2] = 4.000000 p[2][3] = 5.000000

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc dyn-array3.c To run:          ./a.out