Replace subscripts with pointers in C

Question

I've got a C program that I'd like to have pointers do the calculations instead of the subscripts. Here's the code:

/* rain */
#define TWLV 12
#define YRS 5 
main()

{
     static float rain [YRS][TWLV] = {
{10.2, 8.1, 6.8, 4.2, 2.1, 1.8, 0.2, 0.3, 1.1, 2.3, 6.1, 7.4},
{9.2, 9.8, 4.4, 3.3, 2.2, 0.8, 0.4, 0.0, 0.6, 1.7, 4.3, 5.2},
{6.6, 5.5, 3.8, 2.8, 1.6, 0.2, 0.0, 0.0, 0.0, 1.3, 2.6, 4.2},
{4.3, 4.3, 4.3, 3.0, 2.0, 1.0, 0.2, 0.2, 0.4, 2.4, 3.5, 6.6},
{8.5, 8.2, 1.2, 1.6, 2.4, 0.0, 5.2, 0.9, 0.3, 0.9, 1.4, 7.2}
        };
/* initialize rainfall data for 1970-74 */
int year, month;
float subtot, total;

printf( " YEAR  RAINFALL (inches)\n");
for ( year = 0, total = 0; year < YRS; year++ )
{ /* for each year, sum rainfall for each month */
    for ( month = 0, subtot = 0; month < TWLV; month++)
        subtot += rain[year][month];
    printf("%5d, %15.1f\n", 1970 + year, subtot);
    total += subtot; /*total for all years */
}
printf("\nThe yearly average is %.1f inches. \n\n", total/YRS);
printf("MONTHLY AVERAGES:\n\n");
printf("Jan Feb Mar Apr May Jun Jul Aug Sep Oct");
printf("Nov Dec\n ");

for ( month = 0; month < TWLV; month++)
{ /* for each month, sum rainfall over years */
for ( year = 0, subtot =0; year < YRS; year++)
    subtot += rain[year][month];
printf("%4.1f ", subtot/YRS);
}
printf("\n");
}

I'm struggling with arrays / pointers being used interchangably so if someone could provide an explanation that would be great.

Thanks!

And your definition of main() is wrong, are you using a very old book? — Iharob Al Asimi
– Iharob Al Asimi, Commented Oct 2, 2015 at 1:51
main() should be int main() and should have a return value. — Mike -- No longer here
– Mike -- No longer here, Commented Oct 2, 2015 at 2:04
main is an int - and it returns an int. Always. Pls: why are you not happy, specifically, with subscripts? — jim mcnamara
– jim mcnamara, Commented Oct 2, 2015 at 2:04

Davislor · Accepted Answer · 2015-10-02 03:35:52Z

0

If the type of your array is float rain[YRS][TWLV], you can replace every instance of rain[i][j] with *(&rain[0][0] + (i*TWLV) + j). This won’t actually gain you anything, because the compiler does it anyway. However, you can also declare for ( float* p = &rain[0][0]; p < &rain[0][0] + YRS*TWLV; ++p ) and that might be faster because it eliminates pointer arithmetic inside the loop.

answered Oct 2, 2015 at 3:35

Davislor

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